Respiration
I. Respiratory Quotient: A. Volume of CO2 taken up exactly balances the volume of CO2 releases B. Respiratory quotient or RQ is the ratio of CO2 to O2. II. Formation of Hexose Sugars from Reserve Carbohydrates A. Storage and Degradation of Starch: Amylases hydrolyze nonbranched
amylose chains to maltose. Starch Phosphorylase converts amylase to
glucose-1-phosphate. The action of these enzymes on amylopectin
(amylose with side chains linked alpha 1->6)leave a dextrin, the
branch linkages of which are hydrolyzed by Debranching Enzyme.
Maltose is hydrolyzed to glucose by Maltase. 1. Degradation of starch to glucose requires three enzymes a. alpha-amylase, beta-amylase, starch phosphorylase b. Amylases use one H2O for each bond cleaved (hydrolase enzymes) c. Large molecules are synthesized by one series of reactions
(pathways) and are broken down by another. Enzymes that make
amylase are controlled in the opposite way from enzymes that
degrade amylase. d. Amylases are most active in germinating seeds. 2. Alpha-amylase a. Randomly attacks alpha 1,4 bonds b. Leads to maltose (glucose-glucose) c. Cannot attack alpha 1,6 bonds in amylopectin d. Amylopectin digestion stops when branched dextrins of short
chain lengths still remain e. Activated by Ca2+ 3. Beta-amylase a. Hydrolyzes starch into Beta-maltose b. Works from the nonreducing ends (4' side) 4. Starch Phosphorylase a. Begin at nonreducing end b. Incorporation of phosphate (phosphorolytic enzyme) c. Reaction is reversible in vitro 5. Alpha 1,6 branch linkages hydrolyzed by debranching enzymes. a. Three main types b. pullulanase, isoamylase, limit dextrinase 6. Maltose is degraded by maltase enzyme 7. All starch degradation occurs in chloroplasts or amyloplasts.
Breakdown of the resulting hexoses begins in the cytosol. Hexoses
derived from starch are converted to triose phosphates
(3 carbon phosphates) in the plastids. These are
3-Phosphoglyceraldehyde (PGA) and dihydroxyacetone-phosphate
(DHAP). PGA and DHAP are moved by a phosphate carrier into the
cytosol. B. Hydrolysis of Fructans 1. Little is known about their metabolism 2. Hydrolyzed by b-fructofuranosidase 3. Specificity of b-2,1 and b-2,6 links 4. Fructose can undergo respiration directly, sucrose must first be
split into glucose and fructose. C. Hydrolysis of Sucrose: 1. Invertases: found in the cytosol, vacuole, and cell walls. a. Cytosol Invertase: alkaline type with a pH optimum near 7.5 b. Vacuole and Cell Wall Invertase: acidic invertases with pH
optima of 5 or less c. Cell-wall Invertase: hydrolyzes incoming, translocated sucrose
into glucose and fructose, absorbed by sink cells 2. Sucrose Synthase: main enzyme that degrades sucrose in
starch-storage or in rapidly growing tissues that are converting
translocated sucrose to cell-wall polysaccharides. 3. In slow-growing and mature cells, invertase is more important
in sucrose degradation and provides glucose and fructose for
respiration. III. Glycolysis: central pathway of glucose catabolism A. Three routes of glucose catabolism: 1. Aerobic glycolysis: complete degradation of glucose to CO2 and H2O 2. Anaerobic glycolysis: glucose reduced to lactate 3. Alcoholic fermentation: glucose to ethanol production B. Four Important Functions: 1. Converts one hexose molecule into two molecules of pyruvic acid,
and partial oxidation of hexose occurs 2. Production of ATP 3. Formation of molecules that can be removed from the pathway to
synthesize several other constituents of which the plant is made. 4. Pyruvate produced can be oxidized in mitochondria to yield
relatively large amounts of ATP, much more than is produced
in glycolysis. C. Two Phases of Glycolysis: 1. Preparatory phase of glycolysis: a. Serves to collect the carbon chains of all the metabolized
hexoses in the form of one common product b. glyceraldehyde 3-phosphate 2. Second phase: a. Promoted by five enzymes b. Represents the "pay off" of glycolysis c. Energy freed when 2 molecules of glyceraldehyde 3-phosphate are (1) convert into 2 molecules of pyruvate (2) conserved by coupled phosphorylation of 4 molecules of ADP
to ATP. (3) substrate level phosphorylation 3. 3 different type of chemical transformations a. Degradation of the carbon skeleton: pathway of carbon atoms b. Phosphorylation of ADP to ATP: pathway of phosphate groups c. Transfer of hydrogen atoms or electrons: NAD+ -> NADH 4. Enzymes of glycolysis are present in dissolved forms in the cell
cytosol. 5. Enzymes promoting the oxygen-requiring phase of carbohydrate
oxidation a. Located in the mitochondrial membrane in eukaryotic cells b. Located in the plasma membrane of prokaryotic cells D. Glycolysis takes place via phosphorylated intermediates 1. Nine metabolic intermediates between glucose and pyruvate are
all phosphorylated compounds 2. Three functions: a. Phosphate groups are completely ionized at pH 7. (1) have a net negative charge (2) cell membranes generally impermeable to molecules with
electric charge (a) molecules cannot escape cell (b) glucose, lactate, pyruvate: have specific transport systems b. Essential intermediates in the enzymatic conservation of
metabolic energy (1) phosphate is ultimately transferred to ADP to yield ATP. c. Phosphate groups serve as recognition or binding groups (1) required for the proper fit to the active sites of enzymes. 3. Nearly all the glycolytic enzymes require Mg2+ for activity E. First phase of glycolysis results in cleavage of the hexose chain 1. Phosphorylation of Glucose (deltaGo' = -4.0 kcal/mol) a. first step: D-glucose molecule is primed b. phosphorylation at the 6 position: glucose 6-phosphate c. expense of ATP d. Hexokinase with Mg2+ (1) promotes phosphorylation of common hexoses (glu, fru, man) (2) occurs in different. isozyme forms (3) differ with respect to kinetic properties 2. Conversion of Glucose 6-Pi to Fructose 6-Pi (deltaGo'= +0.4 kcal/mol) a. Phosphoglucoisomerase (1) shift in the carbonyl oxygen from carbon atom 1 to carbon
atom 2 (2) requires Mg2+ (3) specific for glucose 6-Pi and fructose 6-Pi 3. Phosphorylation of Fructose 6-Pi to Fructose 1,6-Bis(Di)phosphate In plants there are two ways that this occurs.
(1) ATP-phosphofructokinase (ATP-PFK) which uses two ATP in the beginning preparation steps of glycolysis (2) Pyrophosphate Phosphofructokinase (PPi-PFK) which uses
one ATP in the preparation steps. ATP-PFK route is involved
in maintenance respiration by cells that are not rapidly
growing. PPi-PFK route is more adaptive and can
increases or decrease in importance depending on developmental
processes and environmental conditions. a. Second of the 2 priming reactions of glycolysis b. Phosphofructokinase: requires Mg2+ c. Essentially irreversible (DGo'= - 3.4 kcal/mol) d. Second control point in glycolysis (1) regulatory enzyme (2) most complex known (3)accelerates when cell's supply of ATP becomes depleted (a) or there is an excess of ADP and AMP (4) inhibited whenever cell has ample ATP 4. Cleavage of Fructose 1,6-Bisphosphate (deltaGo'= + 5.73kcal/mol) a. fructose bisphosphate aldolase (1) catalyzes a reversible aldo condensation b. two different triose phosphates, (1) glyceraldehyde 3-phosphate (aldose) (2) dihydroxyacetone phosphate (ketose) c. does not require Mg2+ 5. Interconversion of the Triose Phosphates (deltaGo'= +1.83kcal/mol) a. Gly 3-Pi directly degraded b. DHAP can rapidly and reversibly convert into G3P c. Triose phosphate isomerase F. The Second Phase of Glycolysis Is Energy-Conserving 1. Energy-conserving phosphorylation steps a. 1 glucose = 2 glyceraldehyde 3-phosphate b. 2 glyceraldehyde 3-Pi = 2 pyruvate + 4 ATP c. net yield of ATP = 2 2. Oxidation of Glyceraldehyde 3-Phosphate to 3- Phosphoglyceroyl Phosphate a. first of 2 energy-conserving reactions b. glyceraldehyde phosphate dehydrogenase (deltaGo'= +1.5 kcal/mol) c. Coenzyme NAD+ is hydrogen acceptor (1) oxidized form of nicotinamide adenine dinucleotide (2) contains vitamin nicotinamide 3. Transfer of Phosphate from 3-Phosphoglyceroyl Phosphate to ADP a. Phosphoglycerate kinase (1) transfers high-energy phosphate group (2) from the carboxyl group of 3-Phosphoglyceroyl phosphate b. Generation of ATP coupled to an enzyme transformation of a
substrate (1) to ADP = 3-phosphoglycerate (deltaGo'= -4.5 kcal/mol) (2) metabolic intermediate, is a substrate-level phosphorylation. 4. Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate a. phosphoglycerate mutase (deltaGo'= +1.06 kcal/mol) b. Mg2+ essential c. mutase is used to designate enzymes catalyzing intramolecular
shifts of functional groups 5. Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate a. second reaction: high-energy phosphate compound generated b. enolase: reversible removal of a molecule of water
(deltaGo'= +.00 kcal/mol) 6. Transfer of the Phosphate Group from PEP to ADP a. Last step transfer of high-energy phosphate group from PEP to ADP b. pyruvate kinase: substrate-level phosphorylation (1). regulatory enzyme 7. Reduction of Pyruvate to Lactate a. Important junction point b. Aerobic conditions (1) NADH is reoxidized to NAD+ by O2 (2) 2 molecules of NADH formed in cytosol (a) are reoxidized to NAD+ (b) by transfer of their electrons to the electron-transport chain (c) located in the mitochondria (3) electrons are passed to oxygen, reducing it to form H2O G. "Feeder" pathways lead from glycogen and other carbohydrates into
the Central Glycolytic Pathway 1. D-glucose units of the outer branches of glycogen and starch a. glycogen phosphorylase or starch phosphorylase and
phosphoglucomutase b. Removes one glucose at a time in the form of glu 1-Pi c. Occurs in intracellular condition in which the phosphate conc.
is high (1) Glycogen phosphorylase reaction proceeds only in the
direction of degradation to yield glu 1-Pi. d. Terminal a(1-4) glycosidic linkage at the nonreducing end of the
glycogen branch undergoes phosphorolysis (1) Removal of the terminal glucose residue by attack of phosphate (2) To yield a-D-glucose 1-phosphate. 2. Further degradation alpha(1-6) glucosidase: two reactions a. First: enzyme removes three remaining glucose residues
and transfers them to the end of some other outer branch b. Second: hydrolyses of the alpha(1-6) linkage at the branch
point, yielding a molecule of D-glucose and making
available another length of glucose residues c. Phosphoglucomutase: glu 1-Pi to glu 6-Pi 3. Other monosaccharides enter the glycolytic sequence a. D-Fructose (1) phosphorylated by hexokinase to give fru 6-Pi (2) major pathway in muscles and the kidney (3) liver however fructose gains entry by enzyme fructokinase (a) phosphorylation of fructose, fru 1-Pi (b) fru 1-Pi cleaved to: D-glyceraldehyde and DHAP by aldolase (c) DHAP used directly (d) D-glyceraldehyde is phosphorylated by ATP with triose kinase
to form D-glyceraldehyde 3-phosphate b. D-galactose 1. first phosphorylated with ATP by galactokinase: D-galactose 1-Pi 2. converted to epimere at carbon atom 4 to D-glucose 1-Pi as
follows: (a) uridine diphosphate (UDP) functions as a coenzyme-like
carrier of hexose groups (b) human liver, galactose 1-Pi reacts with UDP-glucose to yield
UDP-D-galactose and glucose 1-Pi (c) Enzyme is: UDP-glucose:a-D-galactose 1-phosphate
uridylyltransferase (d) galactose residue of UDP-D-galactose is then enzymatically
epimerized at carbon 4 to yield UDP-D-glucose by the enzyme UDP-glucose 4-epimerase (e) UDP-glucose is then cleaved by UDP-glucose pyrophosphorylase
to D-glu 1-Pi (f) converted into glu 6-Pi by phosphoglucomutase 3. Sequence used in reverse in the mammary gland for the synthesis
of D-galactose, required in the formation of lactose or milk
sugar 4. galactosemia: UDP-glu:a-D-gala 1-Pi uridylyltransferase is
genetically defective (a) D-galactose and D-gala 1-Pi accumulate in the blood and
tissues (b) liver and other organs become enlarged, vision becomes
impaired because of cataracts, mental retardation (c) deficiency appear in infants c. D-mannose 1. phosphorylated at the 6 position by hexokinase (man 6-Pi) 2. isomerized by phosphomannoisomerase to yield fru 6-Pi VI. Fermentation (Anaerobic Respiration) A. Anaerobic conditions (Lactic acid or Ethanol) 1. Enzymes: a. Ethanol: (1) Pyruvic Acid Decarboxylase (2) Alcohol dehydrogenae b. Lactic Acid: (1) Lactic acid dehydrogenase: uses NADH to form L-lactate
(deltaGo'=-6.0 kcal/mol) (2) Uses NADH to reduce pyruvic acid to lactic acid 2. NADH generated by glycolysis cannot be reoxidized by O2 3. NADH must be reoxidized to NAD+ B. Alcoholic fermentation differs from glycolysis in its terminal steps 1. Yeast and other microorganisms that ferment glu to Ethanol and CO2 a. enzymatic pathway of glu degradation is identical to that
described for anaerobic glycolysis 2. In yeast: two alternate enzymatic reaction
a. first, pyruvate resulting from the breakdown of glu loses its
carboxyl group by the action of pyruvate decarboxylase b. acetaldehyde is reduced to ethanol (1) with NADH derived from glyceraldehyde 3-Pi dehydrogenation
furnishing the reducing power (2) through the action of alcohol dehydrogenase C. Electron flow from substrates to oxygen is the source of ATP energy 1. Each turn around the citric acid cycle a. four pairs of hydrogen atoms are removed b. isocitrate, a-ketoglutarate, succinate, malate 2. donate their electrons to the electron-transport chain and become
H+ ions 3. electrons are transported along a chain until reach cytochrome aa3
(cytochrome oxidase) VII. The Citric Acid Cycle A. Oxidation of glucose to CO2 and H2O releases more energy than
glycolysis 1. glucose -> 2 lactate + 2H+ (deltaGo'= -47.0 kcal/mol) glucose + 6O2 -> 6CO2 + 6H2O (deltaGo' = -686 kcal/mol) 2. Anaerobically: lactate still contains 93% available energy B. Primary functions of the Krebs/Citric Acid Cycle: 1. Reduction of NAD+ and ubiquinone to the electron donors NADH and
ubiquinol, which are subsequenctly oxidized to yield ATP 2. Direct synthesis of a limited amount of ATP (one ATP for each
pyruvate oxidized) 3. Formation of carbon skeletons that are used to synthesize
amino acids that, in turn, are converted into larger molecules. C. Pyruvate must be oxidized to Acetyl-CoA and CO2 1. Carbohydrates, Fatty Acid, and Amino Acid are oxidized to CO2 and
H2O via citric acid cycle a. First: carbon backbones are degraded to acetyl-CoA (1) form in which the citric acid cycle accepts fuel 2. Pyruvate dehydrogenase complex: in mitochondria of eukaryotes,
cytoplasm of prokaryotes a. overall reaction: Pyruvate + NAD+ + CoA-SH -> acetyl-CoA + NADH + H+ + CO2 b. Pyruvate undergoes oxidative decarboxylation: (1) carboxyl group is removed as a molecule of CO2 (2) Acetyl group appears as acetyl-CoA (3) Two hydrogen atoms appear as NADH and H+ 3. Sequential action of three different enzymes a. pyruvate dehydrogenase b. dihydrolipoyl transacetylase c. dihydrolipoyl dehydrogenase 4. Five different coenzymes: a. thiamine pyrophosphate (TPP) b. flavin adenine dinucleotide (FAD) c. coenzyme A (CoA) d. nicotinamide adenine dinucleotide (NAD+) e. lipoic acid 5. Organized into a multienzyme cluster a. four vitamins required in human nutrition are vital (1) thiamine (in TPP) (2) riboflavin (in FAD) (3) pantothenic acid (in CoA) (4) nicotinamide (in NAD+) b. lipoic acid (1) an essential vitamin or growth factor for several
microorganisms 6. Sequence of five steps: a. step 1: pyruvate loses its carboxyl group - pyruvate
dehydrogenase b. step 2: transfer of H atoms and the acetyl group to core enzyme
dihydrolipoyl transacetylase c. step 3: molecule of CoA-SH reacts with the acetyl derivative to
yield acetyl-S-CoA d. step 4: reduced from of dihydrolipoyl transacetylase (1) acted upon by dihydrolipoyl dehydrogenase (2) transfer of hydrogen atoms to the FAD prosthetic group of
dihydrolipoyl dehydrogenase. e. step 5: the reduced FAD group of dihydrolipoyl dehydrogenase
transfers hydrogen to NAD+, forming NADH D. Citric Acid Cycle is a circular enzyme system 1. Sir Hans Krebs, Nobel Prize 1953 for citric acid cycle E. Citric Acid Cycle has eight steps 1. Citrate Is Formed by Condensation of Acetyl-CoA with Oxaloacetate a. First reaction: condensation of acetyl-CoA with oxaloacetate to
from citrate b. CoA-SH formed in this reaction free to participate in the
oxidative decarboxylation of another pyruvate c. Citrate synthase is a regulatory enzyme: rate-limiting step 2. Citrate is converted into isocitrate via cis-aconitate a. aconitase: reversible transformation of citrate into isocitrate b. intermediary formation of tricarboxylic cis-aconitate c. reversible addition of H2O d. isocitrate (product)is transformed into the subsequent steps of
the cycle. e. aconitase contains iron and acid-labile sulfur atoms arranged
in a cluster called an iron-sulfur center 3. Isocitrate is dehydrogenated to yield a-ketoglutarate and CO2 a. isocitrate is dehydrogenated to a-ketoglutarate and CO2 by
isocitrate dehydrogenase b. two different isocitrate dehydrogenases (NAD+, and NADP+) (1) NAD+ found in mitochondria (2) NADP+ found in mitochondria. and cytosol (3) require Mg2+ or Mn2+ (4) ADP positive modulator 4. a-Ketoglutarate is oxidized to succinate and CO2 a. a-ketoglutarate undergoes oxidative decarboxylation to form
succinyl-CoA and CO2 b. a-ketoglutarate dehydrogenase complex c. virtually identical to the pyruvate dehydrogenase reaction d. three enzymes (analogous to pyruvate system) e. enzyme-bound thiamine pyrophosphate, Mg2+, coenzyme A, NAD+,
FAD, and lipoic acid 5. Conversion of Succinyl-CoA into Succinate a. energy-conserving coupled reaction b. phosphorylation of guanosine diphosphate (GDP) to GTP c. succinyl-CoA synthetase d. substrate-level phosphorylations e. GTP formed by succinyl-CoA synthetase, donates its terminal
phosphate group to ADP to form ATP a. nucleoside diphosphokinase 6. Dehydrogenation of Succinate to Fumarate a. succinate formed from succinyl-CoA is dehydrogenated to fumarate b. flavoprotein succinate dehydrogenase c. covalently bound flavin adenine dinucleotide, hydrogen acceptor d. succinate dehydrogenase is competitively inhibited by malonate 7. Fumarate Is Hydrated to Form Malate a. reversible hydration of fumarate to L-malate b. fumarate hydratase: commonly called fumarase c. requires no coenzyme 8. Malate Is Dehydrogenated to Form Oxaloacetate a. the NAD-linked L-malate dehydrogenase b. dehydrogenation of L-malate to oxaloacetate F. Conversion of Pyruvate to Acetyl-CoA Is Regulated 1. Rate of the citric acid cycle is regulated first by the rate of
formation of acetyl-CoA a. arises from oxidation of pyruvate and oxidation of Fatty Acid b. when ATP conc. in mitochondria. is high (1) ample acetyl-CoA and Krebs cycle intermediates available, (2) formation of acetyl-CoA is slowed down. 2. ATP serves as a stimulatory modulator to activate an auxiliary
enzyme, pyruvate dehydrogenase kinase. a. uses ATP to phosphorylate a specific serine residue in the
pyruvate dehydrogenase b. yields inactive form pyruvate dehydrogenase phosphate 3. When ATP levels decline, a. reactivation by hydrolytic removal of inhibitory phosphate group
from pyruvate dehydrogenase by pyruvate dehydrogenase phosphate
phosphatase b. stimulated by rise in conc. of free Ca2+ (1) important metabolic messenger (2) conc. rises at times of ATP need 4. pyruvate dehydrogenase complex is regulated allostericly a. strongly inhibited by ATP, acetyl-CoA, and NADH (products
of pyruvate dehydrogenase reaction) G. Citric Acid Cycle is regulated 1. First reaction set the overall pace of the cycle (citrate synthase) a. controlled by concentration of: (1) acetyl-CoA (2) oxaloacetate (3) succinyl-CoA (a) inhibits cit. synthase by decreasing affinity for acetyl-CoA b. Fatty Acid precursors of acetyl-CoA inhibit citrate synthase by
allosteric effects c. In some cells citrate and NADH are inhibitors 2. Isocitrate to Â-ketoglutarate and CO2 a. regulated through the allosteric stimulation of NAD-linked enzyme by ADP b. NADH and NADPH are inhibitory modulators of isocitrate
dehydrogenase 3. a-ketoglutarate dehydrogenase complex is inhibited by its product
succinyl-CoA H. Citric Acid Cycle intermediates are used for other metabolic
purposes and can be replenished 1. Citric acid cycle is an amphibolic pathway a. function in oxidative catabolism of carbohydrates, FATTY ACID,
and AMINO ACID b. also first stage in biosynthetic pathways (1) a-ketoglutarate, succinate, and oxaloacetate, (a) precursors of AMINO ACID 2. Special enzymes replenish pool of cycle intermediates - anaplerotic
reactions a. enzymatic carboxylation of pyruvate by CO2 to form oxaloacetate (1) pyruvate carboxylase (2) when citric acid cycle is deficient in oxaloacetate or any of
the other intermediates (3) pyruvate carboxylated to produce more oxaloacetate (4) requires energy, ATP b. pyruvate carboxylase is a very complex enzyme (1) regulatory enzyme (a) acetyl-CoA positive allosteric modulator (b) acetyl-CoA in excess - stimulates the pyruvate carboxylase
reaction 3. Pyruvate carboxylase reaction important anapeloric reaction in
liver and kidney 4. In heart and muscles: phosphoenolpyruvate carboxykinase a. breakdown of PEP (from glycolysis) b. furnishes energy for carboxylation to yield oxaloacetate and
generates GTP I. Glyoxylate Cycle Is a Modification of the Citric Acid Cycle 1. Plants and some microorganisms 2. Acetyl groups source of intermediates required to synthesize.
Carbon skeletons of carbohydrates 3. Citric acid cycle operates in 2 modes a. carry out standard cycle b. specialized modification called glyoxylate cycle 4. Acetyl-CoA condenses with oxaloacetate to form citrate a. citrate synthase 5. Usual conversion of citrate to isocitrate with aconitase 6. Isocitrate to succinate and glyoxylate with isocitrate lyase a. succinate may be converted to oxaloacetate via fumarate and
malate b. oxaloacetate then converted to PEP with PEP carboxykinase c. PEP is then used as the precursor for gluconeogenesis 7. Glyoxylate then condenses with acetyl-CoA to yield malate with
malate synthase 8. Malate dehydrogenated to oxaloacetate a. condensation with another molecule of acetyl-CoA 9. Does not occur in animals a. lack the enzymes isocitrate lyase and malate synthase 10. active in germinating plant seeds a. converts acetyl residues from FATTY ACID metabolism to glucose b. occurs in cytoplasmic organelles called glyoxysomes VIII. Electron-Transferring Reactions Are Oxidation-Reduction Reactions A. Electron-donating molecule is called the reducing agent or reductant B. electron-accepting molecule is the oxidizing agent or oxidant C. Reducing and oxidizing agents function as conjugate reductant-oxidant pairs D. Electrons are transferred from one molecule to another in one of 4
ways: 1. Electrons may be transferred directly a. Fe2+ + Cu2+ -> Fe3+ + Cu+ b. Fe transfers its electrons to Cu 2. Electrons may be transferred in the form of hydrogen atoms a. AH2 = A + 2e- + 2H+ b. AH2 + B -> A + BH2 3. Electrons may be transferred from an electron donor to an
acceptor a. in the form of a hydride ion (:H-) b. which bears 2 electrons, c. as in the case of NAD-linked dehydrogenases 4. Electrons transfer by direct combination of an organic reductant
with oxygen a. give a product in which the oxygen is covalently incorporated b. R-CH3 + 1/2O2 = R-CH2-OH E. Neutral term: Reducing equivalent is used to designate a single
electron equivalent participating in an oxidoreduction reaction F. Each conjugate redox pair has a characteristic standard potential 1. The convention is to express the standard potentials of conjugate
redox pairs as reduction potentials: a. assign increasingly negative values to systems that have
increasing tendencys to lose electrons b. assign increasingly positive values to systems that have
increasing tendencies to accept electrons. 2. List in order of increasing potential, ie: in the order of
decreasing tendency to lose electrons G. Free-Energy Changes Accompany Electron Transfers 1. The Eo' values of various redox couples a. used to predict the direction of flow of electrons from
one redox couple to another b. when both are present under standard conditions and a catalyst
is available. 2. Catalyst does not alter the direction of flow 3. Electrons tend to flow from a relatively electronegative
conjugate redox pair to a more electropositive electron acceptor a. result of the loss of free energy 4. 52.6 kcal of free energy released when a pair of electrons passes
from NADH to oxygen under standard conditions a. enough to synthesize three molecules of ATP, b. requires input of 3(7.3) = 21.9 kcal under standard conditions. 5. Energy diagram showing: a. standard potentials of electron carrier of the respiratory chain b. the direction of electron flow is always "downhill" toward oxygen H. There are many electron carriers in the electron-transport chain 1. Respiratory chain of the mitochondria contains electron-carrying
proteins a. act in sequence to transfer electrons from substrate to oxygen 2. 15 or more chemical groups in the electron-transport chain a. can accept and transfer reducing equivalents in sequence 3. Different kinds of e- carrying groups associated with proteins a. NAD b. FMN c. ubiquinone or coenzyme Q (1) isoprenoid lipid-soluble quinone which functions in
association with one or more proteins d. 2 different kinds of iron-containing proteins (1) iron-sulfur centers (Fe-S) (2) cytochromes e. copper of cytochrome aa3 (cytochrome oxidase) 4. All water insoluble and are in the inner mitochondria membrane I. Pyridine nucleotides have a collecting function 1. e- pairs entering the respiratory chain arise from dehydrogenases a. use the coenzymes NAD+ or NADP+ as e- acceptors 2. NAD(P)-linked dehydrogenases 3. pyridine-linked dehydrogenases remove 2 hydrogen atoms from their
substrates a. one of these is transferred as a hydride ion (:H+) to NAD+ or
NADP+ b. the other appears as H+ in the medium J. NADH dehydrogenase accepts electrons from NADH Ubiquinone Is a Lipid-Soluble Quinone Cytochromes Are Electron-Carrying Heme Proteins Incomplete Reduction of Oxygen Causes Cell Injury Electron Carriers Always Function in a Specific Sequence Electron-Transport Energy is Conserved by Oxidative Phosphorylation K. Energetics: Glycolysis, Krebs Cycle, and Electron-Transport System 1. Glycolysis yields 2 ATP and 2 NADH per hexose used a. NADH oxidized by the electron transport system yields 2 ATP b. Glycolysis contributes a total of 6 ATP per hexose or per two
pyruvate 2. Citric Acid Cycle a. 8 NADH per hexose within the mitochondrial matrix b. oxidative phosphorylation each of these NADH yields 3 ATP, or
24 per hexose. c. Each ubiquinol yields 2 ATP by oxidative phosphorylation, or
4 per hexose d. Total contribution of Citric Acid cycle is 30 ATP 3. 6 ATP + 30 ATP = 36 ATP per hexose 4. Efficiency of respiration in terms of how much energy in glucose
can be trapped in the terminal phosphate bond of ATP. a. The standard Gibbs free energy change (DG’o) for complete
oxidation of one mole of glucose or fructose at pH 7 is
-2,870 kJ ( -686 kcal). b. The deltaGo’ for hydrolysis of the terminal phosphate in a mole
of ATP is -31.8 kJ (-7.6 kcal) or -1,140 kJ in 36 moles of ATP. c. The efficiency is about -1140/-2870 or 40%. d. The remaining 60% lost as heat. IX. Cyanide-Resistant Respiration A. Cyanide (CN-), Azide (N3-) and Carbon monoxide (CO): negative ions B. Cyanide-resistant mitochondria have an alternative branch
in the electron-transport pathway at the first step involving
ubiquinol (UQH2) 1. Terminal oxidase has a much lower affinity for O2 2. Little or no oxidative phosphorylation 3. Leads mainly to production of heat a. heat production is beneficial to certain plants b. pollination ecology of arum lilies: Saurmatum guttatum and
Symplocarpus foetidus (skunk cabbage) C. The alternative pathway operates as an overflow mechanism
to remove electrons when the cytochrome pathway becomes saturated
by rapid glycolysis and Krebs-cycle activities 1. The alternative pathway results in decreased efficiency
of respiration in plants in proportion to the pathway’s activity. X. The Pentose Phosphate Pathway A. Glycolysis and PPP occur mainly in the cytosol, the two pathways
are interwoven 1. in PPP: NADP+ is the electron acceptor 2. in Glycolysis NAD+ is the electron acceptor B. Glucose-6-phosphate dehydrogenase is inhibited non-competitively
by NADPH. 1. Chloroplasts: isozyme of glucose-6-phosphate dehydrogenase exists.
PPP operates during darkness, light inactivates the enzyme,
preventing degradation of glucose-6-phosphate and allowing the
C3 (Calvin cycle) to operate faster. 2. PPP is an alternative route to compounds degraded by glycolysis. C. Three other functions of PPP: 1. NADPH is produced 2. Erythrose-4-phosphate is produced: essential starting reactant
for production of numerous phenolic compounds such as
anthocyanins and lignin 3. ribose-5-phosphate is produced: precursor of ribose and
deoxyribose units in nucleotides. XI. Production of molecules used for synthetic processes A. In all plants (day and night) there is fixation of CO2(HCO3-)
into oxaloacetate and malate by PEP carboxylase and malate
dehydrogenase. These reactions replenish organic acids that are
converted to larger molecules and used in the Citric Acid Cycle
(Krebs cycle). XII. Biochemical Control of Respiration A. Entry of glucose residues into the glycolytic sequence is regulated 1. Rates of the central catabolic pathways adjust themselves to the
needs of the cell for ATP a. ATP used for biosynthetic reactions, b. for active transport processes, c. or for contractile or mechanical work. d. degradation products are important precursors or intermediates
in other aspects of metabolism B. Regulation of glycolysis: glycolytic sequence itself is
regulated at two major points 1. Two major regulatory points: a ATP-Phosphofructokinase (ATP-PFK) (1) allosteric enzyme (2) regulated by concentration of substrates ATP and fru 6-Pi,
and products ADP and fru 1,6-BisPi (a) ATP, citrate, and PEP most important inhibitory modulators (b) ADP, AMP, Mg2+, phosphate, are slightly inhibitory (5) Pi and fru 1,6-BisPhosphate most active stimulatory modulators b. Pyruvate Kinase: secondary control point (1) allosteric enzyme (2) occurs in at least three isozyme forms (3) differ in their (a) tissue distribution (b) response to modulators (4) High ATP conc.: affinity of pyruvate kinase for PEP is low (5) Pyruvate kinase inhibited by acetyl-CoA and by long-chain
Fatty Acid 2. NAD+/NADH ratio also a regulator a. O2 is important in oxidizing NADH b. good aeration favors glycolysis 3. Plants contain PPi-PFK and that it (along with the ATP-PFK)
catalyzes formation of fructose-1,6-bisphosphate. Animals lack
PPi-PFK. Bacteria have PPi-PFK. Plants contain
fructose-2,6-bisphosphate. Fructose-2,6-bisphosphate is
an activator of PPi-PFK. If glycolysis is favored, then
sucrose formation will be depressed because both processes
compete for the same fructose-6-phosphate in the cytosol. C. Control of Respiration in Mitochondria 1. Major controlling factor was the concentration of ADP in the
mitochondria 2. If concentration is relatively high, oxidation phosphorylation
is fast 3. Pyruvate dehydrogenase is phosphorylated (inactive) or
dephosphorylated (active) 4. If mitochondria have accumulated much ATP, this ATP slows the
Citric Acid Cycle (Krebs cycle) and therefore all subsequent
respiratory processes in mitochondria 5. An abundance of pyruvate formed in glycolysis can overcome the
effect of high ATP levels and can keep mitochondrial respiration
going. D. Control of the Pentose Phosphate Pathway 1. Glucose-6-Phosphate dehydrogenase: in chloroplasts the isozyme
is inhibited by NADPH formed in light and is inactivated
in light by the ferredoxin-thioredoxin system. 2. Isozyme present in the cytosol is also inhibited by NADPH 3. Processes that favor conversion of NADPH to NADP+ increase PPP 4. Two processes: (1) oxidation of NADPH by the electron-transport
system, (2) oxidation during biosynthesis of fatty acids and
isoprenoid compounds such as carotenoids and sterols XIII. Factors Affecting Respiration A. Substrate Availability 1. Respiration depends on the presence of an available substrate 2. Lower-shaded leaves respire slower than upper-full sun
leaves. 3. Difference in starch and sugar contents resulting from unequal
photosynthetic rates is responsible for the lower
respiratory rates of shaded leaves. B. Oxygen Availability 1. O2 supply influences respiration, magnitude of its
influence differs among plant species and within
organs of the same plant. 2. Importance of intercellular spaces for gaseous diffusion 3. Intercellular spaces represent significant amounts of the total
tissue volume a. values for roots 2%-45% have been observed. Higher values
more common among wetland plants. 4. Diffusion of O2 through intercellular spaces from
leaves to roots is important in moving O2 and other
gases through plant tissues more rapidly than expected for
organisms with no lungs or hemoglobin to help transport gases. a. flooding over long periods of time is toxic to plants(anoxia) 5. Tolerance also exist in trees a. hypoxic - under reduced oxygen levels 6. Morphological adaptation of roots to hypoxia is formation of
aerenchyma tissue. a. Aerenchyma is produced following collapse and lysis of some
mature cortex cells, so it is a tissue with large air spaces. 7. Roots commonly respond to hypoxia by faster glycolysis and
fermentation. a. Injurious effects of hypoxia are caused by several metabolic
imbalances resulting from insufficient oxygen b. retard transport of cytokinin growth regulators c. insufficient absorption of mineral slats d. leaf wilting e. slower photosynthesis and carbohydrate translocation f. decreased permeability of roots to water g. accumulation of toxins caused by microbes around the roots. 8. Supply of ATP is limited because the electron-transport
system and Citric Acid Cycle (Krebs cycle) cannot function
without oxygen a. Products of fermentation (ethanol, lactic acid, malic acid,
glycerol, accumulate to some extent 9. Rice and barnyard grass: a. seeds of these plants put coleoptile up first rather than
radicle b. produce ATP from a rapid fermentation during anoxia c. Proteins synthesized under anoxia are pyruvic acid decarboxlase
and alcohol dehydrogenase (fermentation enzymes). d. Allows the shoot to emerge into air, become green and
photosynthetic, and transfer oxygen to the roots. 10. Seeds exhibit fermentation during the normal imbibition
of water that leads to germination. a. Group 1: were sensitive, and germination would not occur at O2
partial pressures below 2 kilopascals (2% O2) b. Group 2: was much ore resistant, and germination was not
stopped until the O2 partial pressure decreased below about
0.1 kPa. 11. Lower O2 concentrations cause a rise in CO2 production
(Pasteur Effect) a. O2 allostericaly inhibits ATP-PFK b. Glycolysis can bypass ATP-PFK when PPi-PFK is used c. Pasteur Effect: causes a decrease in carbohydrate reserves
in plants during soil flooding. Faster rate of glycolysis under
hypoxic conditions. d. Pasteur effect also has some practical importance in storage
of fruits and vegetables. (1) prevent extensive sugar loss and overripening (2) decreasing O2 (3) CO2 is also added to the air (4) Temperature is lowered closer to the freezing (5) CO2 inhibits action of a fruit-ripening hormone, ethylene (6) Low concentrations of O2 also slow ethylene production C. Temperature 1. Rate of O2 penetration into cells limits respiration at
high temperatures at which chemical reactions could otherwise
proceed. a. Diffusion rates increase for O2 and CO2 as temperature increases b. The Q10 for physical processes is about 1.1 c. Temperature does not speed diffusion of solutes through water
very much 2. With rises in temperature to 40oC , the rate of respiration decreases,
especially if plants are maintained for long periods under such
conditions. The enzymes begin to be denatured. D. Type and Age of Plant 1. There is good correlation between the rate of growth of a
particular cell type and its respiration rate. a. Inactive seeds and spores have the lowest respiration rates. b. Changes in the protoplasm, especially desiccation, shut
off metabolism 2. Age of the plant influences respiration a. Large burst in respiratory activity as dry seeds absorb
water and germinate. b. Respiration remains high during rapid vegetative growth
but then drops before flowering. c. Respiration in mature plants is in young leaves
and roots and in the growing flowers. 3. Changes in respiration occur during development of ripening
fruits. a. Respiration rate is high when fruit are young when cells are
dividing and growing. Respiration rate gradually declines. b. Gradual decrease in respiration is reversed by a sharp increase,
know as the climacteric (1) usually coincides with full ripeness and flavor of fruits (2) ethylene that stimulate ripening c. Some fruits (citrus fruits, cherries, grapes, pineapples,
and strawberries) do not show the climacteric.