EXAM 3: CAC Flashcards by Olivia Kasselain

cellular respiration

complete oxidation

cells consume O2 and produce CO2

captures energy stored in lipids and amino acids

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3 stages of cellular respiration

acetyl-coa production

acetyl-coa oxidation

electron transfer and oxidative phosphorylation

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glycolysis location

cytosol

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citric acid cycle location

mitochondrial matrix (except succinate dehydrogenase on inner membrane)

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oxidative phosphorylation location

inner membrane

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pyruvate to acetyl-CoA

required for pyruvate from glycolysis to enter CAC

not required for fatty acid catabolism

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result of reaction

pyruvate —> acetyl-Coa

oxidative decarboxylation of pyruvate

first carbons of glucose to be fully oxidized

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what catalyzes pyruvate to acetyl-coa

pyruvate dehydrogenase complex

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pyruvate dehydrogenase complex

5 cofactors

prosthetic groups: TPP, lipoyllysine, FAD

coenzymes: NAD+, CoA-SH

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coenzyme A

not a permanent part of enzymes structure

associate, fulfill reaction, dissociate

in this rxn, carries and accepts acetyl groups

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pyruvate dehydrogenase complex (PDC)

multiple copies of 3 enzymes

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advantages of multienzyme complex in PDC

short distance between catalytic sites, allows channeling of substrates from one catalytic site to another

channeling minimizes side reactions

regulation of activity of one subunit affects entire complex

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sequence of events in oxidative decarboxylation of pyruvate

step 1: pyruvate is decarboxylated to an aldehyde; prosthetic group TPP attaches
=hydorxyethyl TPP

step 2: lipoyl cofactor is reduced (disulfide bonds break) and binds the aldehyde to form thioester
oxidizes aldehyde

step 3: first main product of acetyl-CoA

step 4: reduced lipoyllysine is reoxidized (recycled) with reduction of FAD to FADH2

step 5: regeneration of oxidized FAD cofactor forms reduced NADH (product 2)

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net result of citric acid cycle

acetyl-CoA + 3NAD+ FAD + GDP + Pi + 2H2O

2 CO2 + 3NADH + FADH2+ GTP + CoA + 3H+

energy captured by electron transfer to NADH and FADH2

GTP can be converted to ATP

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citric acid cycle name

citrate is made first

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TCA cycle name

tricarboxylic acid cycle; first 2 molecules made have 3 carboxyl groups

citrate, isocitrate

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krebs cycle name

hans krebs in 1937

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CAC: Step 1

acetyl-CoA + oxaloacetate + H2O —> citrate + CoA-SH

C-C bond formation by condensation of acetyl-CoA and oxaloacetate

citrate synthase

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CAC Step 1: citrate synthase reaction

condensation of acetyl-Coa and oxaloacetate

rate limiting step of CAC

activity largely depends on [oxaloacetate]

favorable, irreversible (regulated by substrate availability and product inhibition)

ordered sequential; oxaloacetate then acetyl-coa

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CAC: Step 2

citrate —> cis-aconitate + H2O —> isocitrate

isomerization by dehydration / rehydration

aconitase

CAC Step 2: aconitase reaction

isomerizes molecule by removing then adding h2O

elimination of H2O from citrate gives cis c=c bond
addition of H2O to cis-aconitrate is stereospecific
citrate = tert alc; poor substrate for oxidation
isocitrate = secondary alc,good substrate

unfavorable,reversible; low product

why is isocitrate a better substrate for oxidation?

2 hydrogens to remove; one on the alc, and one on the alcohol the carbon is attached to

CAC: Step 3

isocitrate + NAD+ —> a-ketoglutarate + NADH + CO2

oxidation decarboxylation of isocitrate

isocitrate dehydrogenase

CAC Step 3: isocitrate dehydrogenase reaction

catalyzes oxidative decarboxylation

generates NADH
lose carbon as CO2 (complete oxidation)

oxidation converts alcohol to ketone; transfers hydride to NAD+

carboxyl group on C3 leaves

favorable, irreversible; product inhibition and ATP allosteric regulation

CAC: Step 4

a-ketoglutarate + CoA-SH + NAD+ —> succinyl-CoA + CO2 + NADH final oxidative decarboxylation alpha-ketoglutarate dehydrogenase complex

CAC Step 4: alpha-ketoglutarate dehydrogenase rxn

net full oxidation of all carbons of acetyl-CoA after two turns: complete oxidation of all carbons from glucose ketone oxidized to thioester; NAD+ reduced succinyl-CoA has a higher energy thioester bond favorable, irreversible; product inhibition

a-ketoglutarate dehydrogenase

complex similar to pyruvate dehydrogenase same coenzymes, identical mechanisms; active sites different for diff sized substrates

CAC: Step 5

succinyl-CoA + GDP + Pi —> GTP + CoA-SH + succinate generation of GTP through thioester bond cleavage succinyl-CoA synthetase

CAC Step 5: succinyl-CoA synthetase reaction

substrate level phosphorylation energy of breaking thioester bond allows incorporation of Pi to make GTP - Pi displaces CoA to make succinyl phosphate - Pi transferred to make phospho-enzyme intermediate; succinate leaves - Pi added to GDP to produce GTP can be converted to ATP favorable, reversible; low product

Step 5 isozymes in brain, muscle, heart

use ADP, make ATP

Step 5 isozymes in liver, kidney

GTP, GDP

CAC: Step 6

succinate + FAD —> FADH2 + fumarate oxidation of alkane to alkene succinate dehydrogenase

CAC Step 6: succinate dehydrogenase rxn

in inner membrane oxidation of alkane to alkene; reduction of FAD to FADH2 reversible; low product concentration

why isn’t NAD+ used in step 6 in the succinate dehydrogenase reaction?

reduction potential is too low

CAC: Step 7

fumarate + OH- —> carbanion TS + H+ —> L-malate hydration across a double bond fumarase

CAC Step 7: fumarase reaction

stereospecific addition of water is trans, forms L-malate - OH- adds fumarate first, then H+ to carbanion - cannot distinguish between inner carbons, so either can gain OH- favorable, reversible; low product

CAC: Step 8

L-malate + NAD+ —> NADH + H+ + oxaloacetate oxidation of alcohol to ketone malate dehydrogenase

CAC Step 8: malate dehydrogenase reaction

final step alcohol on malate oxidized to ketone; NAD+ reduced regenerates oxaloacetate for citrate synthesis unfavorable, reversible; oxaloacetate kept very low by citrate synthesis to pull reaction forward

how many NADH are made for CAC?

3 per cycle; 6 per glucose

how many FADH2 made per CAC?

1 per cycle; 2 per glucose

how many GTP/ATP made per CAC***?

1 per cycle; 2 per glucose

how many Co2 formed in CAC?

2 per cycle; 4 per glucose

4 reactions in CAC with regulation

pyruvate dehydrogenase citrate synthase isocitrate dehydrogenase alpha-ketoglutarate dehydrogenase

regulatory mechanism in CAC

activation: substrate availability NAD+, AMP, ADP inhibition: product accumulation ATP, NADH enzymes may be associated to allow channeling

amphibolic intermediates

intermediates can go into other pathways but if they are taken out, cycle isn’t completed, and oxaloacetate will run out

catabolic

further break down

anabolic

makes other molecules

anapleurotic reactions

intermediates in CAC can be used in biosynthetic pathways (removed from CAC) must replenish the intermediates in order for the cycle and central metabolic pathway to continue 3 carbon intermediates (pyruvate, phosphoenolpyruvate) are carboxylated to form 4 carbon intermediates (oxaloacetate, malate) malate is one step away from oxaloacetate