Biochem Citric Acid Cycle

what is cellular respiration

process by which the pyruvate produced by glycolysis is further oxidized to h2o and co2

what metabolite links two central catabolic pathways, glycolysis and the CAC

pyruvate

what does the pyruvate oxidation decarboxylation do?

turns pyruvate into acetyl coa via pyruvate dehydrogenase complex

what is the purpose of the mitochondrial pyruvate carrier?

a H+ coupled pyruvate specific symporter located in the inner mitochondrial membrane that moves pyruvate from cytosol into mitochondria

pyruvate oxidative decarboxylation includes three steps, name them

decarboxylation, oxidation, and transfer to coa

name the pyruvate dehydrogenase complex enzymes

E1 = pyruvate dehydrogenase
E2 = dihydrolipoyl transacetylase
E3 = dihydrolipoyl dehydrogenase

what is the structure of the pyruvate dehydrogenase complex

E2 core surrounded by multiple and variable numbers of E1 and E3

what is the first step of pyruvate oxidative decarboxylation

e1 decarboxylates pyruvate using cofactor thiamine pyrophosphate or tpp
- co2 is reduced and hydroxyethyl-tpp is formed

what is step two of pyruvate oxidative decarboxylation

hydroxyethyl group is transferred to e2
- e2 uses a lipoamide cofactor
- forms acetyl lipoamide and tpp is regenerated

what is step three of pyruvate oxidative decarboxylation

acetyl transfer to coa
prod a reduced lipoamide and acetyl-coa

what is step four of pyruvate oxidative decarboxylation

e2 is restored
lipoamide is re-oxidized by e3’s disulfide and FAD cofactor

what is step five of pyruvate oxidative decarboxylation

restoration of e3
electrons move to fad to form fadh2
electrons move from fadh2 to nad to form nadh and h+
e3 is restored to its oxidized state

what is substrate channeling

the passage of intermediates from one enzyme directly to another without release
the five reaction sequence of the PDH complex is an example of substrate channeling

how does the pdh complex act as a substrate channeling mechanism

the lipoyllysyl arm of e2 physically swings intermediates between the active sites of e1, e2, and e3

what does one round of the CAC produce

3 nadh, 1 qh2, 1 gtp or atp

what is step 1 of the cac

formation of citrate via citrate synthase
- oxaloacetate binds to citrate synthase = induces conformational change that creates a binding site for acetyl-coa
- acetyl coa binds and citrate synthase catalyzes the condensation to form citryl coa
- citryl coa triggers another structural change that completes the active site formation
- citryl coa is cleaved into citrate and coa-sh

what is step two of the cac

formation of isocitrate via aconitase
- aconitase removes h2o from citrate = prod cis-aconitate intermediate
- h2o is added back stereospecifically = prod isocitrate
- the iron sulfur center in aconitase acts both in the binding of substrate to the active site and in the catalytic addition/removal of h2o

what is step three of the cac

formation of alpha-ketoglutarate via isocitrate dehydrogenase
- isocitrate dehydrogenase oxidizes isocitrate = forms oxalosuccinate intermediate and nadh
- oxalosuccinate is stabilized by Mn2+
- oxalosuccinate loses co2 = forms alpha-ketoglutarate

what is step four of the cac

formation of succinyl-coa via alpha-ketoglutarate dehydrogenase complex
- one carbon is released as co2 from alpha-ketoglutarate and the remaining four carbon fragement is attached to coa-sh = prod succinyl-coa

what is step five of the cac

formation of succinate via succinyl-coa synthetase
- succinyl coa synthetase catalyzes the breakage of the thioester bond of succinyl coa = prod succinate
- the energy released by breaking the bond is used to prod gtp or atp
- coa sh is also regenerated

what is nucleoside diphosphate kinase

catalyzes the reversible conversion of gtp and atp

what is step six of the cac

formation of fumarate via succinate dehydrogenase
- succinate dehydrogenase oxidizes succinate = prod fumarate
- 2 electrons are accepted by fad cofactor = prod fadh2

what is malonate

strong competitive inhibitor for succinate dehydrogenase

what is step seven of the cac

formation of malate via fumarase
- fumarase catalyzes the reversible hydration of fumarate to L-malate

what is step eight of the cac

formation of oxaloacetate via L-malate dehydrogenase
- malate’s hydroxyl group is oxidized to a carbonyl = prod oxaloacetate

what are the activity changes of the cac in tumors?

• downregulation of mitochondrial pyruvate carrier

• inactivation of the pdh complex and succinate dehydrogenase

• oncometabolites = stimulate tumor growth by acting through specific GFCRs in plasma membrane