Chapter 13: Krebs Cycle Flashcards
what is the energy produced in the krebs cycle conserved as?
conserved as reducing power when NAD+ and ubiquinone are reduced to NADH and QH2
why is the krebs cycle not exclusively a catabolic pathway for the oxidation of acetyl CoA?
- oxaloacetate, citrate, alpha ketoglutarate and succinyl CoA all lead to biosynthetic pathways
- the pool of intermediates can be replenished by the catabolism of amino acids or fatty acids
why is the krebs cycle considered a multi-step catalyst reaction?
- only small amounts of each intermediate are needed to convert large quantities of acetyl CoA to products
- the rate at which the krebs cycle metabolizes acetyl CoA is very sensitive in changes in the concentration of intermediates
where is the pyruvate dehydrogenase complex located?
mitochondria
how is pyruvate transported from the cytoplasm into the mitochondria so it can be converted to acetyl CoA?
pyruvate translocase transports pyruvate in symport with H+ across the inner membrane
why can molecules cross the outer membrane of the mitochondria?
presence of porins
How is oxaloacetate, which can be a substrate for gluconeogenesis, transported to the cytoplasm from the mitochondria?
oxaloacetate is converted to phosphoenolpyruvate (catalyzed by PEPCK) and a PEP transporter relocates it across the membrane
how is acetyl CoA transported from the mitochondria to the cytoplasm for fatty acid synthesis?
- tricarboxylic acid transporter that exports citrate
- in the cytoplasm, citrate is converted to oxaloacetate and acetyl CoA
how do species that lack mitochondrial PEPCK export oxaloacetate?
- malate-aspartate shuttle
- oxaloaceate is converted to malate by malate dehydrogenase
a large complex of enzymes and cofactors that catalyzes the conversion of pyruvate to acetyl CoA, so it can enter the krebs cycle
pyruvate dehydrogenase complex
is the conversion of pyruvate to acetyl CoA an oxidation or reduction?
- oxidation
- NAD+ is concomitantly reduced
what are the 3 enzymes complexes of pyruvate dehydrogenase?
- pyruvate dehydrogenase (E1)
- Dihydrolipoyl transacetylase (E2)
- Dihydrolipoyl dehydrogenase (E3)
what is the prosthetic group of E1?
TPP
what is the prosthetic group of E2?
lipoamide
what is the prosthetic group of E3?
FAD
what are the cosubstrates of the pyruvate dehydrogenase complex?
- HS-CoA
- NAD+
What is the role of TPP?
binds pyruvate in E1, catalyzing decarboxylation to form hydroxyethyl-TPP
what is the role of lipoamide?
- E2 swinging arm
- hydroxyethyl is transferred to lipoamide
- transfer of acetyl group to HS-CoA leaves lipoamide in reduced dithiol form
what is the role of FAD?
- E3
- becomes reduced in regeneration of oxidized lipoamide
describe the swinging arm mechanism
- Lipoamide accepts acyl group from E1, disulfide reduced to thiol and thioester
- Acyl group transferred to CoA; lipoamide fully reduced to dithiol
- E3 oxidizes dithiol back to disulfide
where is the pyruvate dehydrogenase complex located in eukaryotes?
mitochondrial maxtrix
where is the pyruvate dehydrogenase complex located in bacteria?
cytosol
How is E1 regulated?
- pyruvate dehydrogenase kinase
- activated by: NADH and Acetyl CoA
- inhibited by: NAD+, HS-CoA, ADP, Pyruvate
- pyruvate dehydrogenase is active when dephosphorylated
How is E2 regulated?
- activated by: HS-CoA
- inhibited by: acetyl CoA
How is E3 regulated?
- activated by: NAD+
- inhibited by: NADH
what are the starting molecules of the krebs cycle?
- acetyl CoA
- oxaloacetate
- NAD+
what are the products of the krebs cycle?
- 2 CO2
- 3 NADH
- FADH2 (QH2)
- GTP (ATP)
- oxaloacetate
how many ATP equivalents are produced per acetyl CoA in the krebs cycle?
10 ATP equivalents
how many ATP equivalents are produced per glucose in the krebs cycle?
20 ATP equivalents
what is the first reaction of the krebs cycle?
- reaction of acetyl CoA with oxaloacetate to form citrate and HS-CoA
- catalyzed by citrate synthase (transfers acetyl group)
- delta G= -32
why does the 1st reaction of the krebs cycle have a negative delta G?
due to the hydrolysis of the high energy thioester bond
instead of being couple to ATP synthesis, the large negative G of the first step in the krebs cycle is used to……?
ensure that the reaction happens even when oxaloacetate concentrations are low
what is the 2nd reaction of the krebs cycle?
- conversion of citrate to isocitrate
- catalyzed by aconitase
- near equilibrium reaction
- isomerization required since citrate is a tertiary alcohol so it cannot be further oxidized
what helps aconitase in the binding of citrate?
iron-sulfur cluster
what is the 3rd step of the krebs cycle?
- conversion of isocitrate to alpha-ketoglutarate
- decarboxylation catalyzed by isocitrate dehydrogenase
- NAD+ reduced to NADH
- CO2 formed
what is the 4th step of the krebs cycle?
- conversion of alpha-ketoglutarate to succinyl CoA
- catalyzed by alpha-ketoglutarate dehydrogenase complex
- oxidative decarboxylation
- reaction analogous to pyruvate dehydrogenase
- NAD+ reduced to NADH
- CO2 formed
what is the 5th step of the krebs cycle?
- conversion of succinyl CoA
- catalyzed by succinyl CoA synthetase
- substrate level phosphorylation –> formation of GTP
what is the 6th step of the krebs cycle?
- conversion of succinate to fumarate (pro-chiral)
- catalyzed by succinate dehydrogenase complex (inhibited by malonate)
- FAD reduced to FADH2
- contains iron-sulfur clusters
- protons and electrons are moved to ubiquinone, forming QH2
- same enzyme complex as complex II in ETC
what is the 7th step of the krebs cycle?
- conversion of fumarate to malate
- catalyzed by fumarase
- near equilibrium
- addition of water
what is the 8th step of the krebs cycle?
- conversion of malate to oxaloacetate
- catalyzed by malate dehydrogenase
- NAD+ reduced to NADH
- delta G = + 29
how does the conversion of malate to oxaloacetate proceed if the delta G is + 29?
the low concentration of oxaloacetate drives the reaction
what are the regulation points of the krebs cycle?
- citrate synthase
- isocitrate dehydrogenase
- alpha-ketoglutarate dehydrogenase
how is citrate synthase regulated?
inhibited by: ATP and NADH
how is isocitrate dehydrogenase regulated?
- allosterically activated by: Ca2+ and ADP
- allosterically inhibited by: NADH
how is the alpha-ketoglutarate dehydrogenase complex regulated?
activated by: Ca2+
what is the glyoxylate pathway/shunt?
- route that bypasses the two CO2 producing reactions of the krebs cycle
- one of the ways that fatty acids can produce glucose
what happens at the end of the glyoxylate pathway?
both malate and oxaloacetate can be converted into phosphoenolpyruvate, whch is the product of phosphoenolpyruvate carboxykinase, the first enzyme in gluconeogenesis
