BCH_3053_Chapter19_S24
Chapter 19: Citric Acid Cycle
Cellular Respiration
cells consume oxygen and produce carbon dioxide
provides more ATP from glucose than glycolysis
captures energy stored in lipids and amino acids
occurs in three stages
acetyl-CoA production
acetyl-CoA oxidation (citric acid cycle)
electron transfer and oxidative phosphorylation
Central Role of the Citric Acid Cycle
three processes for anaerobic metabolism
citric acid cycle
electron transport
oxidative phosphorylation
metabolism = catabolism (breakdown) + anabolism (build)
amphibolic: plays role in catabolism and anabolism
central metabolic pathway
Eukaryotes
glycolysis occurs in cytoplasm
citric acid cycle occurs in mitochondrial matrix
except succinate dehydrogenase (inner membrane)
oxidative phosphorylation occurs in inner membrane
Aerobic Metabolism
catabolism of proteins, fats, and carbohydrates
Stage 1: acetyl-CoA production
oxidation of fatty acids, glucose, and amino acids
yields acetyl-CoA
generates ATP, NADH, and FADH2
conversion of pyruvate to acetyl-CoA
oxidative decraboxylation of pyruvate
remaining two carbons enter citric acid
acetyl-CoA used to synthesize lipids
catalyzed by pyruvate decarboxylase complex
short distance between catalytic sites allows channeling
channeling minimizes side reactions
activity of complex is subject to ATP regulation
Stage 2: acetyl-CoA oxidation (acetyl-CoA to citrate)
generates NADH, FADH2, and 1 GTP
Reaction 1: catalyzed by citrate synthase
acetyl-CoA + OAA + water = citrate
first committed step in the citric acid cycle
conformational change upon binding of OAA
open conformation
free enzyme does not have binding site
closed conformation
binding of OAA creates site for acetyl-CoA
reactive carbanion is protected
Reaction 2: isomerization of citrate by aconitase
citrate is a poor substrate for oxidation with tertiary alcohol
elimination of water from citrate gives cis C=C
adding water to cis-aconitate is stereospecific
isocitrate is a good substrate for oxidation with secondary alcohol
Reaction 3: oxidative decarboxylation
d-isocitrate to a-ketoglutarate with NAD+
catalyzed by isocitrate dehydrogenase
oxalosuccinate intermediate
Reaction 4: oxidative decarboxylation
a-ketoglutarate to succinyl-CoA with CoA-SH and NAD+
catalyst: a-ketoglutarate dehydrogenase complex
similar to pyruvate dehydrogenase complex
thymine, lipoic acid, CoA, FAD, NAD+
Summary of Stage 2 reactions 1-4
pyruvate to succinyl-CoA
introduction of 2 carbons as acetyl-CoA
loss of 2 carbons as carbon dioxide
Reaction 5: phosphorylation of GDP
succinyl-CoA converted to succinate with GDP and Pi
forms CoA-SH and GTP
catalyzed by succinyl-CoA synthetase
Reaction 6: FAD-dependent oxidation
succinate to fumarate
catalyzed by succinate dehydrogenase
inner mitochondrial membrane
Reaction 7: stereospecific trans hydration
formation of alcohol from double bond
catalyzed by fumarase
specific for fumarate to L-malate
Reaction 8: oxidation to complete cycle
L-malate to oxaloacetate (OAA)
catalyzed by malate dehydrogenase
citrate synthase consumes OAA, driving forward reaction
le chatelier’s principle
Sequence of reactions in citric acid cycle
C-C bond formation makes citrate
isomerization followed by hydration
oxidative decarboxylation to give 2 NADH
substrate-level phosphorylation to give GTP
dehydrogenation to give reduced FADH2
hydration
dehydrogenation to give NADH
Stage 3: electron transport and oxidative phosphorylation
electrons carried by NADH and FADH2 funneled into mitochondelectron carriers
electron flow drives production of ATP
Summary of Citric Acid Cycle
one molecule of pyruvate oxidized to three molecules of carbon dioxide
oxidative decarboxylation
oxidation reactions accompanied by reductions involving NAD+/FAD to NADH/FADH
GDP phosphorylated to 1 molecule of GTP
generates 30-32 ATP in total
Fate of Citric Acid Cycle Components
oxidation of acetyl-CoA to carbon dioxide (catabolic)
key intermediates in biosynthesis (anabolic)
carbohydrates, fatty acids, and amino acids
danger that intermediates can be depleted (dual role of cycle)
citrate → fatty acids
a-ketoglutarate → glutamate → amino acids and purines
succinyl-CoA → porphyrins, heme
oxaloacetate → amino acids/pyrimidines and PEP → glucose or amino acids
Anaplerotic Reactions
Pyruvate carboxylase (liver and kidney)
catalyzed by pyruvate carboxylase
allosterically activated by acetyl-CoA
inactive in absence of acetyl-CoA
most important anaplerotic reaction
PEP carboxykinase (heart and skeletal muscle)
PEP carboxylase (plants, yeast, and bacteria)
Malic enxyme (wide distribution)
Regulation of the Citric Acid Cycle
production of acetyl-CoA by pyruvate dehydrogenase complex
entry of acetyl-CoA in citric acid cycle as catalyzed by citrate synthase
first committed step of citric acid cycle
Regulation of pyruvate dehydrogensaae complex
allosteric inhibitors and activators
activity is off when fuel energy is high
activity is on when fuel energy is low
ATP, acetyl-CoA, NADH, fatty acids inhibit
AMP, CoA, NAD+, Ca2+ activate
phosphorylation at specific Ser
phosphate inactivates pyruvate dehydrogenase complex
Other citric acid cycle enzymes are allosterically activated and inhibited
citrate synthase
inhibited by NADH, succinyl-CoA, citrate, ATP
activated by ADP
isocitrate dehydrogenase
inhibited by ATP
activated by ADP
a-ketoglutarate dehydrogenase
inhibited by NADH and succinyl-CoA
Summary
Citric acid cycle is important catabolic process
makes GTP and reduces cofactors that could yield ATP
Citric acid cycle is important anabolic process
Pyruvate dehydrogenase converts pyruvate into acetyl-CoA
several factors involved in reactions that harness energy from pyruvate
rules of organic chemistry rationalize reactions