What needs to happen before the citric acid cycle can begin
acetyl CoA synthesis from pyruvate
Glucose goes to how many molecules of pyruvate?
2
Name the three enzymes in the pyruvate dehase complex
pyruvate dehydrogenase
dihyrolipoyl transacetylase
dihydrolipoyl dehase
larger numbers of dimers of each of 3 enzymes in the complex
What are some of the advantages of the pyruvate dehase complex?
channel substrates and minimize side reactions
minimal diffusion of substrates
coordinated control of reactions
What two things can acetyl CoA do after it's produced?
enter the citric acid cycle or it can be used to synthesize storage lipids
Where does glycolysis occur?
in the cytoplasm
where does the citric acid cycle occur?
in the mitochondrial matrix
where does oxidative phosphorylation occur?
in the inner membrane of the mitochondria
Name the five coenzymes involve in the synthesis of acetyl CoA from pyruvate
TPP
Lipoic acid
FAD
NAD
CoA-SH
TPP a coenzyme in the reaction of pyruvate to acetyl coA is attached to what enzyme in the pyruvate dehydrogenase complex?.
E1 - pyruvate dehydrogenase
Lipoic a coenzyme in the reaction of pyruvate to acetyl coA is attached to what enzyme in the pyruvate dehydrogenase complex?
through what type of linkage?
E2 via Lys residue
E2 - dihyrolipoyl transacetylase
FAD a coenzyme in the reaction of pyruvate to acetyl coA is attached to what enzyme in the pyruvate dehydrogenase complex?
E3 - dihydrolipoyl dehase
NAD is a substrate for which enzyme in the pyruvate dehydrogenase complex?
E3
Pyruvate Dehydrogenase mechanism:
Step 1?
decarboxylation of pyruvate to an aldehyde (CHOH--Ch3) via TPP in E1
CO2 is given off in the first step
Pyruvate Dehydrogenase mechanism:
Step 2?
transfer to oxidized lipoic acid in E2
lipoic acid gets reduced to acetylated form
(oxidation to an acetyl group (S - C(=O) - Ch3) from aldehyde)
Pyruvate Dehydrogenase mechanism:
Step 3?
CoA-SH comes in and acetyl CoA is formed (given off)
lipoic acid has two SH groups now
Pyruvate Dehydrogenase mechanism:
Step 4?
lipoic acid gets oxidized via FAD of E3 (FAD gets reduced to FADH2)
Pyruvate Dehydrogenase mechanism:
Step 5?
regeneration of oxidized FAD cofactor via NAD
FADH2 + NAD+ ----> NADH + H+
What two things inhibit pyruvate dehydrogenase?
Also, what enzyme inhibits it? What reverses this?
NADH and Acetyl CoA (feedback)
Kinase phosphorylates it to inactivate it and a phosphatase reverses this
What's the net reaction of the citric acid cycle?
CH3-C(=O)-S-CoA + 3 NAD+ + 1 FAD + Pi + GDP + 2 H2O ---> 2 CO2 + CoA + 3 NADH + FADH2 + GTP + 3 H
The 3 NADH and 1 FADH2 get put into the e- transport chain to make 11 ATP
Citric Acid Cycle
Step 1:
Reactant(s):
Product:
Enzyme:
Type of Rxn:
Citric Acid Cycle
Step 1:
Reactant(s): Acetyl-CoA (2 carbons) + oxaloacetate (4 carbons)
Product: Citrate ( 6 carbons)
Enzyme: citrate synthase
Type of Rxn: condensation
only cycle reaction with C-C bond formation
essentially irreversible process
Acetyl-CoA + oxaloacetate -> citrate + CoA-SH
H2O
Citric Acid Cycle
Step 2:
Reactant(s):
Product:
Enzyme:
Type of Rxn:
Citric Acid Cycle
Step 2:
Reactant(s): citrate
Product: isocitrate
Enzyme: aconitase
Type of Rxn: dehydration then hydration
citrate a tertiary alcohol is a poor substrate for oxidation
first step is to form cis-aconitate via the loss of water to form a cis bond
then stereospecific addition of H2O to cis-aconitate gives isocitrate
citrate -> cis-aconitate + H2O -> isocitrate
Citric Acid Cycle
Step 3:
Reactant(s):
Product:
Enzyme:
Type of Rxn:
Citric Acid Cycle
Step 3:
Reactant(s): isocitrate
Product: alpha-ketoglutarate
Enzyme: isocitrate dehydrogenase
Type of Rxn: oxidative decarboxylation
NAD+ gets reduced and alcohol gets oxidized
decarboxylation through loss of CO2
Citric Acid Cycle
Step 4:
Reactant(s):
Product:
Enzyme:
Type of Rxn:
Citric Acid Cycle
Step 4:
Reactant(s): alpha-ketoglutarate
Product: succinyl-CoA
Enzyme: alpha-ketoglutarate dehydrogenase complex
Type of Rxn: oxidative decarboxylation
enzyme uses same coenzymes as pyruvate dehase complex, identical mechanisms
oxidative again NAD+ -> NADH
C5 -> C4 + CO2
CoA-SH and NAD+ added
Citric Acid Cycle
Step 5:
Reactant(s):
Product:
Enzyme:
Type of Rxn:
Mechanism?
Citric Acid Cycle
Step 5:
Reactant(s): succinyl-CoA
Product: succinate
Enzyme: succinyl CoA synthetase
Type of Rxn: substrate level phosphorylation
GDP + Pi added -> GTP + CoA-SH
Mechanism:
1) P-succinate intermedate
2) His-P intermediate
Citric Acid Cycle
Step 6:
Reactant(s):
Product:
Enzyme:
Type of Rxn:
where is the enzyme located?
What's attached to the enzyme?
what does this transfer electrons to?
Citric Acid Cycle
Step 6:
Reactant(s): succinate
Product: fumarate
Enzyme: succinate dehydrogenase
Type of Rxn: dehydrogenation
enzyme in mitochondrial membrane
FAD attached to enzyme
FADH2 passes electrons to coenzyme Q
QH2 can be used to make ATP
Citric Acid Cycle
Step 7:
Reactant(s):
Product:
Enzyme:
Type of Rxn:
Citric Acid Cycle
Step 7:
Reactant(s): fumarate
Product: malate
Enzyme: fumarase
Type of Rxn: hydration
alpha hydration... highly stereospecific
fumerate -> L-malate
Net effect: trans addition of water
Citric Acid Cycle
Step 8:
Reactant(s):
Product:
Enzyme:
Type of Rxn:
Citric Acid Cycle
Step 8:
Reactant(s): L-malate
Product: oxaloacetate
Enzyme: malate dehydrogenase
Type of Rxn: dehydrogenation
oxidation of OH to carbonyl NAD+ gets reduced
thermodynamically unfavorable reaction
oxidation occurs because the concentration of oxaloacetate is very low as it is continuously used to make citrate
What type of molecule is citrate?
prochiral
molecule isn't chiral but aconitase enzyme only acts on one of the CH2-COO- groups
In the citric acid cycle pathway what three enzymes are regulated?
Citrate synthase
isocitrate dehase
alpha-ketoglutarate dehase
Flux through the citric acid cycle pathway is controlled by what three things?
substrate availability (acetyl-CoA and oxaloacetate)
inhibition by accumulating products
allosteric activators and inhibitors
The conversion of pyruvate to Acetyl-CoA involves the pyruvate dehydrogenase enzyme complex.
What things block this enzyme?
What things facilitate this enzyme?
Block: ATP, acetyl-CoA, NADH, fatty acids
Things that this reaction produces inhibit it...this reaction eventually leads to ATP and fatty acid synthesis via Acetyl-CoA. And Acetyl-CoA and NADH are products of this reaction. NADH via the redoxidation of FAD which gets reduced from the oxidation of lipoic acid.
Activate: AMP (low energy conditions), CoA, NAD+, Ca++
only once activated by NAD+ and AMP
The conversion of Acetyl-CoA and Oxaloacetate to Citrate involves the citrate synthase enzyme.
What things block this enzyme?
What things facilitate this enzyme?
Block: citrate, NADH, succinyl-CoA, ATP
Acitvate: ADP
The conversion of Isocitrate to alpha-ketoglutarate involves the isocitrate dehydrogenase enzyme
What things block this enzyme?
What things facilitate this enzyme?
Block: ATP
Activate: Ca++, ADP
only enzyme control spot that isn't blocked by it's product or NADH
The conversion of alpha-ketoglutarate to succinyl-CoA involves the alpha-ketoglutarate dehydrogenase complex
What things block this enzyme?
What things facilitate this enzyme?
Block: Succinyl CoA, NADH
Activate: Ca++
only one not blocked by ATP