Citric acid cycle regulation
Learning objectives:
1. The regulation process employed within the citric acid cycle
2. Understand each of these processes
3. How each of these citric acid cycles can be used as a representative example of such processes
4. Why is the citric acid cycle so tightly controlled through these processes.
Background to the citric acid cycle
3 names:
- Citric acid cycle
- Krebs cycle
- Tricarboxylic acid cycle (TCA)
Cycle features:
- Occurs in mitochondria
- Reduces 3 NAD+ to NADH
- Reduces 1 FAD to FADH2
- Converts an acetyl group to 2 CO2 molecules (decarboxylation)
- The cycle is catalytic
- Classified as a cycle because can take 1 oxaloacetate molecule used throughout and regenerated in cycle
- Many cycle enzymes perform stereospecific reactions
From Citrate synthase to succinyl CoA synthetase, the enzymes are stereospecific
LO1. THE REGULATION PROCESS EMPLOYED WITHIN THE CITRIC ACID CYCLE
LO2. UNDRSTAND EACH OF THESE PROCESSES
LO3. HOW EACH OF THE CITRIC ACID CYCLES IS SO TIGHTLY CONTROLLED THROUGH THESE PROCESSES
Pre-cycle reaction
Pyruvate to acetyl-CoA
Components:
- Pyruvate
Last product of glycolysis
Passes from cytosol to mitochondrial matrix
- Pyruvate carrier
- Coenzyme A
Comprised of:
1. Adenosine-3’-phosphate
2. Panthoenic acid residues
3. Β-mercaptoethanolamine residues
- NAD+
, Nicotinamide adenine dinucleotide
- Enzyme: pyruvate dehydrogenase (PD)
A multienzyme complex = a group of enzymes non-covalently bound together catalysing more than 1
reaction in a metabolic pathway
Enzyme complex members
o Pyruvate dehydrogenase (E1)
o Dihydrolipoyl trasacetylase (E2)
o Dihydrolipoyl dehydrogenase (E3)
o Each member is a subunit of the whole complex
Eukaryotic PD
o Core of 60 E2 subunits as a dodecahedron
o 30 E1 dimers and 6 E3 dimers around core
Multienzyme complex advantages:
o Rate increase
o Intermediate channelled to successive enzymes
o Reactions are coordinated in the process
Citrate synthatase
Produce citrate from oxaloacetate and acetyl-CoA
Citrate synthase kinetics (rate of reaction)
- 1 substrate must bind before the other
- Oxaloacetate binds first
- Crystal structure
Binding of citrated shows part of the active site in the enzyme
Citrate is an inhibitor of the enzyme (example of product inhibition)
With oxaloacetate bound the protein structure changes (example of induced fit)
Excludes solvent from active site
Creates binding site for acetyl-CoA
Confirms enzyme kinetics
Isocitrate dehydrogenase (ID)
Produces α-ketoglutarate from isocitrate
First oxidative decarboxylation
Involves NAD+, Mg2+, Mn2+ in catalysis
Learning objectives:
1. The regulation process employed within the citric acid cycle
2. Understand each of these processes
3. How each of these citric acid cycles can be used as a representative example of such processes
4. Why is the citric acid cycle so tightly controlled through these processes.
Background to the citric acid cycle
3 names:
- Citric acid cycle
- Krebs cycle
- Tricarboxylic acid cycle (TCA)
Cycle features:
- Occurs in mitochondria
- Reduces 3 NAD+ to NADH
- Reduces 1 FAD to FADH2
- Converts an acetyl group to 2 CO2 molecules (decarboxylation)
- The cycle is catalytic
- Classified as a cycle because can take 1 oxaloacetate molecule used throughout and regenerated in cycle
- Many cycle enzymes perform stereospecific reactions
From Citrate synthase to succinyl CoA synthetase, the enzymes are stereospecific
LO1. THE REGULATION PROCESS EMPLOYED WITHIN THE CITRIC ACID CYCLE
LO2. UNDRSTAND EACH OF THESE PROCESSES
LO3. HOW EACH OF THE CITRIC ACID CYCLES IS SO TIGHTLY CONTROLLED THROUGH THESE PROCESSES
Pre-cycle reaction
Pyruvate to acetyl-CoA
Components:
- Pyruvate
Last product of glycolysis
Passes from cytosol to mitochondrial matrix
- Pyruvate carrier
- Coenzyme A
Comprised of:
1. Adenosine-3’-phosphate
2. Panthoenic acid residues
3. Β-mercaptoethanolamine residues
- NAD+
, Nicotinamide adenine dinucleotide
- Enzyme: pyruvate dehydrogenase (PD)
A multienzyme complex = a group of enzymes non-covalently bound together catalysing more than 1
reaction in a metabolic pathway
Enzyme complex members
o Pyruvate dehydrogenase (E1)
o Dihydrolipoyl trasacetylase (E2)
o Dihydrolipoyl dehydrogenase (E3)
o Each member is a subunit of the whole complex
Eukaryotic PD
o Core of 60 E2 subunits as a dodecahedron
o 30 E1 dimers and 6 E3 dimers around core
Multienzyme complex advantages:
o Rate increase
o Intermediate channelled to successive enzymes
o Reactions are coordinated in the process
Citrate synthatase
Produce citrate from oxaloacetate and acetyl-CoA
Citrate synthase kinetics (rate of reaction)
- 1 substrate must bind before the other
- Oxaloacetate binds first
- Crystal structure
Binding of citrated shows part of the active site in the enzyme
Citrate is an inhibitor of the enzyme (example of product inhibition)
With oxaloacetate bound the protein structure changes (example of induced fit)
Excludes solvent from active site
Creates binding site for acetyl-CoA
Confirms enzyme kinetics
Isocitrate dehydrogenase (ID)
Produces α-ketoglutarate from isocitrate
First oxidative decarboxylation
Involves NAD+, Mg2+, Mn2+ in catalysis
