The citric acid cycle and pyruvate dehydrogenase Flashcards
What are the other names for the citric acid cycle?
Kreb’s cycle
Tricarboxylic acid cycle (TCA)
How much NADH and FADH2 is generated in each turn of the citric acid cycle?
3 NADH
1 FADH2
How much CO2 is produced in the citric acid cycle?
2 molecules per turn for each acetyl group
Which enzyme catalyses the conversion of pyruvate to acetyl CoA?
Pyruvate dehydrogenase
What does the pyruvate dehydrogenase complex consist of?
3 enzymes and 5 co-enzymes
Why does pyruvate dehydrogenase made of multiple enzymes?
By co-localising these enzymes, side reactions are minimised and the overall rate is increased
How does PDH convert pyruvate to acetyl CoA?
Each reaction has to be coupled to ensure the free energy released during the loss of CO2 is coupled to the subsequent generation of acetyl-CoA and NADH
To achieve this a flexible arm (lipoamide) tethers the acetyl group to transfer it between two active sites, as well as transferring reducing potential to a third site
Catalysis REQUIRES a chemical cofactor – THIAMINE diphosphate (from vitamin B1)
What are the 3 stages of the reaction catalysed by the enzyme PDH?
Decarboxylation
Oxidation
Transfer to CoA
Which parts of metabolism have mostly cytosolic enzymes?
Glycolysis, pentose phosphate pathway and fatty acid synthesis
Which parts of metabolism have mostly mitochondrial enzymes?
Citric Acid Cycle, β-oxidation and the respiratory chain
Which part of the mitochondria contains the enzymes of β-oxidation and most of the Citric Acid Cycle?
What is an important exception to this?
Matrix
Succinate dehydrogenase
Why is it necessary to have an anaplerotic pathway alongside the citric acid cycle?
Give an example of an anaplerotic reaction
If the cycle is used to generate new compounds then carbon is lost from the cycle
Pyruvate + CO2 + ATP + H2O → oxaloacetate + ADP + Pi + 2H+
How much energy is derived from 1 NADH and 1 FADH2 molecule?
NADH = 2.5 ATP
FADH2 = 1.5 ATP
How much ATP is generated by oxidative glycolysis?
5 molecules per glucose
Why do we not get 2.5 ATP per NADH generated by glyceraldehyde-3-phosphate?
We ‘lose energy’ getting NADH into the mitochondria (only get 1.5 ATP per NADH)
How much ATP is generated by the citric acid cycle from 1 molecule of glucose?
25 (2 cycles per glucose)
How much ATP is generated by oxidative glycolysis and the citric acid cycle together?
30
Why is acetyl CoA seen as the point of no return for glucose derived carbon?
At this point the carbon source cannot be converted back into glucose for use in the brain.
This is controlled by pyruvate dehydrogenase
What is the role of PDH kinase?
Phosphorylates the E1 of the complex and deactivates PDH
What is the role of PDH phosphatase?
Dephosphorylates the complex and activates PDH
What inhibits PDH kinase?
What is the effect of this?
Pyruvate
Ensures PDH is ‘on’ when pyruvate conc is high
What activates PDH phosphatase?
What is the effect of this?
Ca2+ and insulin in adipocytes
Stimulates PDH during exercise (Ca2+) and feeding (for lipid synthesis)
How do substrates and products of the citric acid cycle regulate PDH?
Regulated by the ratio of NADH/NAD+ and acetyl CoA/CoA
Turn ‘off’ PDH if lots of NADH and acetyl CoA
(via stimulation of the inhibitory kinase)
What are the effects of a PDH phosphatase deficiency?
Pyruvate dehydrogenase is always phosphorylated
(Inactive)
Glucose is processed to lactic acid.
(Unremitting lactic acidosis)
Malfunctioning of many tissues
especially the central nervous system
What allosterically inhibits citrate synthase?
ATP
When is citric acid cycle regulation via citrate synthase important?
In periods of starvation to promote gluconeogenesis
Why does inhibition of citrate synthase by ATP help to fuel the brain in periods of starvation?
Oxaloacetate is diverted to gluconeogenesis
Acetyl CoA is used to make ketone bodies
What inhibits iso-citrate dehydrogenase?
High NADH/NAD+ ratio typical of the fed state
ATP
What stimulates iso-citrate dehydrogenase?
ADP
What inhibits α-ketoglutarate dehydrogenase?
Its products; succinyl CoA and NADH
What stimulates α-ketoglutarate dehydrogenase?
Ca2+
Which enzymes regulate metabolic control of the citric acid cycle?
- Pyruvate dehydrogenase
- Citrate synthase
- Iso-citrate dehydrogenase
- α-ketoglutarate dehydrogenase
What is the purine nucleotide cycle?
The breakdown of ATP during strenuous exercise to generate fumarate (through several steps!) to prime the cycle with more oxaloacetate
What are the symptoms of Patients with deficiencies in the purine nucleotide cycle?
Muscle cramps during exercise
How can the rate of the citric acid cycle be measured?
Oxygen consumption
Labelling of the intermediates – ‘chase the label experiments’
-using radioactive carbon isotopes
How is oxidative metabolism measured in the brain?
Magnetic resonance imaging (MRI) relies on imaging the brain by detecting hydrogen nuclei largely in water.
Paramagnetic substances will modify this signal.
Deoxyhaemoglobin is paramagnetic, while oxyhaemoglobin is diamagnetic.
We can use MRI as a tool for monitoring blood oxygenation through the paramagnetism of deoxyhaemoglobin.
Increased neuronal activity is indicated by increased blood flow.
