The Krebs cycle
The Krebs cycle, the Tricarboxylic acid cycle (the TCA cycle) and the citric acid cycle all refer to the same thing. We will use all the terms interchangeably. The cycle is named after its discoverer, Sir Hans Krebs. The cycle contains the six carbon tricarboxylic acid, citric acid (citrate), as the first formed component.
1784 Citrate was discovered in lemon juice
1900- 1920 It was found to be widespread in nature. It was shown that minced muscle catalysed the transfer of H atoms (H+ + e-) from certain organic acids to the dye methylene blue. (Methylene blue is a redox dye - it changes colour depending on whether it is reduced or oxidised). The organic acids that could be used were succinate, malate, citrate. The enzymes used were called dehydrogenases.
1930's Measurement of the oxygen uptake rate of minced tissues showed that these acids were all rapidly oxidised to carbon dioxide.
Szent-Gyorgyi in Hungary assembled these observations into the sequence:
Succinate Fumarate Malate Oxaloacetate
Martius and Knoop showed that there was another sequence:
Citrate Isocitrate Ketoglutarate Succinate
1937 The contribution of Hans Krebs was to realise that these sequences were linked together as a cycle of reactions. There are very few cycles in biology and he was awarded the Nobel prize for this insight.
The reactions occur in the inner compartment (the matrix) of the mitochondria in eukaryotic cells. Some of the enzymes are associated with the inner membrane.
1) It is the final oxidative step in the catabolism of carbohydrates, fatty acids and amino acids.
2) It provides a flow of simple carbon compounds into anabolic processes.
3) It functions as a major source of energy, generating some ATP and a lot of NADH.
(The NADH can be oxidised in the respiratory chain to generate much more ATP).
Link with glycolysis
The end product of glycolysis is the 3 carbon compound pyruvate. The enzyme pyruvate dehydrogenase converts pyruvate, CoA and NAD+ into Acetyl CoA, CO2 and NADH. The acetyl CoA feeds the 2 carbon acetyl group into the cycle. Pyruvate dehydrogenase is a complex consisting of 3 distinct enzymes and needs 5 coenzymes.
There are 8 different reactions. In summary, these are:
1) condensation of acetyl CoA (2C) with oxaloacetate (4C) to form 6C citrate by the enzyme citrate synthase (citrate-condensing enzyme).
2) formation of isocitrate via cis-aconitate, using the enzyme aconitase. This involves removal of water, then its addition, resulting in movement of the OH group.
3) Oxidative decarboxylation of isocitate to ketoglutarate (5C) and CO2 by isocitrate dehydrogenase.
4) Oxidative decarboxylation of ketoglutarate to succinyl CoA (4C) and CO2 by ketoglutarate dehydrogenase.
5) conversion of succinyl CoA to succinate by succinate thiokinase. This involves hydrolysis with phosphorylation.
6) oxidation of succinate to fumarate (removal of 2H to leave a double bond) by succinate dehydrogenase.
7) Hydration of fumarate to malate by fumarase.
8) Oxidation of malate to oxaloacetate by malate dehydrogenase.
Steps 3,4 & 8 generate NADH., 6 generates FADH2 , 5 generates GTP (or ATP) for one turn of the cycle.
Regeneration of these cofactors
NAD+ and FAD are regenerated by the electron transport chain. GDP can be regenerated by transferring a phosphate to ADP from GTP by the enzyme nucleoside diphosphate kinase.
GTP + ADP -----> GDP + ATP
Relation to lipid metabolism
Fatty acid chains are broken down into 2C fragments by the process of beta-oxidation. This 2C fragment is acetyl CoA, which feeds in directly into the Krebs cycle. Biosynthesis of fatty acids is the reversal of this chopping off process.
Relation to protein metabolism
Proteins are broken down to amino acids. These undergo transamination and deamination reactions to give Krebs cycle intermediates. For example aspartate has a structure very similar to oxaloacetate. Valine and methionine feed in as succinate.
With regard to biosynthesis of the amino acids the important precursors are ketoglutarate and oxaloacetate.
The yield of ATP from glucose
We are now in a position to calculate how much ATP is released from a molecule of glucose. This is assuming that NADH and FADH2 are oxidised using oxygen and that no intermediates are taken off for biosynthesis.
The first problem is that the inner mitochondrial membrane is impermeable to NADH. NADH generated in the cytoplasm cannot enter the mitochondrion to reach the respiratory chain. The answer is to transport the electrons carried by NADH to the respiratory chain, rather than the NADH itself. There may of course be an energetic price to pay for this transport.
There are two shuttles that can be used.
1) The glycerol 3 phosphate shuttle (used by skeletal muscle)
In this shuttle, cytosolic NADH, generated by glycolysis passes its electrons across the membrane via glycerol 3 phosphate. This forms FADH2, rather than forming NADH inside the mitochondrion. This reduces the yield of ATP that can be made.
2) The malate-aspartate shuttle
This forms aspartate to carry electrons across the membrane into the matrix, generating NADH. Malate diffuses outwards. Used by liver, kidney and heart.
So the ATP yield will depend on the shuttle used to transport the electrons across the membrane.
Let us assume 1 molecule of NADH generates 3ATP, FADH2 generates 2ATP.
glucose to pyruvate (x2) 2ATP used up, 4 released 2ATP
2NADH formed 6ATP (4ATP)
(depends on shuttle)
NADH dehydrogenase (2pyruvate ---> 2 Ac CoA)
Krebs cycle (1 turn) gives 3NADH, 1FADH2, 1GTP
2 turns gives 6 NADH 18ATP
2 FADH2 4ATP
2 GTP 2ATP
If Krebs cycle intermediates are used up for biosynthesis, they need to be replenished to enable the cycle to continue. Anaplerotic (filling up) reactions are used to replace them.
For example instead of being broken down to Acetyl CoA pyruvate can be carboxylated to oxaloacetate using CO2 and ATP by the enzyme pyruvate carboxylase. This oxaloacetate is used to replenish any intermediates drawn off for biosynthesis.