Question

Kreb’s cycle is also called metabolic sink as it is a common pathway for:
(a) Carbohydrates, fats and proteins
(b) Carbohydrates, fats only
(c) Carbohydrates and organic acids only
(d) proteins and fats only

Answer 1

Hint: H.A Kreb was a German-born British biochemist who discovered the Krebs cycle for which he was awarded the Nobel Prize in 1953. This cycle is also known as the citric acid cycle or Tricarboxylic Acid Cycle, in short, TCA cycle, named after the first stable product formed.

Complete answer:
This eight-step cycle occurs in the matrix of mitochondria. This cycle is common to all the metabolites and thus not only the carbohydrates but also fats and proteins digested and absorbed by the body can enter this cycle after being converted to acetyl CoA. This is the reason Kreb’s cycle is also called metabolic sink as it is a common pathway for carbohydrates, fats and proteins that acts as a respiratory substrate and releases energy.
The 8-step pathway proceeds this way:
- Acetyl CoA condenses with four carbon molecules ‘oxaloacetate’ to yield the first stable product ‘citrate’ with a release of CoA subunit.
- It is followed by an isomerisation reaction where a water moiety is first removed only to add later. This is done to move the hydroxyl group from one carbon atom to its neighbour. The product of this reaction is Isocitrate.
- Isocitrate is oxidised to ɑ-ketoglutarate while losing a molecule of carbon dioxide.
-The second molecule of carbon dioxide is released while oxidising ɑ-ketoglutarate to succinyl CoA by addition of a CoA group.
- The cleavage of the thioester bond in the succinyl CoA releases CoA while ADP is phosphorylated to ATP. This reaction yields Succinate.
- Two hydrogen atoms are removed from succinate to produce fumarate.
- A molecule of water is added to fumarate to introduce a hydroxyl group, generating malate.
- Oxaloacetate is regenerated by dehydrogenation or removal of a hydrogen molecule from malate.

So, the correct answer is ‘Carbohydrates, fats and proteins.’

Note:
- All three steps of aerobic respiration- glycolysis, pyruvate oxidation, and Krebs cycle, releases energy from the respiratory substrates in the form of ATP, NADH, and ${ FADH }_{ 2 }$.
- The free energy released in the form of NADH and ${ FADH }_{ 2 }$ needs to be converted to usable forms of energy i.e ATP, for the cell. Electrons are transferred from NADH and ${ FADH }_{ 2 }$ through a series of electron carriers present on the inner mitochondrial membrane. This is described as oxidative phosphorylation or electron transport chain(ETC).