The number of substrate level phosphorylations in one turn of citric acid cycle is

🌀 Concept:
Cellular respiration is a catabolic reaction taking place in the cells, and the Krebs cycle is a part of Cellular Respiration.
Cellular respiration has four parts:
→ Glycolysis – Partial oxidation of a glucose molecule to form 2 molecules of pyruvate in the cytosol.
→ Formation of Acetyl CoA – Pyruvate enters the mitochondrial matrix and undergoes oxidative decarboxylation to form two molecules of Acetyl CoA.
→ Krebs cycle (TCA cycle or Citric Acid Cycle)
→ Electron Transport System and Oxidative Phosphorylation: In the inner membrane of mitochondria, ATP is generated when electrons are transferred from energy-rich molecules like NADH and FADH2 (produced in earlier steps) to molecular O2 by a series of electron carriers.

🔍 Explanation:
Let us see the Krebs cycle:
→ Formation of Citric Acid:
The tricarboxylic acid (TCA) cycle, also known as the Krebs cycle, begins with the condensation of the acetyl group with oxaloacetic acid and water to form citric acid in the presence of the enzyme citrate synthase.

→ Substrate-Level Phosphorylation and Formation of α-Ketoglutaric Acid and Succinyl CoA:
Further, two steps of decarboxylation occur, leading to the formation of α-ketoglutaric acid and succinyl CoA.
Succinyl CoA is converted into succinic acid, and a molecule of guanosine triphosphate (GTP) is formed. This process is called substrate-level phosphorylation.

In other reactions, GTP is converted into GDP, and ATP is synthesized from ADP.

→ Formation of Oxaloacetic Acid:
In the remaining steps of the citric acid cycle, succinyl CoA is oxidized to oxaloacetic acid (OAA) for the cycle to continue.

In a single turn of the Krebs cycle, a single GTP molecule is synthesized, and substrate-level phosphorylation occurs only once.
At three places in the cycle, NAD+ is reduced to NADH + H+, and at one place, FAD+ is reduced to FADH2.