Answer to Question #177232 in Biochemistry for Min

Question

What is the overall chemical changes that occur during one complete turn of the Krebs cycle?

Answer 1

Krebs cycle is also known as citric acid cycle or tricarboxylic acid (TCA) cycle. Krebs cycle is the main source of energy for cells and an important part of aerobic respiration. This cycle converts the available energy of acetyl coenzyme (acetyl CoA) into the reducing power of nicotinamide adenine dinucleotide (NADH).

#STEPS OF KREB CYCLE: -

1-The Krebs cycle begins when Coenzyme A transfers its 2-carbon acetyl group to the 4-carbon compound oxaloacetate to form the 6-carbon molecule citrate.

2-The citrate is isomerized by a dehydration sequence to yield isocitrate or the citrate is rearranged to form an isomeric form, isocitrate.

3-The 6-carbon isocitrate is oxidized and a molecule of carbon dioxide is removed producing the 5-carbon molecule alpha-ketoglutarate. During this oxidation, NAD⁺ is reduced to NADH + H⁺ 

4-Alpha-ketoglutarate is oxidized, carbon dioxide is removed, and coenzyme A is added to form the 4-carbon compound succinyl CoA. During this oxidation, NAD⁺ is reduced to NADH + H⁺.

5-CoA is removed from succinyl-CoA to produce succinate. The energy released is used to make guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and Pi by substrate-level phosphorylation. GTP can then be used to make ATP.

6-Succinate is oxidized to fumarate. During this oxidation, FAD is reduced to FADH₂.

7-Water is added to fumarate to form malate 

8-Malate is oxidized to produce oxaloacetate, the starting compound of the citric acid cycle. During this oxidation, NAD⁺ is reduced to NADH + H⁺.

#OVERALL CHEMICAL REACTION FOR KREBS CYCLE IS: -

   2 acetyl groups + 6 NAD⁺ + 2 FAD + 2 ADP + 2 P_i

   yields 4 CO₂ + 6 NADH + 6 H⁺ + 2 FADH₂ + 2 ATP

The citric acid cycle provides a series of intermediate compound that donates protons and electrons to the electron transport chain by way of the reduced coenzymes NADH and FADH_2. The electron transport chain then generates additional ATP_S  by  oxidative phosphorylation. The citric acid cycle also produces 2 ATP by substrate phosphorylation  

#KREBS CYCLE PRODUCTS: -Before the Krebs cycle begins, a glucose molecule must be converted to acetyl-coA. This process yields 2acetyl-coA molecules to be fed into the cycle. Thu, the cycle proceeds twice per original glucose, yielding twice the products as shown

ONE TCA CYCLE TURN YIELDS 7 PRODUCTS: -

·     GTP

·     3NADH

·     FADH₂ which is converted to UQH₂  in the presence of coenzyme Q(ubiquinone)

·     2 CO₂ (carbon dioxide)