Question

Soalan 3/ Question 3 a) Kitar asid sitrik bermula dengan kondensasi asetil-KoA bersama oksaloasetat. Jelaskan TIGA (3) sumber-sumber yang mungkin bagi asetil-KoA tersebut. [3 markah] b) Apakah nama enzim yang menghubungkan glikolisis dan kitar asid sitrik? Jelaskan tindakbalas yang dijalankan oleh enzim tersebut. [3 markah] c) Jelaskan secara ringkas bagaimana kitar sitrik asid dikawalselia. Kenalpasti enzim pengawalseliaan utama dan faktor-faktor yang menentukan samada fluks kitaran meningkat atau menurun. [6 markah] a) The citric acid cycle begins with the condensation of acetyl-CoA with oxaloacetate, Describe THREE (3) possible sources for the acetyl-CoA. [3 marks] b) What is the name of the enzyme that connects glycolysis and citric acid cycle? Describe the reaction catalysed by the enzyme. [3 marks] c) Briefly describe how the citric acid cycle is regulated. Identify the key regulatory enzymes and the factors that determine whether the flux of the cycle increases or decreases. [6 marks] 17/19 B DDD D

Answer 1

a. Acetyl-CoA is produced by (1) the pyruvate dehydrogenase complex, (2) β oxidation of fatty acids, or (3) degradation of certain amino acids.

the pyruvate dehydrogenase complex - One of the primary sources of acetyl-CoA is from the breakdown of carbohydrates by glycolysis which yield pyruvate which is decarboxylated by the enzyme pyruvate dehydrogenase complex and generate acetyl-CoA.

CH₃C(=O)C(=O)O⁻pyruvate + HSCoA + NAD⁺ → CH₃C(=O)SCoAacetyl-CoA + NADH + CO₂

β oxidation of fatty acids - Acetyl-CoA is also produced by the breakdown of lipids (by β-oxidation). In β-oxidation the fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric acid cycle.

Degradation of certain amino acids - Amino acids that are degraded to acetyl CoA or acetoacetyl CoA are termed ketogenic amino acids because they can give rise to ketone bodies or fatty acids. In some organisms the amino acid threonine is degraded to form acetyl-CoA.

b. The enzyme that connects the glycolysis and citric acid cycle is Pyruvate dehydrogenase complex (PDC). It is a complex of three enzymes that converts pyruvate into acetyl-CoA by a process called pyruvate decarboxylation. This is a transition reaction that connects glycolysis to the citric acid (Krebs) cycle. The PDC converts the two molecules of pyruvate from glycolysis into two molecules of acetyl Coenzyme A (acetyl-CoA) and 2 molecules of carbon dioxide. Each pyruvate molecule loses a carboxylic group in the form of carbon dioxide. The remaining two carbons are then transferred to the enzyme CoA to produce Acetyl CoA.

c. The citric acid cycle – also known as the TCA cycle or the Krebs cycle is regulated in a variety of ways including:

Metabolites: The products of the cycle provide negative feedback on the enzymes that catalyse it. The citric acid cycle is regulated by the concentration of ATP and NADH. The key control points in the regulation of citric acid cycles are the enzymes isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. Isocitrate dehydrogenase is allosterically stimulated by ADP, which enhances the affinity of the enzyme for substrates. The binding of isocitrate, NAD⁺, Mg²⁺ and ADP is mutually cooperative. Therefore NADH inhibits the enzyme iso-citrate dehydrogenase by directly displacing NAD⁺, ATP, too, is inhibitory. A second control site in the citric acid cycle is α-ketoglutarate dehydrogenase. This enzyme is inhibited by succinyl CoA and NADH, the products of the reaction that it catalyzes and also inhibited by a high energy charge.

Citrate: Citrate is the first product of the citric acid cycle, can also inhibit phosphofructokinase (PFK) a key enzyme in glycolysis. If citrate builds up, this is a sign that glycolysis can slow down, because the citric acid cycle is backed up and doesn't need more fuel. This reduces the rate of production of pyruvate and therefore of acetyl-coA.

Calcium: Calcium also plays an important role in the regulation of the citric acid cycle by activating pyruvate dehydrogenase, NAD⁺-dependent isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase.