Understanding the Citric Acid Cycle: Process, Significance, and Role in Energy Production, Exercises of Biochemistry

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TCA CYCLE

STEPS

REGULATION AND

SIGNIFICANCE

Introduction

• The citric acid cycle is the central metabolic hub of the cell.
• It is the final common pathway for the oxidation of fuel molecule such as amino acids, fatty acids, and carbohydrates.
• I n eukaryotes, the reactions of the citric acid cycle take place inside mitochondria, in contrast with those of glycolysis, which take place in the cytosol.

Alcoholic Fermentation

Overview of the Citric Acid

Cycle

The citric acid cycle (Krebs cycle, tricarboxylic acid cycle) includes a series of oxidation- reduction reactions in mitochondria that result in the oxidation of an acetyl group to two molecules of carbon dioxide and reduce the coenzymes that are reoxidized through the electron transport chain, linked to the formation of ATP.

Overview of the Citric Acid Cycle

o Three hydride ions (hence, six electrons) are transferred to three molecules of nicotinamide adenine dinucleotide (NAD+), whereas one pair of hydrogen atoms (hence, two electrons) are transferred to one molecule of flavin adenine dinucleotide (FAD). o The function of the citric acid cycle is the harvesting of high- energy electrons from carbon fuels.

Citric acid cycle and requirement of

oxygen

Oxygen is required for the citric acid cycle indirectly in as much as it is the electron acceptor at the end of the electron- transport chain, necessary to regenerate NAD+ and FAD.

Role of oxaloacetate in citric acid cycle

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• The four-carbon molecule, oxaloacetate that initiates the first step in the citric acid cycle is regenerated at the end of one passage through the cycle.
• The oxaloacetate acts catalytically: it participates in the oxidation of the acetyl group but is itself regenerated.
• Thus, one molecule of oxaloacetate is capable of participating in the oxidation of many acetyl molecules.

Reactions of the Citric Acid Cycle

• Step-1 Formation of Citrate- The citric acid cycle begins with the condensation of a four- carbon unit, oxaloacetate, and a two-carbon unit, the acetyl group of acetyl CoA. Oxaloacetate reacts with acetyl CoA and H2O to yield citrate and CoA.

Step-2-Formation of Isocitrate

intermediate.

• Citrate is isomerized into isocitrate to enable the six-carbon unit to undergo oxidative decarboxylation.
• The isomerization of citrate is accomplished by a dehydration step followed by a hydration step.

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Step-2-Formation of Isocitrate

Step-3- Formation of α- Keto Glutarate

Respiratory chain-linked oxidation of isocitrate proceeds almost completely through the NAD+-dependent enzyme. 15

Step-4-Formation of Succinyl Co A

o (^) α-Ketoglutarate undergoes oxidative

decarboxylation in a reaction catalyzed by a multi-enzyme complex similar to that involved in the oxidative decarboxylation of pyruvate.

o (^) The α- -ketoglutarate dehydrogenase

complex requires the same cofactors as the pyruvate dehydrogenase complex—thiamine diphosphate, lipoate, NAD+, FAD, and CoA— and results in the formation of succinyl-CoA.