TCA Cycle Final Study

Location of the TCA cycle

the mitochondrial matrix

What is the rational as to why the TCA cycle must occur?

The overall goal of the cycle is to completely oxidize acetate (as Acetyl-CoA) into 2 CO_2​ by cleaving a carbon-carbon bond usually occurs between α- and β-carbons relative to a carbonyl group. However, acetate lacks a β-carbon, making direct carbon-carbon bond cleavage impossible. To overcome the lack of a β-carbon, Acetyl-CoA (2C) is condensed with Oxaloacetate (4C) to form Citrate (6C) in order to be cleaved.

Step 1 of TCA

Acetyl CoA(2C) and Oxaloacetate(4C) are combined and form Citrate(6C) with the help of Citrate Synthase

• This is an irreversible condensation Reaction

Step 2 of TCA

Citrate(6C) is converted to Isocitrate(6C) by the enzyme Aconitase through a two-step process involving dehydration and rehydration.

• hydroxyl group is moved

Step 3 of TCA

Isocitrate(6C) is oxidized to α-ketoglutarate(5C) by the enzyme Isocitrate Dehydrogenase, resulting in the reduction of NAD+ to NADH and the release of CO₂

• irreversible

Step 4 of TCA

α-ketoglutarate(5C) is oxidized to Succinyl CoA(4C) by the enzyme α-ketoglutarate Dehydrogenase, generating NADH and releasing CO₂.

Step 5 of TCA

Succinyl CoA(4C) is converted to Succinate(4C) by the enzyme Succinyl-CoA Synthetase, generating GTP or ATP and releasing CoA.

• substrate level phosphorylation

• tissue specificity

Step 6 of TCA

Succinate(4C) is oxidized to Fumarate(4C) by the enzyme Succinate Dehydrogenase, producing FADH2.

• alkane to alkene

• Inner mitochondrial matrix

Step 7 of TCA

Fumarate(4C) is hydrated to Malate(4C) by the enzyme Fumarase, adding a water molecule.

Step 8 of TCA

Fumarate(4C) is hydrated to Malate(4C) by the enzyme Fumarase, utilizing water.

• Regenerates Oxaloacetate

Tissue Specificity in Step 5

• Liver and Kidney: Express the GTP-forming isoform (GDP+Pi​→GTP), where GTP is primarily used to fuel gluconeogenesis.

• Heart and Brain (and Muscle): Express the ATP-forming isoform (ADP+Pi​→ATP), producing ATP directly for cellular energy/mechanical contraction.

TCA Cycle Toxins

1. Fluroacetate

2. Arsenite

3. Malonate

Fluoroacetate

Rat Poison

• used as a metabolic inhibitor that disrupts the TCA cycle by converting to fluorocitrate, which inhibits aconitase in step 2

Arsenite

A toxic compound that inhibits the enzyme alpha-ketoglutarate dehydrogenase in the TCA cycle, disrupting energy production and leading to cellular damage.

Malonate

An organic compound that acts as a competitive inhibitor to succinate dehydrogenase

Amphibolic Nature

Refers to the ability of the TCA cycle to participate in both catabolic and anabolic processes, contributing to energy production as well as biosynthesis.

Cataplerosis

The process of removing intermediates from the TCA cycle for biosynthetic pathways elsewhere, thus supporting anabolic reactions and energy production.

Cataplerosis Examples: Citrate

• Citrate → Fatty Acid Synthesis: Citrate leaves the mitochondrial matrix to the cytoplasm to supply acetyl-CoA for making fatty acids and lipids.

Cataplerosis Examples: α-Ketoglutarate

• α-Ketoglutarate → Amino Acid Synthesis: Converted into glutamate, glutamine, and other amino acids.

Cataplerosis Examples: Succinyl-CoA

• Succinyl-CoA → Heme Synthesis: Used to construct porphyrins and heme groups.

Cataplerosis Examples:Oxaloacetate

• Oxaloacetate → Gluconeogenesis & Amino Acids: Converted to phosphoenolpyruvate (PEP) to synthesize glucose, or converted to aspartate and asparagine.

Anaplerosis

The process of replenishing TCA cycle intermediates that have been extracted for biosynthesis. Without enough oxaloacetate, acetyl-CoA cannot condense to form citrate

Anaplerosis examples

• Pyruvate Carboxylation (The primary anaplerotic reaction): Catalyzed by Pyruvate Carboxylase (PC), which converts pyruvate directly into oxaloacetate

• Amino Acid Degradation: The breakdown of amino acids directly replenishes intermediates like α-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate.

• Odd-Chain Fatty Acid Oxidation: Converts the remaining 3-carbon unit (propionyl-CoA) into succinyl-CoA.