Citric Acid Cycle Summary

Overview

This lecture covers the steps, main products, and significance of the citric acid cycle (Krebs/TCA cycle) in cellular energy production.

The citric acid cycle occurs in the mitochondrial matrix and uses acetyl CoA from sugars and fats.
The cycle produces ATP, NADH, FADH2, and carbon dioxide as main outputs.
NADH and FADH2 proceed to the electron transport chain for further ATP production.
The cycle is also called the TCA cycle or Krebs cycle.

Step 1: Citrate synthase combines acetyl CoA (2C) with oxaloacetate (4C) to make citrate (6C).
Step 2: Aconitase converts citrate into isocitrate.
Step 3: Isocitrate dehydrogenase converts isocitrate to α-ketoglutarate (5C), releasing CO2 and producing NADH.
Step 4: α-Ketoglutarate dehydrogenase converts α-ketoglutarate to succinyl CoA (4C), releasing CO2 and producing NADH.
Step 5: Succinyl CoA synthetase converts succinyl CoA to succinate, generating GTP (converted to ATP).
Step 6: Succinate dehydrogenase converts succinate to fumarate, producing FADH2.
Step 7: Fumarate hydratase converts fumarate to malate.
Step 8: Malate dehydrogenase converts malate to oxaloacetate, producing NADH.

For each glucose, two acetyl CoA are produced, resulting in two cycles.
Each glucose yields 4 CO2, 6 NADH, 2 FADH2, and 2 ATP from the cycle.
NADH and FADH2 act as electron carriers for additional ATP synthesis.
Proteins and fats can also be metabolized to enter the citric acid cycle as intermediates or acetyl CoA.

Citric Acid Cycle (Krebs/TCA Cycle) — Major metabolic pathway to oxidize acetyl CoA for energy.
Acetyl CoA — Two-carbon molecule entering the cycle, derived from sugars or fats.
NADH/FADH2 — Electron carriers produced during the cycle.
Oxaloacetate — Four-carbon molecule that initiates and is regenerated by the cycle.
GTP/ATP — High-energy compounds produced during the cycle.