Krebs Cycle Overview and Key Steps

Study Notes

Overview

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a crucial part of cellular respiration.
It's a series of eight enzyme-catalyzed reactions that occur in the mitochondrial matrix of eukaryotic cells.
It plays a central role in the metabolism of carbohydrates, fats, and proteins.
The cycle begins with acetyl-CoA, a two-carbon molecule derived from the breakdown of glucose, fatty acids, and amino acids.

Key Steps and Reactions

Step 1: Formation of Citrate: Acetyl-CoA combines with oxaloacetate (a four-carbon molecule) to form citrate (a six-carbon molecule).
Steps 2-8: A series of oxidation-reduction reactions, decarboxylations, and isomerizations follow, converting citrate to oxaloacetate through a cyclical process.
Key intermediates include isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, malate.
During these reactions, electrons are released and carried by NAD+ and FAD, producing NADH and FADH2.
Carbon dioxide (CO2) is released in two steps.
GTP or ATP is produced from succinyl-CoA.
Oxaloacetate is regenerated at the end of the cycle, enabling the cycle to continue.

Role of Enzymes

Each step in the Krebs cycle is catalyzed by a specific enzyme.
These enzymes are vital for the efficient and controlled operation of the cycle.
The activity and regulation of these enzymes are crucial for maintaining metabolic homeostasis.

Energy Production

The Krebs cycle is a major source of energy for the cell.
The energy released during the oxidation reactions is captured as high-energy electron carriers (NADH and FADH2).
These electron carriers then donate their electrons to the electron transport chain, which generates ATP through oxidative phosphorylation.
A single turn of the Krebs cycle yields 1 ATP (or GTP), 3 NADH, and 1 FADH2.
These energy-rich molecules store energy for later use in the form of ATP.

Regulation of the Krebs Cycle

The Krebs cycle is regulated to meet the energy demands of the cell.
The rate of the cycle is controlled by the availability of substrates (acetyl-CoA and oxaloacetate) and the feedback inhibition of key enzymes by high concentrations of ATP, NADH, and succinyl-CoA.
The availability of O2 also affects the cycle's activity, as NADH and FADH2 cannot pass on their electrons to the electron transport chain without oxygen.

Connection to Other Metabolic Pathways

The Krebs cycle connects various metabolic pathways.
It serves as a central hub for the metabolism of carbohydrates, fats, and proteins.
Intermediates of the cycle can be used as precursors for the synthesis of other molecules, such as amino acids and certain lipids.

Importance for Cellular Activities

The Krebs cycle is essential for various cellular processes, including energy production, biosynthesis, and cell growth.
It is a vital pathway that sustains the cell's functions.
Disruptions in the Krebs cycle can lead to metabolic dysfunction and various diseases.

Description

This quiz covers the Krebs cycle, also known as the citric acid cycle, highlighting its importance in cellular respiration and metabolism. You will learn about the key steps, the series of reactions involved, and the crucial intermediates formed during this cycle. Test your knowledge on the essential functions and processes occurring in the mitochondrial matrix.