This is “Overview of Stage II of Catabolism”, section 20.3 from the book Introduction to Chemistry: General, Organic, and Biological (v. 1.0). For details on it (including licensing), click here.

For more information on the source of this book, or why it is available for free, please see the project's home page. You can browse or download additional books there. You may also download a PDF copy of this book (72 MB) or just this chapter (5 MB), suitable for printing or most e-readers, or a .zip file containing this book's HTML files (for use in a web browser offline).

Has this book helped you? Consider passing it on:
Creative Commons supports free culture from music to education. Their licenses helped make this book available to you.
DonorsChoose.org helps people like you help teachers fund their classroom projects, from art supplies to books to calculators.

20.3 Overview of Stage II of Catabolism

Learning Objective

  1. Describe the role of acetyl-CoA in metabolism.

A metabolic pathwayA series of biochemical reactions by which an organism converts a given reactant to a specific end product. is a series of biochemical reactions by which an organism converts a given reactant to a specific end product. As you will learn in Section 20.5 "Stage II of Carbohydrate Catabolism" through Section 20.7 "Stage II of Protein Catabolism", there are specific metabolic pathways—which are different for carbohydrates, triglycerides, and proteins—that break down the products of stage I of catabolism (monosaccharides, fatty acids, and amino acids) to produce a common end product, acetyl-coenzyme A (acetyl-CoA) in stage II of catabolism.

Acetyl-CoA is shown in Figure 20.10 "The Structure of Acetyl-Coenzyme A (Acetyl-CoA)". The acetyl unit, derived (as we will see) from the breakdown of carbohydrates, lipids, and proteins, is attached to coenzyme A, making the acetyl unit more reactive. Acetyl-CoA is used in a myriad of biochemical pathways. For example, it may be used as the starting material for the biosynthesis of lipids (such as triglycerides, phospholipids, or cholesterol and other steroids). Most importantly for energy generation, it may enter the citric acid cycle and be oxidized to produce energy, if energy is needed and oxygen is available. The various fates or uses of acetyl-CoA are summarized in Figure 20.11 "Cell Chemistry".

Figure 20.10 The Structure of Acetyl-Coenzyme A (Acetyl-CoA)

Figure 20.11 Cell Chemistry

Acetyl-CoA plays a variety of roles in cell chemistry.

Concept Review Exercises

  1. What is a metabolic pathway?

  2. What vitamin is required to make coenzyme A?

Answers

  1. A metabolic pathway is a series of biochemical reactions by which an organism converts a given reactant to a specific end product.

  2. pantothenic acid

Key Takeaway

  • Acetyl-CoA is formed from the breakdown of carbohydrates, lipids, and proteins. It is used in many biochemical pathways.