CHAPTER 22: Unit 1. Metabolic Pathways Introduction

In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reactions catalyzed by enzymes.

When we eat food, the polysaccharides, lipids and proteins are digested to smaller molecules that can be absorbed into the cells of our body. As glucose, fatty acids an amino acid are broken down further, the energy is transferred to molecules in our cells such as adenosine triphosphate (ATP). Our cells use the energy stored in ATP when they do work such as contracting muscles, synthesizing large molecules, sending nerve impulses, and moving substances across cell membranes. All the chemical reactions that take place in cells to break down or build molecules are known as metabolism.

A metabolic pathway is a series of linked reactions each catalyzed by a specific enzyme.

Metabolic pathways and cycles are reaction chains where chemical products become the substrate for the next step. All substrates are chemically transformed in reactions that belong to either pathways (if the reactions are aligned in linear fashion) or metabolic cycles (if the moieties of the reactions are preserved). The term substrate oxidation is used for substrate degradation ultimately leading to CO₂ production. However, such an oxidative pathway may be interrupted for several reasons, such as in the case of glucose oxidation, where limited oxygen results in the production of lactate rather than water and CO₂. Some but not all substrates can also be converted to building blocks for biosynthetic processes. This chapter will describe the main metabolic pathways and cycles of cardiac substrate metabolism. It will also address their interactions and their contributions to meet cardiac energy requirements.

There are three metabolic pathways that provide our muscles with energy: the phosphagen pathway, the glycolytic pathway, and the oxidative pathway. The phosphagen pathway dominates high power, short duration efforts: things that take less than 10 seconds but require a huge power output.

In humans, the most important metabolic pathways are glycolysis – glucose oxidation in order to obtain ATP. citric acid cycle (Krebs’ cycle) – acetyl-CoA oxidation in order to obtain GTP and valuable intermediates. oxidative phosphorylation – disposal of the electrons released by glycolysis and citric acid cycle.

So, Metabolism is the set of chemical reactions that occur in a cell, which enable it to keep living, growing and dividing. Metabolic processes are usually classified as:

1. Catabolism – obtaining energy and reducing power from nutrients.
2. Anabolism – production of new cell components, usually through processes that require energy and reducing power obtained from nutrient catabolism.

There is a very large number of metabolic pathways. In humans, the most important metabolic pathways are:

1. Glycolysis– glucose oxidation in order to obtain ATP.
2. Citric acid cycle (Krebs’ cycle) – acetyl-CoA oxidation in order to obtain GTP and valuable intermediates.
3. Oxidative phosphorylation – disposal of the electrons released by glycolysis and citric acid cycle. Much of the energy released in this process can be stored as ATP.
4. Pentose phosphate pathway– synthesis of pentoses and release of the reducing power needed for anabolic reactions.
5. Urea cycle – disposal of NH₄⁺ in less toxic forms
6. Fatty acid β-oxidation – fatty acids breakdown into acetyl-CoA, to be used by the Krebs’ cycle.
7. Gluconeogenesis– glucose synthesis from smaller precursors, to be used by the brain.

Reference:
https://www.youtube.com/watch?v=cn5tq2l2ptA