Metabolism And Its Integration (With Diagram)

Read this article to learn about the metabolism and its integration.

Hundreds of reactions simultaneously take place in a living cell, in a well-organized and integrated manner. The entire spectrum of chemical reactions, occurring in the living system, is collectively referred to as metabolism.

A metabolic pathway (or metabolic map) constitutes a series of enzymatic reactions to produce specific products. The term metabolite is applied to a substrate or an intermediate or a product in the metabolic reactions.

Introduction to Metabolism:

Metabolism is broadly divided into two categories (Fig. 67.1).

1. Catabolism:

The degradative processes concerned with the breakdown of complex molecules to simpler ones, with a concomitant release of energy.

2. Anabolism:

The biosynthetic reactions involving the formation of complex molecules from simple precursors. A clear demarcation between catabolism and anabolism is rather difficult, since there are several intermediates common to both the processes.

Catabolism:

The very purpose of catabolism is to trap the energy of the biomolecules in the form of ATP and to generate the substances (precursors) required for the synthesis of complex molecules. Catabolism occurs in three stages (Fig. 67.2).

1. Conversion of complex molecules into their building blocks:

Polysaccharides are broken down to monosaccharide’s, lipids to free fatty acids and glycerol, and proteins to amino acids.

2. Formation of simple intermediates:

The building blocks produced in stage (1) are degraded to simple intermediates such as pyruvate and acetyl CoA. These intermediates are not readily identifiable as carbohydrates, lipids or proteins. A small quantity of energy (as ATP) is captured in stage 2.

3. Final oxidation of acetyl CoA:

Acetyl CoA is completely oxidized to CO₂, liberating NADH and FADH₂ that finally get oxidized to release large quantity of energy (as ATP). Krebs cycle (or citric acid cycle) is the common metabolic pathway involved in the final oxidation of all energy-rich molecules. This pathway accepts the carbon compounds (pyruvate, succinate etc.) derived from carbohydrates, lipids or proteins.

Anabolism:

For the synthesis of a large variety of complex molecules, the starting materials are relatively few. These include pyruvate, acetyl CoA and the intermediates of citric acid cycle. Besides the availability of precursors, the anabolic reactions are dependent on the supply of energy (as ATP or GTP) and reducing equivalents (as NADPH + H⁺).

The anabolic and catabolic pathways are not reversible and operate independently. As such, the metabolic pathways occur in specific cellular locations (mitochondria, microsomes etc.) and are controlled by different regulatory signals.

The terms—intermediary metabolism and energy metabolism—are also in use. Intermediary metabo­lism refers to the entire range of catabolic and anabo­lic reactions, not involving nucleic acids. Energy metabolism deals with the metabolic pathways concerned with the storage and liberation of energy.

Types of Metabolic Reactions:

The biochemical reactions are mainly of four types:

1. Oxidation-reduction.

2. Group transfer.

3. Rearrangement and isomerization.

4. Make and break of carbon-carbon bonds.

These reactions are catalysed by specific enzymes—more than 2,000 known so far.

Methods Employed to Study Metabolism:

The metabolic reactions do not occur in isolation. They are interdependent and integrated into specific series that constitute metabolic pathways. It is, therefore, not an easy task to study metabolisms. Fortunately, the basic metabolic pathways in most organisms are essentially identical. Several methods are employed to elucidate biochemical reactions and the metabolic pathways.

These experimental approaches may be broadly divided into 3 categories:

1. Use of whole organisms or its components.

2. Utility of metabolic probes.

3. Application of isotopes.

The actual methods employed may be either in vivo (in the living system) or in vitro (in the test tube) or, more frequently, both.

1. Use of whole organism or its components:

(a) Whole organisms: Glucose tolerance test (GTT).

(b) Isolated organs, tissue slices, whole cells, subcellular organelles etc., to elucidate biochemical reactions and metabolic pathways.

2. Utility of metabolic probes:

Two types of metabolic probes are commonly used to trace out biochemical pathways. These are metabolic inhibitors and mutations.

3. Application of isotopes.

Integration of Metabolism:

Metabolism is a continuous process, with thousands of reactions, simultaneously occurring in the living cell. However, biochemists prefer to present metabolism in the form of reactions and metabolic pathways. This is done for the sake of convenience in presentation and understanding. We have learnt the metabolism of carbohydrates, lipids and amino acids. We shall now consider the orga­nism as a whole and integrate the metabolism with particular reference to energy demands of the body organism.

Energy Demand and Supply:

The organisms possess variable energy demands; hence the supply (input) is also equally variable. The consumed metabolic fuel may be burnt (oxidized to CO₂ and H₂O) or stored to meet the energy requirements as per the body needs. ATP serves as the energy currency of the cell in this process (Fig. 67.21).

The humans possess enormous capacity for food consumption. It is estimated that one can consume as much as 100 times his/her basal requirements! Obesity, a disorder of over nutrition mostly prevalent in affluent societies, is primarily a consequence of overconsumption.

Integration of Major Metabolic Pathways of Energy Metabolism:

An overview of the interrelationship between the important metabolic pathways, concerned with fuel metabolism depicted in Fig. 67.22, is briefly described here.

1. Glycolysis:

The degradation of glucose to pyruvate (lactate under anaerobic condition) generates 8 ATP. Pyruvate is converted to acetyl CoA.

2. Fatty acid oxidation:

Fatty acids undergo sequential degradation with a release of 2-carbon fragment, namely acetyl CoA. The energy is trapped in the form of NADH and FADH₂.

3. Degradation of amino acids:

Amino acids, particularly when consumed in excess than required for protein synthesis, are degraded and utilized to meet the fuel demands of the body. The glucogenic amino acids can serve as precursors for the synthesis of glucose via the formation of pyruvate or intermediates of citric acid cycle. The ketogenic amino acids are the precursors for acetyl CoA.

4. Citric acid cycle:

Acetyl CoA is the key and common metabolite, produced from different fuel sources (carbohydrates, lipids, amino acids). Acetyl CoA enters citric acid cycle and gets oxidized to CO₂. Thus, citric acid cycle is the final common metabolic pathway for the oxidation of all foodstuffs. Most of the energy is trapped in the form of NADH and FADH₂.

5. Oxidative phosphorylation:

The NADH and FADH₂, produced in different metabolic pathways, are finally oxidized in the electron transport chain (ETC). The ETC is coupled with oxidative phosphorylation to generate ATP.

6. Hexose monophosphate shunt:

This pathway is primarily concerned with the liberation of NADPH and ribose sugar. NADPH is utilized for the biosynthesis of several compounds, including fatty acids. Ribose is an essential component of nucleotides and nucleic acids (note—DNA contains deoxyribose).

7. Gluconeogenesis:

The synthesis of glucose from non-carbohydrate sources constitutes gluconeogenesis. Several compounds (e.g. pyruvate, glycerol, amino acids) can serve as precursors for gluconeogenesis.

8. Glycogen metabolism:

Glycogen is the storage form of glucose, mostly found in liver and muscle. It is degraded (glycogenolysis) and synthesized (glycogenesis) by independent pathways. Glycogen effectively serves as a fuel reserve to meet body needs, for a brief period (between meals).

Regulation of Metabolic Pathways:

The metabolic pathways, in general, are controlled by four different mechanisms:

1. The availability of substrates

2. Covalent modification of enzymes

3. Allosteric regulation

4. Regulation of enzyme synthesis.

The details of these regulatory processes are discussed under the individual metabolic pathways.