Introduction to metabolism
- Hundreds of reactions simultaneously takeplace in a living cell, in a well-organized and integrated
- The entire spectrum of chemical reactions, occurring in the Iiving system, are collectively referred to as metabolism.
- A metabolic pathway (or metabolic map) constitutes a series of enzymatic reactions to produce specific
- The term metabolite is applied to a substrate or an intermediate or a product in the metabolic
- Metabolism is a highly coordinated and purposeful cell activity, in which many multienzyme systems cooperate.
- Metabolism has four specific functions:
- To obtain chemical energy from the degradation of energy rich nutrients from the environment or from captured solar energy.
- To convert nutrient molecules into the building block precursors of cell macromolecules.
- To assemble these building blocks into proteins, nucleic acids, lipids, polysaccharides and other cell components.
- To form and degrade biomolecules required in specialized functions of cells. Although metabolism involves hundreds of different enzyme catalyzed reactions, the central metabolic pathways, are few in number and they are identical in most forms of life.
- Metabolism is broadly divided into two categories.
1. Catabolism : The degradative processes concerned with the breakdown of complex molecules to simpler ones, with a release of energy.
- 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.
- The term amphibolism is also in use for reactions which are both catabolic and anabolic in nature.
Fig: Energy relationship between Catabolic and Anabolic Pathways
- 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
- Catabolism occurs in three stages.
- Conversion of complex molecules into their building blocks:
- Polysaccharides are broken down to monosaccharides, lipids to free fatty acids and glycerol, proteins to amino
- Formation of simple intermediate :
- 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
- 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
- This pathway accepts the carbon compounds (pyruvate, succinate etc.) derived from carbohydrates, lipids or proteins.
Fig: The three stages of catabolism
- Anabolism is the biosynthetic phase of metabolism in which small, simple, precursors are built up into larger and more complex molecules, including lipids, polysaccharides, proteins and nucleic acids.
- 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
- Besides the availability of precursors, the anabolic reactions are dependent on the supply of energy (as ATP ) and reducing equivalents (as NADPH +H+, NADH +H+, FADH2)
- As such, the metabolic pathways occur in specific cellular locations (mitochondria, microsomes ) and are controlled by different regulatory signals.
In general, catabolic pathways are convergent and anabolic pathways are divergent
- The terms-intermediary metabolism and energy metabolism-are also in
- Intermediary metabolism refers to the entire range of catabolic and anabolic reactions, not involving nucleic
- Energy metabolism deals with the metabolic pathways concerned with the storage and liberation of energy.