Every living organism has the fundamental capability to grow and synthesize new cell material. This requires processing of the nutrient molecules taken up by the cell and involves a series of biochemical transformations. The set of biochemical reactions occurring in cell includes degradation, synthesls as well as modification of the molecules.
These chemical reactions, which operate by the living cells are collectively referred to as metabolic reactions and the phenomenon is called metabolism. Thus, metabolism is defined as the sum total of all biochemical reactions carried out by a living cell.
The metabolic reactions are further categorized as
- Catabolism
- Anabolism
- Primary metabolism
- Secondary metabolism
- Intermediary metabolism
Catabolism
Catabolism includes the set of biochemical reactions which involve degradation of the molecules taken up by the cell and generation of substances essential for biosynthesis of cell constituents. The products of catabolism are catabolites.
They, generally, involve formation of various precursor metabolites, energy rich compounds and reducing power. Hence, these metabolic reactions are often called as fuelling reactions, which provide essential fuels required for cellular synthesis. The precursor metabolites provide basic carbon skeleton for the synthesis of building blocks of the called cellular macromolecules.
The energy rich compounds are ATP, GTP, CTP, TTP, UTP and acetyl CoA, which provide necessary form of biochemical energy required to drive various energy requiring blochemical reactions. On the hydrolysis of high energy bond of these compounds, necessary free energy is available for the purpose.
Reducing power, generated during the catabolism. is in form of reduced pyridine compounds. NADH and NADPH. They provide essential reducing conditions required for several blosynthetic as well as assimilatory reactions of the cell.
Anabolism
Anabolism includes the set of blochemical reactions which involve synthesis cellular molecules.These include blosynthesis of
- Building blocks of cellular macromolecules e.g. amino acids, nucleotides, fatty acids, sugars etc.
- Vitamins and coenzymes, which are essential for driving various enzyme catalyzed reactions.
- Cellular macromolecules such as proteins, lipids, nucleic acids, polysaccharides as well as synthesis of cell structural compounds.
Primary metabolism
The part of cellular metabolism which is very much essential for cell growth is termed as the primary metabolism. The products of primary metabolism are called primary metabolites.
The primary metabolism includes the metabolisms associated with generation of energy rich compounds, reducing power, precursor metabolites as well as the synthesis of bullding blocks of cellular macromolecules.
The products of primary metabolism include
- Energy rich compounds such as ATP and others.
- Organic acids such as lactic acid, citric acid, acetic acid etc.
- Organic alcohols and solvents such as ethanol, glycerol. acetone, butanol etc.
- Amino acids, vitamins coenzymes, nucleotides etc.
The primary metabolism, in general, is found operative lin the cell during log phase of the growth.
Secondary metabolism
The part of cellular metabolism, which is not essential for cellular growth is called secondary metabolism. Products of secondary metabolism are called secondary metabolites.
It becomes active during late log phase and stationary phase of the growth. It involves utilization of the excess remains of carbon precursors and energy for synthesis of new molecules which may have secondary role in growth of a cell.
e.g. Antibiotic synthesis, which is not essential for growth. but does help the organisms to survive in the environment in the presence of various antagonistic organisms by destroying them.
Intermediary metabolism
The part of cellular metabolism which occurs after the entry of nutrients into the cell, leading to the synthesis of bullding blocks of the cellular macro molecules is generally referred to as the intermediary metabolism.
Central metabolic pathways
The central metabolic pathways are basically catabolic in nature. These pathways Include Glycolysis, pentose phosphate pathway and TCA Cycle.They participate in generation of energy, reducing power and precursor metabolites required for cellular synthesis.
Role of reducing power in metabolism
All elements, except phosphorus, present in cell are in reduced form. Carbon exists in organic form. Nitrogen is present as amino group. sulfur as úSH group etc. Therefore, all these elements must be reduced at the cellular level of reduction, before they are assimilated in to cell during biosynthetic reactions.
Most of these elements exist in oxidized state in nature. Therefore, before they are assimilated in the cell, they must be reduced. This requires avallability of sultable reducing power. NADPH serves as the principle reducing power in all such assimilatory and biosynthetic reactions of anabolism. In addition to NADPH, flavin coenzyme, FADH2 can also serve as immediate reducing agent in biochemical reactions.
Generation of reducing power :
Reducing power is generated on oxidation of a suitable electron donor during catabolic reactions of metabolism. These electron donors can be either organic or inorganic or both depending on the type of organism. NADPH acts as the reducing power in all anabolic reactions. In addition to NADPH, flavin coenzyme, FADH2 can also serve as reducing power in certain biochemical reactions.
NADPH is generated during oxidative pentose phosphate pathway, where glucose 6 PO4 is oxidized to pentose phosphate in all most all living organisms. However in arche bacteria, where pentose phosphate pathway does not function, alternate glycolytic pathway function to generate NADPH.
NADPH can also be generated in cell by transhydrogenase reaction, where reduced NAD will participate in reduction of NADP. This reaction also helps in maintaining adequate cellular levels of NADH / NADPH ratio. NADH mainly participates In ATP formation, by transferring electron through electron transport chain.
Role of precursor metabolites
Precursor metabolites are the intermediate molecules in the metabolic pathways. They are produced during operation of catabolic pathways. The precursor metabolites can- Provide basic carbon skeleton for the synthesis of all the building blocks required to synthesize macromolecules.
- Undergo oxidation via catabolic pathways to provide ATP and other energy rich compounds that fuel anabolic pathways.
About 150 different low molecular. weight compounds are required for cellular synthesis. They include
1. Building blocks for synthesis of cellular macromolecules. They include
- Amino acids for synthesis of proteins.
- Fatty acids for synthesis of lipids.
- Monosccharides for synthesis of polysaccharides.
- Purines and pyrimidines for synthesis of Nucleic acids
There are only 12 compounds, which act as precursor metabolites. They are virtually the same in all living organisms. They include
- Acetyl CoA
- Pyruvate
- Phospho enol pyruvate (PEP)
- 3 phospho glyceraldehydes (3PGAL)
- Dihydroxy acetone phosphate (DHAP)
- Glucose 6 Phosphate
- Fructose 6 Phosphate
- Erythrose 4 Phosphate
- Ribose 5 Phosphate
- Xylulose 5 Phosphate
- Aplha Keto glutaric Acid (Alpha KG)
- Succinate
The precursor metabolites are the intermediates of three Indispensible pathways of catabolism
- TCA cycle
- Glycolytic or gluconeogenic pathways
- Pentose phosphate pathway
Role of energy rich compounds
To perform all cellular activities, a suitable form of blochemical energy is required by the cell. This biochemical energy is obtalned as the energy rich compounds, which possess high energy rich chemical bonds.
The necessary energy required to drive a blochemical reaction is released on hydrolysis of this energy rich bond. In living cells, a variety of energy rich compounds are formed, which are utilized for general purpose or to drive a specific biochemical set of reactions.
Energy rich compounds of cell
There are mainly two classes of energy rich compounds formed in the cell, which satisfy need of energy requiring reactions. They are
- Compounds having high energy anhydrous phosphoester bond.
- Compounds having high energy thiolester bond.
Compounds having high energy anhydrous phosphoester bond
Most energy rich compounds of the cell belong to this category. They are obtained as nucleoside triphosphate derivatives. These include
- ATP Adenosine triphosphate
- GTP Guanosine triphosphate
- CTP Cytidine triphosphate
- TTP Thymidine triphosphate
- UTP Uridine triphosphate
ATP and its role
ATP is considered as one of the most commonly used energy currency of the cell. It possesses two energy rich anhydrous phosphoester bonds.Hydrolysis of each of this energy rich bond releases 7.3 Keal energy.
ATP + H₂O ➞ ADP + Pi + 7.3 Kcal.
ADP + H₂O ➞ AMP + Pi + 7.3 Kcal.
AMP + H₂O ➞ Adenosine + Pi+ 4 Kcal.
ATP is most commonly required for
- Uptake of nutrients
- Activation of most substrate molecules so that they are able to enter the cell metabolism
- Biosynthesis of most cellular molecules, nucleic acids, chromosome replication and cell division.
Other energy rich compounds and their role
Apart from ATP, various other kinds of energy rich compounds are formed in the cell. They have specialized role in cellular metabolism. Their specialized utilization in specific metabolic reactions may be considered useful for adequate supply of energy for the concerned biosynthetic reactions, so that they can operate at optimal level in the cell. These energy rich compounds and their role are summarised in below table. |
| Energy rich compounds and their role in metabolism. |
