Metabolism in any organism includes two processes anabolism and catabolism. These two processes include all the biochemical reactions of living organisms.
Process # 1. Anabolism:
It is a process in which essential biomolecules required for growth are generated by the utilization of energy. The chief biomolecules required are carbon like glucose, ribose, glycerol, pyruvate, etc. Some of the biomolecules act as the central metabolic intermediates for all types of carbon and nitrogen compounds required for growth.
Some microorganisms can themselves make all the essential organic compounds required for growth as in the case of autotrophs. Such organisms can be grown on simple and chemically defined media. On the other hand, some of the microorganisms which are unable to make most of the organic compounds from atmosphere are called fastidious organisms. These can only be grown on complex media with different growth factors.
For the biosynthesis, (anabolism) of different essential biomolecules, following anabolic processes take place in organisms:
(a) Synthesis of carbohydrates like glucose, sucrose, cellulose, etc.
(b) Synthesis of lipids, glycolipids, phospholipids, etc.
(c) Synthesis of amino acids and protein.
(d) Synthesis of nucleic acids.
(e) Synthesis of other growth requirements like vitamins, hormones, etc.
Process # 2. Catabolism:
All the processes in which the nutrients taken in the form of biomolecules as food are broken down (digested) to release energy are called catabolism. Catabolic processes also convert complex organic compounds, stored in the cells of microorganisms (like glycogen granules, polyphosphate, etc.) to simpler forms.
Each and every organic compound whether it is carbohydrates, protein or fat, can be catabolized according to the requirement of organism. Similar to anabolism certain biomolecules act as link between catabolic and anabolic processes. Apart from these, ATP and NAD (P)H also act as a link between the two types of pathways.
The chief catabolic processes involved m cell metabolisms are:
(a) Glycolysis,
(b) Pentose-phosphate pathway (PPP),
(c) Entner doudoroff pathway (EDP),
(d) Tricarboxylic acid cycle (TCA),
(e) Fermentation,
(f) Glyoxylate cycle,
(g) Lipid hydrolysis,
(h) Protein hydrolysis.
Adenosine Triphosphate (ATP):
ATP is the chief energy carrier molecule in all type of organisms. ATP is required for the anabolic (generative) process of an organism by which organic macromolecules required for the growth are synthesized. ATP is formed by the catabolic (degradative) process in which macromolecules are broken down and energy is generated.
The structure of ATP was first deduced by Lohman in 1930 and confirmed by Alexander Todd in 1948. ATP consists of an adenosine (adenine plus ribose sugar) and three phosphate groups.
These three phosphate groups are bounded by high energy ester and anhydride bonds to adenosine unit and whenever one or two of these phosphate groups are removed from the ATP, large amount of energy is released (Fig. 12.1).
This energy is the instant energy utilized in the various anabolic processes of the cell. When the ATP is utilized in anabolic process, it is broken down to ADP or AMP releasing phosphate and energy. The ATP utilized in anabolic process is replenished by catabolic process by reversal reactions. The trapping of chemical energy, released by the oxidative reactions of the cell, in the form of ATP, is called phosphorylation. There are three type of phosphorylation.
(i) Photophosphorylation:
It occurs in the presence of light by photosynthetic organisms mainly in the photoautotrophs and photo heterotrophs. In the photosynthetic cells, green pigment (chlorophyll) present in the chloroplast or bacteriochlorophylls present in the cell membrane of thylakoids, trap energy from solar radiations which become activated, releasing electrons.
These electrons then pass through a series of earners. The energy thus released is trapped in the form of phosphate bonds in ATPs. Photophosphorylation is of two types.
(a) Cyclic photophosphorylation:
In this type of photophosphorylation electrons released from chlorophyll molecules (due to excitement by light energy) return back to the same chlorophyll molecules by the same route using electron carriers. Such pathway is cyclic in nature as the electrons pass, energy is released which is trapped in the form of ATP.
(b) Non-cyclic photophosphorylation:
In this type of photophosphorylation electrons released from chlorophyll molecules do not return back to the same chlorophyll molecules, but instead are received by nicotinamide adenine dinucleotide phosphate (NADP+).
From NADP electrons enter the electron transport chain whereby oxidative phosphorylation occurs and ATP is formed. The electrons released from chlorophyll molecules are replenished in the chlorophyll molecules by photolysis of water.
2 H2O → 4 H+ + 4e– + O2
Thus, we can see that oxygen is released in this type of photophosphorylation. Oxygen is toxic for anaerobes and, therefore, this type of phosphorylation is not found among anaerobic microorganisms (Fig. 12.2).
(ii) Oxidative Phosphorylation:
In this phosphorylation, the electrons collected by certain electron carriers like NAD+, NADP+ and FAD+ from various sources are passed into electron transport chain. Finally, electrons reach oxygen or some other inorganic molecule (like iron, nitrate, etc.), which act as final electron acceptors. The transfer of electrons from one carrier to another released energy, which is used to generate ATP from ADP.
Oxidative phosphorylation occurs in the inner membrane of mitochondria in eukaryotes and in plasma membrane of prokaryotes. One molecule of NADPH generates three molecules of ATP, when it enters the electron transport chain for oxidative phosphorylation, however, one molecule of FADH generates only two molecules of ATP. This is due to the fact that FADH enters the ETC later than NADPH.
(iii) Substrate Level Phosphorylation:
In substrate level phosphorylation, ATP is generated by the transfer of high energy phosphate bond from any other metabolic compound to ADP, for example:
(iv) Nicotinamide Adenine Dinucleotide Phosphate (NADP+):
Nicotinamide adenine dinucleotide phosphate (NADP) and Nicotinamide adenine dinucleotide (NAD) are the carriers of electrons (protons) in the cells. Therefore, NADP+ and NAD+ serve as the reducing power of the cells in the form of NADP/NADPH2 or an NAD/NADH2.
Reduced substrate + NADP → Oxidised substrate + NADPH2
Oxidised substrate + NADPH2 → Reduced substrate + NADP
NADP or NAD+ functions as coenzymes of a large number of oxidoreductase enzymes. They act as electron acceptors during enzymatic removal of hydrogen atoms from specific substrate molecules. Finally, reduced NADP or NAD i.e. NADPH2 or NADH2 release energy, ATP is generated. In catabolic reactions like respiration, NAD+/NADH act as electron donor, whereas NADP+/NADPH play roles in anabolic reactions.
Structurally, NAD consists of a nicotinamide moiety (the protein which undergoes reversible reduction), two adenine nucleotide. In NADP, a phosphate group is esterified to the second adenine nucleotide at 2-hydroxyl group (Fig. 12.3).
