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The following points highlight the three main heterotrophic reactions operating in different living systems. The reactions are: 1. Reactions in which NADPH2 is Required 2. Reactions Requiring ATP 3. Reactions Requiring no Apparent Energy Source.
1. Reactions in which NADPH2 is Required:
Two examples of this type are malic enzyme catalysed CO2 incorporation in pyruvic acid, and synthesis of isocitric acid from α-ketoglutaric acid catalysed by isocitrate dehydrogenase through reversal of the TCA cycle reaction. In the TCA cycle reaction, isocitrate dehydrogenase catalyses decarboxylation using NAD as coenzyme.
But the carboxylation reaction catalysed by the same enzyme requires NADPH2:
2. Reactions Requiring ATP:
Incorporation of CO2 in the reactions requiring ATP is dependent on biotin. Biotin combines with CO2 to form “active CO2” (see Fig. 8.38). ATP is required for the formation of carboxybiotin. Carboxybiotin then transfers CO2 to the acceptor forming the carboxylated product. Biotin remains linked to the carboxylase.
Suggested steps are:
Examples are propionyI-CoA carboxylase and acetyI-CoA carboxylase reactions:
3. Reactions Requiring no Apparent Energy Source:
The third category of carboxylation reactions is exemplified by phosphoenol pyruvic acid carboxylase and phosphoenol pyruvic acid carboxykinase catalysed reactions.
Since phosphoenol pyruvic acid (PEP) is itself an energy-rich compound, no extra energy is required for carboxylation:
In this reaction energy-rich phosphate bond of PEP is transferred to GDP to produce an energy- rich compound (GTP). An interesting discovery was the presence of the Calvin-Benson cycle enzyme, ribulose bisphosphate carboxylase, in Pseudomonas oxalaticus, a heterotrophic organism. When this organism grows on formate or oxalate both of which are organic compounds, it forms RuBP-carboxylase and fixes CO2 via Calvin-Benson cycle, just like a chemoautotrophic organism.
