Metabolism coupling stoichiometry

Since metabolic coupling is stoichiometric in nature, it seems appropriate to consider the different stoichiometric relationships that must be taken into account in the study of metabolism. These may be grouped into three types reaction stoichiometries, obligate coupling stoichiometries, and evolved coupling stoichiometries. 

Table II summarizes the results for degradation of the CP isomers in East River cultures under sulfate-reducing conditions based on the stoichiometry in equation 11. Sulfate loss in the background controls were subtracted from the cultures to which CPs were added. As noted in Table II, the measured sulfate depletion corresponded to that calculated and provided evidence that CP metabolism was coupled to sulfate reduction. In these studies sulfate reduction is supported by two additional experimental observations. First, molybdate, which is a specific inhibitor of microbial sulfate reduction, was documented to stop the CP degradation. Active controls that did not receive molybdate continued to degrade CP. Second, radiolabeled 35S042 formed 35 S2 in active cultures and not in control cultures (33).

The coupling of reaction sequences through the adenylate system is entirely different from the two types of stoichiometric relationships discussed above. The stoichiometry of each reaction in which ATP is regenerated or used is, of course, fixed by the nature of the reaction, but the number and types of reactions in which ATP is involved have been determined by evolutionary processes, rather than by simple chemical necessity. Indeed the use of ATP as a coupling agent and the evolutionary adjustments of stoichiometric relationships for maximal metabolic advantage are at the very center of biological function. This fact is underscored by the participation of ATP in every extended metabolic sequence. 

The existence of two reaction pathways linking glucose and CO2 that differ in ATP stoichiometry and hence in equilibrium constant is by no means a unique situation. Nearly every metabolic pathway has a large in the forward direction but coexists with an oppositely directed pathway, for which in the opposite direction is large. Metabolic sequences are typically strictly unidirectional, and functional reversal of a metabolic conversion nearly always involves a different reaction pathway with a different ATP coupling coefficient. The biosynthesis and degradation of fatty acids supplies another illustration of this generalization. The conversion of 8 /mmoles of aeetyl-SCoA to 1 /imole of palmityl-SCoA requires 14 /imoles of TPNH, or 56 ATP... 

At the first Symposium on Phosphorous Metabolism, held in Baltimore in the spring of 1951, Barker reviewed the results of studies with enzyme preparations of C. kluyveri as well as complimentary studies in the field of CoA metabolism. Based on a most remarkable critical analysis of the available information, he proposed a mechanism for the oxidation of butyrate by soluble extracts of C. kluyveri (see Fig. 1) in which CoA derivatives of 4-carbon compounds at various states of oxidation were postulated as the active intermediates, and in which cleavage of acetoacetyl CoA to form two equivalents of acetyl-CoA) [the reverse of reaction (26)] was the final step. The observed stoichiometry of one mole each of acetyl-P and acetate as ultimate products of butyrate oxidation [reaction (3)] was explained by the obligatory coupling of butyrate oxidation with reaction (25), needed to regenerate free CoA, and with reaction (27), in which butyryl-CoA is formed. 

Besides the transport, the most imponant processes in biological systems are those related to chemical reactions of metabolism. One of the typical aspects of such reactions is the requirement regarding the apparent stoichiometry of two partially coupled reactions, and the study of the efficiency of such reactions as limited by the constraints of the second law of thermodynamics. 

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