Schematic Presentation of the Citric Acid Cycle The citric acid cycle (tricarboxylic cycle) is often presented ss a ring of the sequence of intermediates. However, some misunderstanding arises from this kind of presentation which is well pointed out by McCilvary' in this way: "The stoichiometry of the citric acid cycle does not involve the production or consumption of oxalacetate or any other constituent of the cycle. The reactions of the citric acid cycle do not provide a route for making oxalseetate from scetyl groups. Failure to sppreciate this point has led intelligent men into serious error in the past, and i t must be kept firmly in mind to avoid the same mistake in the future." In order to avoid this kind of misunderstanding, I have made P,..~.,~ 7 @c" a. new scheme (see figure) of the cycle based on the number of % ' A"!d*A csrbon atoms entering into reactions. So the four-carbon skele% ton of oxslaeetrtte is presented as a four-wheel carrier to be loaded with two carbon atoms of scetyl group to form the six carbon citrate. The sequence of reactions consumes the load of the carrier by transforming i t to two moles of COZand the unloaded carrier is left in the form of oxalacetate whichis reedy to beloaded again with two-carbon scetyl group and t o be used in the sune manner repeatedly.
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R. W., "Biooheniistry, A Functional Approach," 'MCGILVARY, W. B. Saunders, Philadelphia, 1970, p. 222.
R ~ NAFISSY A UNIVER~ITY OF TEHRAN SCHOOL OF MEDICINE
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Journ01 of Chemical Education
