Utilization of Metabolic Energy for Biosyntheses

Lipmann has pointed out that a large number of synthetic reactions have one feature in common formation of more complicated structural units by mechanisms involving the splitting out of water. A few such reactions, which may be considered typical are shown in the following equations. 

All the above reactions, as written, are endergonic, and consequently cannot spontaneously proceed sufficiently far to the right to give reasonable yields of product, unless coupled to some exergonic reaction in such a fashion that the over-all net reaction leads to a negative free energy change. 

The free energy change for the production of an ester bond, or a simple peptide bond, such as are shown in equations 32 and 33, is approximately 3000 cal. per bond. This value is obtained when the free energy change is calculated with the standard state of water taken as pure water, or 55.6 molal. When the standard state of water is taken as 1 molal ( .e., when it is 

The free energy change in reaction 34 is much higher Lipmann has calculated it to be about 16,000 cal. In this reaction, not only is water split out, but a C—C bond of a /3-keto acid is formed. 

Although the postulation of compounds such as acetyl phosphate and the acyl phosphates of the higher fatty acids as intermediates in synthetic reactions is very attractive on thermodynamic grounds, a considerable amount of evidence has been obtained which indicates that these phosphorylated compounds do not themselves play an important physiological part in mammalian tissues. When acetyl phosphate or the higher acyl 

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