BIOSYNTHESES
THE priming reactions described in the preceding chapter are the principal sources of the coin which pays for the performance of cellular work: the pyrophosphate bond of ATP. This coin also pays for the energy required in biosyntheses, with the exception of assimilation phenomena whose mechanism will be studied later (see Part Six). But the priming reactions do not furnish only ATP, they also provide a series of construction materials for these biosyntheses. For example two-carbon fragments in the form of "active acetate" or acetyl-CoA are obtained during the priming reactions from glucose, fatty acids, acetoacetic acid and amino acids.
There are also one-carbon atom materials, together with CO2 which is liberated at numerous points in the priming system. But the most important material of this type is that referred to as "formate" (C1) that is CHO- or active formyl, into which can be converted not only formic acid, formaldehyde and methanol, but also the α-carbon of glycine, the α-carbon of glycollic acid, and α- and ß-carbons of serine, the α-carbon of threonine, the C--2 of histidine, the C--2 of tryptophan, the α-C and the C--6 (or C--2) of phenylalanine and tyrosine, etc. Acetone can be split into an acetyl fragment and a formyl fragment.
In certain bacteria (but not in mammals), pyruvic acid can undergo fission into acetyl phosphate and formic acid (phosphoroclastic reaction), providing C1 units.
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