Unity of Biochemistry 36 dissipated as heat with no power to drive the formation of a bond between X and Y; also, the formation of more water where it is already abundant during the linkage of X and Y is unfavorable. Life’s solution to this problem is both elegant and simple. Instead of transferring the terminal phosphate to water, as in reaction (9), transfer this group to the X-OH in what is called phosphorylation . ATP + X-OH Æ ADP + X-O-P (10) Most of the energy released by breaking the phosphate off of ATP is now recaptured in the bond between the phosphate derivative and the X reactant. Next, the X-O-P, which is said to have been activated , or made more reactive by the addition of the phosphate, interacts with the Y-H to form X-Y; X-O-P + Y-H Æ X-Y + P i (11) Adding reactions (10) and (11): ATP + X-OH + Y-H Æ ADP + X-Y + P i (12) X-Y has formed and ATP has hydrolyzed, but nowhere has water formed (see also fig. 3.7).
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