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F. M. HUENNEKENS AND H. R. WHITELEY
phate group as either partner of the susceptible bond (e.g., acetyl-CoA)
have been discovered.
The present chapter surveys the various "energy-rich" compounds,
which represent potential or actual energy reserves in living systems.
These compounds fall into two general structural categories, as represented in Fig. 1.
O
O
II
II
HO—P—X
R—C—X'
I
OH
(I)
(Π)
FIG. 1. "Energy-rich" structures. I. Derivatives of phosphoric acid; II. Derivatives of carboxylic acid.
The Type I compounds are derivatives of phosphoric acid and
include:
O
II
nucleoside polyphosphates*
X = —O—P—OR
OH
O
inorganic polyphosphates
X = —O—P—OH
OH
O
acyl phosphates
X = —0—C—R
R
enol phosphates
X = —O—C=CH 2
thiol phosphates
X = —SR
NH
amidine phosphates
X = —NH—C—NR 2
The Type II compounds are derivatives of carboxylic acids and include:
acyl thioesters
X' = —SR
/\
acyl imidazoles
X' = —N
N
1
1
* We have omitted from the discussion the nucleotide coenzymes such as DPN,
TPN, flavin adenine dinucleotide, coenzyme A, and uridine diphosphate glucose.
These compounds contain an "energy-rich" pyrophosphate linkage which is not
available for energy-requiring reactions. Ammonium and sulfonium compounds such
as betaine, the thetins, and S-adenosylmethionine (all of which function as methyl
group donors) might also be considered "energy-rich" compounds but are not
anhydrides and, hence, have been omitted from this discussion.