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F. M. HUENNEKENS AND H. R. WHITELEY
tion that invertebrates characteristically have phosphoarginine while
vertebrates have phosphocreatine now also requires modification since
a number of phyla (Porifera, Sipunculoidea, Annelida, Echinodermata,
Hemichordata, and Chordata) have been found to contain both compounds.
B. DERIVATIVES OF CARBOXYLIC ACID
1. Acyl Thioesters
a. Preparation and Identification. Acyl thioesters, whose general
structure is given by the type formula
O
R—C—S—CoA
may be considered as quasi-anhydrides, since thiols (pK = 7-9) are
stronger acids than alcohols (pK= 13-15). As anhydrides, these compounds are similar to acyl or enol phosphates, or to pyrophosphates,
and it is not surprising, therefore, to find that they fall into the category
of "energy-rich" materials. From the viewpoint of energy storage, the
most important members of the acyl thioester group are the many acyl
derivatives of Coenzyme A (Fig. 11,A), particularly acetyl-CoA and
succinyl-CoA. Two additional thioesters, the acyl derivatives of glutathione (Fig. 11,B) and lipoic acid (Fig. 11,C), are quantitatively less
important as energy storage forms, and will be discussed only briefly.
The first acyl thioester available in pure form was acetyl-CoA,
originally called "active acetate." This compound was isolated from
yeast by Lynen and co-workers (377) following its earlier implication
as an intermediate in the acetylation of sulfanilamide (378) and choline
(379). The subsequent discovery that CoA was involved as a transient
carrier of many acyl groups in a wide variety of metabolic reactions
(380-382) left little doubt as to the importance of this coenzyme in
intermediary metabolism.
The total synthesis of CoA has been achieved (383, 383a), and
acyl derivatives of CoA are readily synthesized by several methods:
(a) the reaction of an acid anhydride (384) or mixed anhydride (385)
with the thiol; and (b) the ester exchange reaction (386). Enzymatic
syntheses are also commonly employed, as for example, through the
action of specific "activating enzymes" (387), phosphotransacetylase
(188), or CoA transphorase (388) (described below).
Acyl thioesters react with hydroxylamine (184) at neutral pH to
yield the corresponding acyl hydroxamates, which may be separated
and identified by paper chromatography (162) or measured colori-
F. M. HUENNEKENS AND H. R. WHITELEY
tion that invertebrates characteristically have phosphoarginine while
vertebrates have phosphocreatine now also requires modification since
a number of phyla (Porifera, Sipunculoidea, Annelida, Echinodermata,
Hemichordata, and Chordata) have been found to contain both compounds.
B. DERIVATIVES OF CARBOXYLIC ACID
1. Acyl Thioesters
a. Preparation and Identification. Acyl thioesters, whose general
structure is given by the type formula
O
R—C—S—CoA
may be considered as quasi-anhydrides, since thiols (pK = 7-9) are
stronger acids than alcohols (pK= 13-15). As anhydrides, these compounds are similar to acyl or enol phosphates, or to pyrophosphates,
and it is not surprising, therefore, to find that they fall into the category
of "energy-rich" materials. From the viewpoint of energy storage, the
most important members of the acyl thioester group are the many acyl
derivatives of Coenzyme A (Fig. 11,A), particularly acetyl-CoA and
succinyl-CoA. Two additional thioesters, the acyl derivatives of glutathione (Fig. 11,B) and lipoic acid (Fig. 11,C), are quantitatively less
important as energy storage forms, and will be discussed only briefly.
The first acyl thioester available in pure form was acetyl-CoA,
originally called "active acetate." This compound was isolated from
yeast by Lynen and co-workers (377) following its earlier implication
as an intermediate in the acetylation of sulfanilamide (378) and choline
(379). The subsequent discovery that CoA was involved as a transient
carrier of many acyl groups in a wide variety of metabolic reactions
(380-382) left little doubt as to the importance of this coenzyme in
intermediary metabolism.
The total synthesis of CoA has been achieved (383, 383a), and
acyl derivatives of CoA are readily synthesized by several methods:
(a) the reaction of an acid anhydride (384) or mixed anhydride (385)
with the thiol; and (b) the ester exchange reaction (386). Enzymatic
syntheses are also commonly employed, as for example, through the
action of specific "activating enzymes" (387), phosphotransacetylase
(188), or CoA transphorase (388) (described below).
Acyl thioesters react with hydroxylamine (184) at neutral pH to
yield the corresponding acyl hydroxamates, which may be separated
and identified by paper chromatography (162) or measured colori-