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Enzymatic Hydrolysis and Group Transfer Reactions 109
One example of the use of ATP to activate a carboxylic acid derivative is in
the biosynthesis of acetyl CoA. This is carried out in bacteria by the action of
two enzymes: acetate kinase, which activates acetate as acetyl phosphate using
ATP as a cofactor; and phosphotransacetylase, which transfers the acetyl group
onto coenzyme A (see Figure 5.35). Phosphorylation of serine and tyrosine
residues on the surface of proteins is a very important reaction in cell signalling
pathways. These reactions are catalysed by the serine–threonine and tyrosine–
protein kinases, which also use ATP as coenzyme.
5.7 Enzymatic glycosyl transfer reactions
Carbohydrates fulWl many important roles in biological systems: polysaccharides
are important structural components of plant and bacterial cell walls, and
constitute an important part of the human diet; mammalian polysaccharides,
such as glycogen, are used as short-term cellular energy stores; whilst the
attachment of carbohydrates to mammalian glycoproteins has an important
role to play in cell–cell recognition processes.
Hydrolysis of polysaccharides can be achieved in the laboratory using acid
hydrolysis, as shown in Figure 5.36, since the glycosidic linkage is an acetal
oxonium ion
CH 2 OH
O
RO
HO
OH
OR'
CH 2 OH H
H + /H 2 O O +
RO
O
HO
R'
OH
CH 2 OH
O +
RO
HO
OH
CH 2 OH
O
RO
HO
OH
OH
CH 2 OH
O
RO
HO
OH
OH 2
+
CH 2 OH
O
RO
HO
OH
OR'
b-glycosidase
CH 2 OH
O
RO
HO
OH
OH
+ R'OH
CH 2 OH
O
RO
HO
OH
OR'
a-glycosidase
CH 2 OH
O
RO
HO
OH
OH
Figure 5.36 Non-enzymatic and enzymatic glycoside hydrolysis.
+ R'OH