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108 Chapter 5
Adenosine 5 0 -triphosphate is a thermodynamically unstable molecule, since
the hydrolysis of its phosphoric anhydride linkages is thermodynamically highly
favourable, hence its designation as a ‘high-energy’ source of phosphate. Yet
ATP is reasonably stable in aqueous solution – why The explanation is that
although the hydrolysis of ATP is thermodynamically favourable it is kinetically
unfavourable, particularly the hydrolysis of the phosphodiester groups.
The triphosphate group of ATP can be cleaved at a number of diVerent
points, shown in Figure 5.34, leading to the transfer of either a phosphoryl
group, a pyrophosphoryl group or an adenosine phosphoryl group. The most
common transfer is of a single phosphoryl unit, leaving behind adenosine
5 0 -diphosphate (ADP). This process is used by kinase enzymes in the phosphorylation
of alcohols, and is used by a number of ligase enzymes to activate
carboxyl groups as acyl phosphate intermediates (see Figure 5.35).
NH 2
O −
O −
O −
N
N
− O
P
O
O
P
O
O
P
O
O
O
N
N
R-OPO 3
2− + ADP
R-OPP + AMP
HO
OH
R-OAMP + PP i
Figure 5.34 Structure of ATP and three common modes of phosphoryl transfer.
CO 2
−
CO 2
−
shikimate kinase
HO
OH
OH
ATP
ADP
2− O 3 PO
OH
OH
peptide
O
N
H
OH
O
NH-peptide
Ser/ Thr protein
kinase
ATP
ADP
peptide
O
N
H
OPO 3
2−
O
NH-peptide
O
acetate kinase
O
phosphotransacetylase
O
O −
OPO 2− CoASH
3
ATP ADP
Figure 5.35 Phosphorylation of alcohols and carboxylates by ATP.
SCoA