Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

112 Chapter 5
CH 2 OH
O
RO
HO
OH
O
H
O
H H
O
O−
OR'
O
CH 2 OH
+
O
RO
HO
OH
H
O
O
O
H
OR'
H
O
O
O −
O
CH 2 OH
O R'OH
RO
HO
OH
OH
H
O O
HOH 2 C
HO HO
Figure 5.39 Mechanism of an inverting glycosidase.
O
OH
OPO 3
2−
HOH 2 C
HO HO
O
OH
O
Enz
O −
P O −
O −
fructose-OH
HOH 2 C
HO
HO
Figure 5.40 Sucrose phosphorylase. Black dot indicates the position of 14 C label.
O
OH
O-fructose
derivative. For example, the disaccharide sucrose that we know as table sugar is
synthesised by the enzyme sucrose phosphorylase from glucose-1-phosphate
and fructose, as shown in Figure 5.40.
The overall reaction involves displacement of phosphate by fructose with
retention of stereochemistry at the glycosyl centre, which by analogy with the
above glycosidase enzymes would suggest that this is a double displacement reaction
involving a covalent intermediate. This has been conWrmed by incubation of
the enzyme with sucrose labelled with 14 C at C-1 of the glucose residue, whereupon
in the absence of inorganic phosphate the 14 C label becomes covalently attached to
the enzyme. Addition of phosphate converts the 14 C–glucosyl–enzyme intermediate
into 14 C-glucose-1-phosphate. Other glycosyl transferases utilise glycosyl
diphospho-uridine activated derivatives, which upon glycosylation release uridine
diphosphate (UDP). Theseglycosyl transfer reactions usually occur withinversion
of stereochemistry, via a one-step displacement reaction.
5.8 Methyl group transfer: use of S-adenosyl methionine and
tetrahydrofolate coenzymes for one-carbon transfers
The Wnal example of group transfer reaction that we shall meet is that of methyl
group transfer, and more generally the transfer of one-carbon units. Many
biologically important molecules contain methyl (2CH 3 ) groups attached to
oxygen, nitrogen and carbon substituents which have arisen by transfer of a