Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymatic Transformations of Amino Acids 225
(4) The enzyme threonine synthase catalyses the conversion of l-homoserine
phosphate to l-threonine using PLP as a cofactor. Write a mechanism for
the enzymatic reaction that is consistent with the following observations:
(i) carrying out the enzymatic reaction in H 18
2
O leads to the incorporation of
18 O into the b-hydroxyl group of l-threonine; (ii) incubation of 3S-[3- 3 H]-
homoserine phosphate with the enzyme leads to loss of the 3 H label in the
product, whilst incubation of the 3R-[3- 3 H] isomer leads to retention of
the 3 H label.
2− O 3 PO
H
CO −
2
NH 3
+
threonine
synthase
PLP
OH
H
CO −
2
NH +
3
+ PO 4
3−
(5) Kynureninase is a PLP-dependent enzyme that catalyses the hydrolytic
cleavage of l-kynurenine to give anthranilic acid and l-alanine. Given
that the enzyme catalyses the exchange of the C a hydrogen with solvent,
suggest a mechanism for this enzyme. A sample of stereospeciWcally labelled
l-kynurenine (see below) was incubated with the enzyme in 2 H 2 O, and the
labelled l-alanine product was converted to labelled acetic acid. The
resulting chiral methyl group was found to have the S conWguration.
Deduce whether the enzymatic reaction proceeds with retention or inversion
of conWguration at the labelled centre.
O NH +
3
CO −
CO −
2
2 kynureninase
H NH +
3
+
PLP
NH CO −
2
NH 2 2
L-kynurenine
anthranilic acid
O NH +
3
CO −
2 enzyme
H T
[ 2 H, 3 H]-L-alanine S-[ 2 H, 3 H]-acetate
D
NH 2 O
2