Glutamate Carboxypeptidase II as a Therapeutic Target - Proteinases as Drug Targets - page 72

Scheme 3.1

Simplified GCPII reaction cycle. Important steps/structures during the
hydrolysis of NAAG by human GCPII.

pteroate-poly-g-glutamates. Data from several laboratories (obtained by dif-
ferent experimental protocols) show that GCPII anity towards NAAG is in
the high-nanomolar to low-micromolar range (K M ¼ 75 nM to 1 mM), with the
turnover number (k cat ) ranging from 0.6 s -1 to 5 s -1 . 2,69,71,73-76 GCPII cleaves
polyglutamylated folates with K M values in the range 60-560 nM (k cat ¼ 0.9-2.5
s -1 ) with an apparent preference for folates with more than two g-linked glu-
tamates (Navratil et al., unpublished).
Efforts have also been made to identify novel GCPII substrates. 65,69,72 Glu-
tamate, aspartate, and related dicarboxylic residues are required at the P1
position for ecient substrate hydrolysis. When the P1 glutamate is g-linked (or
b-linked in the case of aspartate) to the C-terminal residue, additional moieties
can be attached to the amino group, allowing the enzyme to process longer
peptides (e.g. folate-g-glutamate). Interestingly, structural and biochemical data
suggest that D-stereoisomers are preferred by GCPII in this instance. The
preferred C-terminal (or P1 0 ) moiety is glutamate, but several other natural
(methionine, alanine) and non-natural (amino acids with aliphatic side chains;
Plechanovova et al., unpublished) amino acids are well tolerated. 65,69,70

3.6.2 Structure
The first experimental structure of GCPII confirmed its expected structural
similarity to transferrin receptor, defined the nature of the dimerization