Chapter 14
The Influence of Fluoro Substituents on the Reactivity of Carboxylic Acids, Amides, and Peptides in Enzyme-Catalyzed Reactions 1
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James K. Coward , John J . McGuire , and John Galivan
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Departments of Medicinal Chemistry and Chemistry, University of Michigan, Ann Arbor, MI 48109 Department of Experimental Therapeutics, Roswell Park Memorial Institute, Buffalo, NY 14263 Wadsworth Center for Laboratories and Research, New York State Department of Health, Albany, NY 12201
Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: February 22, 1991 | doi: 10.1021/bk-1991-0456.ch014
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Two fluorinated analogs of glutamic acid have been studied for their ability to affect the biosynthesis and hydrolysis of folyl- and antifolylpoly-γ-glutamates. Polyglutamates are the predominant intracellular forms involved in folate-mediated one-carbon biochemistry and in the action of clinically useful anticancer agents such as methotrexate. Placement of fluorine atoms at either the 3- or 4- position of glutamate leads to dramatic changes in the rates of the enzymecatalyzed biosynthesis and hydrolysis of these polyglutamates. Studies on the biosynthetic reaction, catalyzed by folypoly-γ-glutamate synthetase, revealed that DL-threo-4-fluoroglutamate is a chain terminating substrate whereas DL-3,3-difluoroglutamate stimulates chain elongation. Synthesis of fluorine-containing oligo-γ-glutamates and investigation of their hydrolysis, catalyzed by γ-glutamyl hydrolase, revealed that the fluorine-containing glutamyl peptides are hydrolyzed very slowly when fluorine is α- or β- to the scissile peptide bond. The vitamin folic acid is, like most other vitamins, an artifact of isolation. In contrast to the fully oxidized pteridine coupled to p-aminobenzoylglutamate found in folic acid, intracellular folates are predominantly γ-glutamyl peptide "conjugates" containing reduced and substituted pteridine moieties (Scheme I). The biochemistry of reduced and substituted pteroyl monoglutamates has been studied extensively (i). However, only recently have similar detailed studies on the biochemistry and pharmacology of folate polyglutamates been carried out (2,5). These studies have led to the hypothesis that intracellular folylpoly-y-glutamate flux involves a balance between synthesis, catalyzed by the enzyme folylpoly-y-glutamate synthetase (FPGS, EC 6.3.2.17), and hydrolysis, catalyzed by γ-glutamyl hydrolase (γ-GH, E C 3.4.22.12). The biochemistry of this flux is depicted in Scheme I. Our interest in the effect of fluorine substitution on polyglutamate metabolism arosefromthe observation that the OL-threo diastereomer of 4-fluoroglutamate
0097-6156/91/0456-0196$06.00/0 © 1991 American Chemical Society
Welch; Selective Fluorination in Organic and Bioorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
14. COWARD ET AL
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Influence of FUioro Substituents
Scheme I Ο ΟΟ,Η C0 H
CNHCH
2
(ρΗ
H N(I:HCH CH CO H 2
2
2
2
2
