Inhibition of serine and proline racemases by substrate-product analogues

D-Amino acids can play important roles as specific biosynthetic building blocks required by organisms or act as regulatory molecules. Consequently, amino acid racemases that catalyze the formation of D-amino acids are potential therapeutic targets. Serine racemase catalyzes the reversible formation of D-serine (a modulator of neurotransmission) from L-serine, while proline racemase (an essential enzymatic and mitogenic protein in trypanosomes) catalyzes the reversible conversion of L-proline to D-proline. We show the substrate-product analogue alpha-(hydroxymethyl) serine is a modest, linear mixed-type inhibitor of serine racemase from Schizosaccharomyces pombe (K-i = 167 +/- 21 mM, K-i ' = 661 +/- 81 mM, cf. K-m = 19 +/- 2 mM). The bicyclic substrate-product analogue of proline, 7-azabicyclo[2.2.1]heptan-7-ium-1-carboxylate is a weak inhibitor of proline racemase from Clostridium sticklandii, giving only 29% inhibition at 142.5 mM. However, the more flexible bicyclic substrate-product analogue tetrahydro-1H- pyrrolizine-7a(5H)-carboxylate is a noncompetitive inhibitor of proline racemase from C. sticklandii (K-i = 111 +/- 15 mM, cf. K-m = 5.7 +/- 0.5 mM). These results suggest that substrate-product analogue inhibitors of racemases may only be effective when the active site is capacious and/or plastic, or when the inhibitor is sufficiently flexible. (c) 2013 Elsevier Ltd. All rights reserved.