Bacterial Glutamate Racemase has High Sequence Similarity with Myoglobins and Forms an Equimolar Inactive Complex with Hemin

Glutamate racemase (EC 5.1.1.3), an enzyme of microbial origin, shows significant sequence homology with mammalian myoglobins, in particular in the regions corresponding to the E and F helices, which constitute the heme binding pocket of myoglobins. Glutamate racemase binds tightly an equimolar amount of hemin, leading to loss of racemase activity. Although this enzyme shows homology with aspartate racemase, the latter does not bind hemin. The glutamate racemase gene of Pediococcus pentosaceus has a 795-nt open reading frame and encodes 265-amino acid residues, which form a monomeric protein (Mr29,000). Neither racemase has cofactors, but they contain essential cysteine residues [Yohda, M., Okada, H. \& Kumagai, H. (1991) Biochim. Biophys. Acta 1089, 234-240].