rucial to characterizing a protein's function. While primary amino acid sequence can be used to predict secondary motifs, the sequence alone yields limited information. Tryptophan synthase β (TRPS β), for example, has only 19% amino acid sequence similarity to OASS, yet all the secondary structural elements in OASS are found in TRPS β in equivalent order and spatial arrangement (Fig. 2). The structural equivalence reflects similar function, for both are PLP-dependent enzymes that catalyze β-replacement reactions in amino acid biosynthesis; TRPS β converts L-serine to L-tryptophan.
             OASS is a homodimeric protein, whereby each single chain monomer is composed of two domains, the N-terminus and the C-terminus. Both domains have a central β-sheet surrounded by several alpha helixes (Fig. 3 and Fig 4). The N and C domains together participate in the homodimeric interaction between the two identical fully folded OASS chains, consisting of 1354 Å2 buried per monomer. Both polar
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