Structure of YMR099cp A. Stereo view ribbon representation of the YMR099cp structure coloured from blue (Nter) to red (Cter). B. Surface mapping of residue conservation of YMR099c. Colouring is from purple (highly conserved) to grey (low conservation). A glucose molecule positioned by superimposition of L. lactis galactose mutarotase-glucose complex is shown in sticks in the putative active site pocket. C. Superimposition of the L. lactis galactose mutarotase (Glu304Gln mutant) (yellow) onto YMR099cp active site (coloured as in Fig. 1B). A sulphate bound to YMR099cp and glucose bound to galactose mutarotase are shown in sticks. For clarity, only YMR099cp residues are labelled.

Despite the generation of a tremendous amount of sequence information over the last decade, more than 40% of well-characterized enzymatic functions still lack associated protein sequences. Assigning protein sequences to documented biochemical functions constitutes an interesting challenge. We illustrate here that structural genomics may be a pertinent approach in addressing these questions. We present the crystal structure of the S. cerevisiae YMR099cp, a protein of unknown function. YMR099cp adopts the same fold as galactose mutarotase and shares the same catalytic machinery necessary for the interconversion of the and anomers of galactose. The structure revealed the presence in the active site of a sulfate ion attached by an "Arginine clamp" made by the side chain from two strictly conserved arginine residues. This sulfate is ideally positioned to mimic the phosphate group of a hexose-6-phosphate. We have subsequently successfully demonstrated that YMR099cp is a hexose-6-phosphate mutarotase with broad substrate specificity. We solved high resolution structures of some substrate enzyme complexes further confirming our functional hypothesis. The metabolic role of a hexose-6-phosphate mutarotase discussed. This work illustrates that structural information has been crucial to assign YMR099cp to the orphan EC activity: hexose phosphate mutarotase.