Colitose is a 3,6-dideoxysugar found in cell surface oligosaccharides of many Gram-negative bacteria. Its synthesis from GDP-mannose follows a pathway unlike that for any other dideoxysugar. Of particular interest are the final two enzymes in the pathway, ColD and ColC. Here we report the crystal structure of GDP-4-keto-6-deoxy-mannose-3-dehydrase (ColD) to 1.9 Å resolution. The quaternary structure is a dimer in which two loops from one subunit form part of the active site of the other subunit. The electron density clearly shows the presence of a PLP-glutamate ketimine intermediate, which occurs prior to the formation of PMP. Interestingly, ColD lacks the conserved lysine residue almost always observed in PLP-dependant enzymes. Instead, the structure shows that the histine in place of the lysine is within hydrogen bonding distance to a carboxyl group of the glutamate ketimine. Coupled with future kinetic experiments, this structure will help elucidate the catalytic mechanism of ColD. We also report progress towards the structural analyses of two other enzymes in the pathway, ColC and ColE.