X-RAY CRYSTALLOGRAPHIC STUDIES OF ENZYMES FOUND IN THE BIOSYNTHETIC PATHWAYS OF THIAMIN AND CYSTEINE, AND THE PURINE NUCLEOSIDE PHOSPHORYLASE SALVAGE PATHWAY

This work encompasses the structure of four different proteins, all of which were solved by X-ray crystallography. The structure of thiazole synthase (Thi4) in Saccharomyces cerevisiae involves the catabolism of another cofactor, nicotinamide adenine dinucleotide (NAD), through a series of catalytic steps that are now understood in light of several structural and biochemical studies. The product of the Thi4 reaction is an adenylated thiazole carboxylate, which is an unprecedented molecule found in primary metabolism. Thi4 also has a flavin adenine dinucleotide (FAD) binding fold, yet does not bind FAD. This implies that an ancient predecessor of Thi4 was able to bind FAD and evolved to bind NAD as a substrate without a change in topology. The biosynthesis of cysteine in Mycobacterium tuberculosis involves a sulfur transfer step that uses a thiocarboxylated protein as the sulfur source. The protein complex structure of the cysteine synthase CysM and sulfur carrier protein CysO gives insight into how this reaction occurs. CysO has a ?-grasp fold and is structurally similar to ubiquitin. The enzyme CysM binds to CysO in a pyridoxal 5?-phosphate dependent reaction with O-acetylserine (OAS) as the substrate to generate a cysteine residue on the C-terminus of CysO. CysM binds CysO through a loop that is ordered when CysO is bound and disordered when CysO is absent. Although there are other examples of proteins that bind to a sulfur carrier protein with a ?-grasp fold, they all differ in their composition of surface electrostatics responsible for protein-protein interactions. The structure of the enzyme purine nucleoside phosphorylase (PNP) from the thermophile Thermotoga maritima is an example of a protein that has the same overall fold as its mesophilic counterpart bovine PNP, yet is stable at considerably higher temperatures. The differences in thermal stability are seen in the fewer number of glycine and greater number of proline residues in TmPNP over bPNP. Also, the protein-protein interactions between monomers in the biological unit of TmPNP contain a greater number of hydrophobic residues, thereby leading tighter packing and greater thermostability