Cofactor binding as protein stability determinant in psychrophilic SHMT

Cofactor binding as protein stability determinant in psychrophilic SHMT Sebastiana Angelaccio, Angela di Bello and Stefano Pascarella Department of Biochemical Sciences “A. Rossi-Fanelli”, “La Sapienza”-University of Rome sebastiana.angelaccio@uniroma1.it Serine hydroxymethyltransferase (SHMT) is a pyridoxal-5’-phosphate (PLP)-dependent enzyme, which belongs to the fold type I superfamily and catalyzes the reversible conversion of L-serine and tetrahydropteroylglutamate (H4PteGlu) to glycine and 5,10-methylenetetrahydropteroylglutamate (5,10-CH2-H4PteGlu). This enzyme represents a good model for analyzing the intricate relationships between activity and stability, since it is ubiquitous in nature and is structurally conserved because of its critical metabolic role. The SHMT from the psychrophilic bacterium Psychromonas ingrahamii (piSHMT) was recently purified, characterized and its apo form crystallized. This enzyme displays interesting catalytic and stability properties similar to those often found in psychrophilic enzymes. The piSHMT is a more efficient biocatalyst compared to E. coli SHMT (eSHMT), in particular for the side reactions where many substrates, tipically -hydroxy--amino acids, represent important compounds in pharmaceuticals, agrochemicals and food additives. piSHMT activity is heat labile and the apparent melting temperature of the protein is 62 °C, lower than of the eSHMT. Interestingly, the difference of the apparent Tm values between the apoenzyme and the holoenzyme is about 20 degrees. Comparative analysis between the structure of the apo form and the homology modelled holo form suggests possible explanations of the strong cofactor stabilization effect on the structure of the piSHMT.