Structural stability of cold-adapted serine hydroxymethyltransferase, a tool for ß-hydroxy-α-amino acid biosynthesis

Serine hydroxymethyltransferase (SHMT) is a pyridoxal-5'-phosphate-dependent enzyme that catalyses the reversible conversion of l-serine and tetrahydropteroylglutamate to glycine and 5,10-methylenetetrahydropteroylglutamate. This enzyme represents a good model for analysing the intricate relationship between activity and stability, since it is ubiquitous in nature and well characterized from different organisms. Besides its physiological role, SHMT catalyses the reversible cleavage of several beta-hydroxy amino acids varying in substituent and stereochemistry at C-beta and, for this reason, it represents a good tool for biotechnological applications. SHMT from the psychrophilic bacterium Psychromonas ingrahamii (piSHMT) displays the interesting feature of having high specific activity in the cleavage of beta-hydroxy amino acids at all temperatures. In the present study, we compare the temperature dependence of psychrophilic piSHMT and mesophilic Escherichia coli SHMT (ecSHMT) in catalysing the physiological hydroxymethyltransferase reaction. We also investigate the structural stability of both enzymes by performing equilibrium unfolding experiments. Unexpectedly, our results show that piSHMT is a less efficient catalyst than ecSHMT in the hydroxymethyltransferase activity at all temperatures. Moreover, the two enzymes have comparable structural stability, with piSHMT showing even higher resistance to chemical denaturation by urea and to inactivation by formaldehyde. This unusual structural stability of piSHMT and its high efficiency at low temperatures as catalyst of beta-hydroxy amino acids cleavage make this enzyme an attractive tool for industrial applications.