Atheromatous plaque is one of the most common cardiovascular-related diseases. Reports show a connection between its development and the levels of homocysteine. In pathological states high levels of homocysteine in the organism can be caused by the malfunction of the methionine synthase pathway. Bacterial methionine synthase (MetH) is a homologue of the human methionine syntase (MS). In this study we aimed to investigate the functional relations between MetH and its cofactor--cobalamine--under stress conditions. We have demonstrated that heat shock proteins (Hsp 70/100 system or HtpG) can protect MetH activity under stress conditions. Moreover, in the presence of cobalamine they can restore the activity of partially denatured methionine synthase.