Biogenesis of Hydrogen Sulfide and Thioethers by Cystathionine Beta-Synthase

Aims: The transsulfuration pathway enzymes cystathionine beta-synthase (CBS) and cystathionine gammalyase
are thought to be the major source of hydrogen sulfide (H2S). In this study, we assessed the role of CBS in
H2S biogenesis.
Results: We show that despite discouraging enzyme kinetics of alternative H2S-producing reactions utilizing
cysteine compared with the canonical condensation of serine and homocysteine, our simulations of substrate
competitions at biologically relevant conditions suggest that cysteine is able to partially compete with serine on
CBS, thus leading to generation of appreciable amounts of H2S. The leading H2S-producing reaction is condensation
of cysteine with homocysteine, while cysteine desulfuration plays a dominant role when cysteine is
more abundant than serine and homocysteine is limited. We found that the serine-to-cysteine ratio is the main
determinant of CBS H2S productivity. Abundance of cysteine over serine, for example, in plasma, allowed for
up to 43% of CBS activity being responsible for H2S production, while excess of serine typical for intracellular
levels effectively limited such activity to less than 1.5%. CBS also produced lanthionine from serine and
cysteine and a third of lanthionine coming from condensation of two cysteines contributed to the H2S pool.
Innovation: Our study characterizes the H2S-producing potential of CBS under biologically relevant conditions
and highlights the serine-to-cysteine ratio as the main determinant of H2S production by CBS in vivo.
Conclusion: Our data clarify the function of CBS in H2S biogenesis and the role of thioethers as surrogate H2S
markers.