Protic small molecule bioregulators

Small molecule gases such as nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S) have long been recognized as endogenous signaling molecules with diverse physiological roles. Often described as "gasotransmitters", these molecules complement other small molecule bioregulators (SMBs) that exert biological function across all kingdoms of life. One underappreciated distinction, however, is that many of these molecules - irrespective of whether or not they are gases in their native states outside of biology - exhibit similar molecular signaling potential mediated by protonation-dependent chemical speciation. In this review, we propose the new cross-cutting classification of protic small molecule bioregulators (PSMBs) to describe molecules in which biological function and reactivity are modulated by protonation state. Examples of PSMBs include the canonical gasotransmitter H₂S, emerging gasotransmitters (H₂Se, HCN), small molecule crosstalk species (e.g., SNO^-, SSNO^-, SO₄^2-, ONOO^-, NO₂^-, SCN^-, OCl^-), and other species where protonation state modulation is accessible at physiological pH. Importantly, these species exist in equilibrium between their neutral and anionic forms, with speciation governed by local pH and molecular environment, directly impacting their membrane nucleophilicity, permeability, redox activity, and interaction with metal centers. We describe the evolutionary origins, biosynthesis, and crosstalk of PSMBs, including roles in redox signaling, post-translational modification, and mitochondrial regulation. Reframing these important molecules in a class defined by their protic ability rather than gaseous state does not diminish prior gasotransmitter designations, but rather serves to recognize commonalities in chemical characteristics that drive the unique biological chemistry and regulation of these species.