Cofactor Dependent Cooperation Between Dimeric hSULT1B1 Subunits

Abstract

The cytosolic sulfotransferases (SULTs) catalyze the transformation of drugs and steroids into sulfate esters, generally “flagging” the molecules for excretion from the cell and body. Despite the variable substrate specificities between hSULT isoforms, all isoforms share a conserved specificity for the sulfonate donor, 3′‐phosphoadenosine 5′‐phosphosulfate (PAPS). Interactions involving the cofactor's 3′‐phosphate ensure the SULTs are not vulnerable to inhibition by other ATP analogues in the cell. However, with this gain in specificity, the SULTs sacrifice catalytic efficiency due to the resulting high affinity for the reaction byproduct, 3′, 5′‐diphosphoadenosine (PAP). We hypothesize the SULTs counteract susceptibility to PAP inhibition by communicating in a cooperative manner between dimer subunits, the result of which is a half‐site reaction mechanism. Molecular dynamic simulations (MDS) of hSULT1B1 dimers indicate the binding of PAP(S) to a single dimer subunit elicits stabilizing effects on its dimeric partner. Coupled with in vitro data showing two distinct PAPS binding affinities for the hSULT1B1 dimer, these data strongly suggest cofactor binding stimulates communication between dimer subunits, altering the cofactor binding affinity of the dimer partner. Further results indicate the presence of PAP actually promotes the binding of PAPS to the SULT dimer, triggering a structural shift that favors the release of PAP from the dimer subunit. Together, the data imply SULT half‐site reactivity results from an oscillating mechanism that promotes PAPS binding and PAP release across the SULT dimerization domain. This oscillatory cycle could be crucial in maintaining SULT catalytic efficiency under physiological PAPS concentrations, which vary throughout the body.