High catalytic rate of the cold-active Vibrio alkaline phosphatase depends on a hydrogen bonding network involving a large interface loop

The role of surface loops in mediating communication through residue networks is still a relatively poorly understood part of cold-adaptation of enzymes, especially in terms of their quaternary interactions. Alkaline phosphatase (AP) from the psychrophilic marine bacterium Vibrio splendidus (VAP) is characterized by an analogous large surface loop in each monomer, referred to as the large-loop, that hovers over the active site of the other monomer. It presumably has a role in VAP high catalytic efficiency that accompanies extremely low thermal stability. We designed several different mutagenic variants of VAP with the aim of removing inter-subunit interactions at the dimer interface. Breaking the inter-subunit contacts from one residue in particular (Arg336) caused diminished temperature stability of the catalytically potent conformation and a drop in catalytic rate by a half. The relative B-factors of the R336L crystal structure, compared to the wild-type, confirmed increased surface flexibility in a loop on the opposite monomer, but not in the large-loop. Contrary to expectations, the observed reduction in stability with an expected increase in dynamic mobility resulted in reduced catalytic rate. This contradicts common theories explaining high catalytic rates of enzyme from cold-adapted organisms as being due to reduced internal cohesion bringing increased dynamic flexibility to catalytic groups. The large-loop increases the area of the interface between the subunits through its contacts and may facilitate an alternating structural cycle demanded by a half-of-sites reaction mechanism through stronger ties, as the dimer oscillates between high affinity (active) or low phosphoryl-group affinity (inactive).