H-loop Histidine Catalyzes ATP Hydrolysis in the E. coli ABC-Transporter HlyB

TL;DR

This study reveals that the conserved H-loop Histidine H662 in E. coli HlyB acts as a proton relay, facilitating ATP hydrolysis by serving as a general acid and base, which clarifies its mechanistic role in ABC transporter function.

Contribution

The paper provides a detailed mechanistic insight into how H-loop Histidine H662 catalyzes ATP hydrolysis in E. coli HlyB using quantum mechanical and molecular mechanical calculations.

Findings

H662 acts as a proton relay in ATP hydrolysis.

Without H662, proton transfer barriers are significantly higher.

H662 functions as a chemical linchpin in the catalytic process.

Abstract

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters form a family of molecular motor proteins that couple ATP hydrolysis to substrate translocation across cell membranes. Each nucleotide binding domain of ABC-transporters contains a highly conserved H-loop Histidine residue, whose precise mechanistic role to motor functions has remained elusive. By using combined quantum mechanical and molecular mechanical calculations, we showed that the conserved H-loop residue H662 in E. coli HlyB, a bacterial ABC transporter, can act first as a general acid and then as a general base to facilitate proton transfers in ATP hydrolysis. Without the assistance of H662, direct proton transfer from the lytic water to ATP results in a greatly elevated barrier height. Our findings suggest that the essential function of the H-loop residue H662 is to provide a "chemical linchpin" that shuttles protons between reactants through a relay mechanism, thereby catalyzing ATP hydrolysis in HlyB.