Loss of a conserved salt bridge in bacterial glycosyl hydrolase BgIM-G1 improves substrate binding in temperate environments

Dipali Mhaindarkar,Raphael Gasper,Natalie Lupilov,Eckhard Hofmann,Lars I Leichert

doi:10.1038/s42003-018-0167-7

Dipali Mhaindarkar, Raphael Gasper + Show 3 more

Open Access

https://doi.org/10.1038/s42003-018-0167-7

Copy DOI

Journal: Communications Biology	Publication Date: Oct 17, 2018
Citations: 12	License type: open-access

Affiliation: Ruhr University Bochum

Abstract

Salt bridges are the strongest electrostatic interactions in proteins. They substantially contribute to a protein’s structural stability. Thus, mutations of salt bridges are typically selected against. Here, we report on the evolutionary loss of a highly conserved salt bridge in the GH1 family glycosyl hydrolase BglM-G1. BglM-G1’s gene was found in the bacterial metagenome of a temperate, seasonally cold marine habitat. In BglM-G1, arginine 75 is replaced by a histidine. While fully retaining β-glucosidase activity, BglM-G1 is less heat stable than an H75R variant, in which the salt bridge was artificially re-introduced. However, the Km toward its substrates was lower in wild type, leading to an overall higher catalytic efficiency. Our results indicate that this loss of the salt bridge leads to higher flexibility in BglM-G1’s active site, trading structural stability at high temperatures, a trait not needed in a temperate, seasonally cold habitat, for a more effective catalytic activity.

Highlights

Salt bridges are the strongest electrostatic interactions in proteins
Our data indicate that the natural mutation to histidine at position 75 leads to the loss of an extensive electrostatic network, including a salt bridge in the active site region and implies that evolution of BglM-G1 traded a decrease in thermal stability for improved catalytic efficiency
Among the 132 glycoside hydrolase family 1 (GH1) family members in this dataset, we found only two sequences which featured a substitution of the invariant arginine 75 to a histidine

Summary

Introduction

Salt bridges are the strongest electrostatic interactions in proteins. They substantially contribute to a protein’s structural stability. Our results indicate that this loss of the salt bridge leads to higher flexibility in BglM-G1’s active site, trading structural stability at high temperatures, a trait not needed in a temperate, seasonally cold habitat, for a more effective catalytic activity. The otherwise invariant arginine at position 75 changed to a histidine in this protein, while the other 36 amino acids deemed necessary for the function and structural integrity of members of this protein family remained unchanged This mutation decreased stability at high temperatures but increased catalytic efficiency toward its substrates. Our data indicate that the natural mutation to histidine at position 75 leads to the loss of an extensive electrostatic network, including a salt bridge in the active site region and implies that evolution of BglM-G1 traded a decrease in thermal stability for improved catalytic efficiency. No activity against any of the tested substrates that contained α-glycosidic bonds was detected

Methods

Results

Conclusion