Halophiles and their enzymes: negativity put to good use

Abstract

Halophilic microorganisms possess stable enzymes that function in very high salinity, an extreme condition that leads to denaturation, aggregation, and precipitation of most other proteins. Genomic and structural analyses have established that the enzymes of halophilic Archaea and many halophilic Bacteria are negatively charged due to an excess of acidic over basic residues, and altered hydrophobicity, which enhance solubility and promote function in low water activity conditions. Here, we provide an update on recent bioinformatic analysis of predicted halophilic proteomes as well as experimental molecular studies on individual halophilic enzymes. Recent efforts on discovery and utilization of halophiles and their enzymes for biotechnology, including biofuel applications are also considered.

Figure 1. Distribution of protein isoelectric points predicted from genome sequences

Percent total protein is plotted versus pI in 0.1 increments for Halorubrum lacusprofundi (red), Acetohalobium arabaticum (blue), Debaryomyces hansenii (black line), and Escherichia coli (black dots).

Figure 2. Comparison of surface acidic and basic amino acids in β-galactosidases

A model structure is shown for the enzyme from Halorubrum lacusprofundi (A) and crystal structure for that of Thermus thermophilus (B), with backbone in white in ribbon form and surface acidic (red) and basic (blue) groups shown using space-filling models. The net surface charges are −65 (A) and −4 (B).