生化学と分析生化学

概要

Stability of Halophilic Proteins in Hyper Saline Brine: [2Fe-2S] Ferredoxin as a Paradigm

Amal K Bandyopadhyay

Apart from normal or mesophilic environment, organisms are found in extreme of salinity, and other hostile environments in the earth. Extreme halophiles thrive as pure culture in their natural environments of saturated salt as other microbes can’t venture to grow. Over the evolution, these microorganism grew up with specialized transport-devices to solve the problem of osmoregulation. As a consequence whole of their biochemical machinery started functioning in this highly saline brine situation that mesophile cannot withstand. Intensive researches are thus carried out over the last fifty years to understand salt dependent properties of these proteins and enzymes. Ferredoxin is a small soluble protein that functions as electron carrier in decarboxylation reactions in cytoplasm in conjunction with an oxidoreductase. Two of its representatives from Halobacterium marismortui (HmFd) and Halobacterium salinarum (HsFd) are extensively studied. Atomic structures of HmFd and HsFd reveal that halo adaptation is largely mediated by a hyper acidic inserted domain of some 24 residues long at N-terminus region. By designed kinetics and thermodynamics experiments it was demonstrated that HsFd indeed adapted in high salt and requires ≥1.5M salt to retain its overall structural integrity. While non-specific electrostatic effect is operative at ≤0.25M salt, higher salt promote salt-bridge and hydrophobic stability. At intermediate salt where Hofmeister effects of specific ion interactions are operative, HsFd forms a hydrophobic collapsed intermediate whose structural properties differs from its native state in saturated salts. Thus intuitively, HsFd in its native state seems to entertain a post Hofmeister like effect in that wide modulation of tertiary interactions might occur.