Q68. Water balance in extreme halophiles such as Halobacterium is maintained
by cell surface glycoproteins consisting of
(A) glycine and lysine.
(B) lysine and histidine.
(C) glycine.
(D) aspartate and glutamate.

Water balance in extreme halophiles such as Halobacterium is maintained by highly acidic cell-surface glycoproteins that are rich in aspartate and glutamate, so the correct option is (D). These negatively charged residues help stabilize proteins and the cell envelope in saturated salt environments and support osmotic balance by working together with high intracellular KCl.​​

Introduction

Water balance in extreme halophiles such as Halobacterium depends on specialized adaptations, including an acidic proteome and negatively charged cell surface glycoproteins that enable survival in hypersaline environments. These archaeal cells accumulate high intracellular KCl and use aspartate- and glutamate-rich S-layer glycoproteins to maintain structural stability and prevent protein precipitation at very high external salt concentrations.​

Correct Option Explained: (D) Aspartate and Glutamate

• Extreme halophiles in the genus Halobacterium live in saturated brines and must prevent water loss and protein denaturation caused by high NaCl.​

• Their proteome, including the S-layer cell surface glycoprotein, is enriched in acidic amino acids, primarily aspartate and glutamate, which provide a strong net negative charge.​

• This excess negative charge interacts favorably with the surrounding cations (Na⁺, K⁺), keeps proteins soluble, and stabilizes the cell envelope in hypersaline conditions, thus contributing to water balance and structural integrity.​

• Genomic and proteomic analyses of Halobacterium species show a marked overrepresentation of aspartic acid and glutamic acid codons and a deficit of basic amino acids such as lysine and arginine, underlining the selective advantage of an acidic surface.​

Therefore, cell surface glycoproteins consisting of aspartate and glutamate correctly describe the composition relevant to water balance and salt adaptation in extreme halophiles, making option (D) correct.​

Why Other Options Are Incorrect

Option (A): Glycine and Lysine

• Glycine is small and uncharged at physiological pH, while lysine is a basic, positively charged amino acid.​

• Halophilic adaptations in Halobacterium are characterized by a deficit of basic residues (like lysine and arginine), not an enrichment, because high positive charge would destabilize proteins in high ionic strength and favor aggregation or precipitation.​

• Although glycine can appear in turns and flexible regions, it does not by itself confer the strong negative surface charge needed to maintain water balance in hypersaline environments.​

• Thus, cell surface glycoproteins dominated by glycine and lysine would not provide the characteristic acidic, salt-stable properties seen in extreme halophiles, so option (A) is incorrect.​

Option (B): Lysine and Histidine

• Both lysine and histidine are basic amino acids; lysine is positively charged, and histidine can carry a positive charge near physiological pH.​

• Halobacterium proteins show a reduction in basic amino acids and a strong shift toward acidic residues (aspartate and glutamate), which is the opposite of what a lysine- and histidine-rich surface would imply.​

• A cell surface glycoprotein dominated by lysine and histidine would carry net positive charge, which is not the observed strategy for halophilic protein stability in hypersaline media.​

• Consequently, option (B) contradicts established halophilic proteome characteristics and is incorrect.​

Option (C): Glycine

• Glycine alone is neutral and does not provide strong ionic interactions with environmental cations, so it cannot generate the highly negative surface that stabilizes proteins in saturated salt.​

• While glycine may occur in halophilic proteins, the key signature of Halobacterium’s adaptation is not high glycine content, but rather a global shift toward acidic residues.​

• Acidic residues such as aspartate and glutamate are essential for maintaining solubility and activity in high-salt conditions; glycine alone cannot substitute for this charged surface.​

• Therefore, option (C), which focuses solely on glycine, fails to capture the crucial acidic nature of halophilic glycoproteins and is incorrect.​

Key Takeaways for Exams

• Extreme halophiles like Halobacterium maintain water balance by:

  • Accumulating high intracellular KCl as a compatible solute.​

  • Encoding an acidic proteome, including S-layer cell surface glycoproteins enriched in aspartate and glutamate, which remain soluble and active in high salt.​

• For MCQs, any mention of acidic amino acids (aspartate, glutamate) on the protein surface should immediately suggest halophilic adaptation and correct water balance strategy in hypersaline conditions, aligning with option (D) here.​