Unique evolution of Bivalvia arginine kinases

Area of Science:

• Biochemistry
• Molecular Evolution
• Enzymology

Background:

• Certain bivalve clams possess unique 80 kDa arginine kinases (AKs) with a two-domain structure, differing from typical 40 kDa AKs.
• These bivalve AKs lack conserved residues Asp(62) and Arg(193), crucial for substrate stabilization in other AKs.
• Despite lacking these residues, bivalve AKs exhibit higher enzyme activity.

Purpose of the Study:

• To investigate the molecular basis for the higher enzyme activity in bivalve AKs.
• To determine the amino acid sequences of AKs from Scapharca broughtonii and Crassostrea gigas.
• To understand the evolutionary divergence of bivalve AKs.

Main Methods:

• cDNA sequencing to derive amino acid sequences of AKs from Scapharca broughtonii and Crassostrea gigas.
• Enzyme activity assays on native and recombinant AKs.
• Comparative analysis of amino acid sequences and enzyme kinetics.

Main Results:

• Asp(62) and Arg(193) are conserved in Scapharca AK but replaced by Asn and Lys in Crassostrea AK.
• Crassostrea AK and recombinant enzymes showed significantly higher activity compared to other molluskan AKs.
• Mutational analysis revealed that Asp(62) and Arg(193) replacements in Scapharca AK did not impair enzyme activity, unlike in typical AKs.

Conclusions:

• Bivalve AKs exhibit unique molecular evolution, diverging phylogenetically from other molluskan AKs.
• The loss of the stabilizing function of residues 62 and 193 in bivalve AKs correlates with increased enzyme activity.
• This suggests a distinct evolutionary strategy in bivalves for enhancing AK function.