The switch between two conformations of adenylate kinase

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Adenylate kinase (AK) is a crucial enzyme in cellular energy metabolism.
• AK catalyzes the reversible transfer of a terminal phosphate group between adenine nucleotides.
• Understanding AK's conformational flexibility is key to elucidating its catalytic mechanism.

Purpose of the Study:

• To refine and compare the crystal structures of two interconvertible forms of porcine muscle adenylate kinase.
• To investigate the molecular basis of structural transitions in adenylate kinase.
• To correlate structural changes with enzyme function, particularly substrate binding and phosphoryl transfer.

Main Methods:

• X-ray crystallography to determine the 3D structure of crystal form B at 3.3 Å resolution.
• Comparative analysis of crystal form B with previously determined crystal form A.
• Analysis of amino acid residue protonation states, specifically His36, and their impact on protein structure.

Main Results:

• Refined structure of porcine adenylate kinase crystal form B reported.
• Identified His36 protonation as the trigger for interconversion between crystal forms A and B.
• Observed significant conformational changes, including N-terminal alpha-helix unwinding and active center cleft opening.
• Detachment of the glycine-rich loop (residues 15-22) from the protein core upon conversion to form B.

Conclusions:

• The two observed conformations of adenylate kinase represent distinct states relevant to its function in solution.
• Protonation-dependent structural changes, particularly involving the glycine-rich loop, are critical for phosphoryl transfer.
• These conformations likely represent snapshots of the enzyme during the substrate binding process, offering insights into the catalytic mechanism.