Scientific and Educational Reports of the Faculty
of Science and Technology, Kochi University
Vol. 9 (2026), No. 1
Chihaya Baba1, Daichi Yano2, Yumeto Otsuka1, Kouji Uda1, Tomohiko Suzuki1, Shigeru Sugiyama1
1Faculty of Science and Technology, Kochi University.
2Department of Applied Chemistry and Biotechnology, National Institute of Technology, Wakayama College.
Arginine kinase (AK) from the ciliate Paramecium tetraure-lia comprises four isozymes, among which PtAK3 uniquely exhibits substrate inhibition, despite sharing high sequence identity with the other family members. To gain structural insight into this unusual catalytic property, we investigated the transition-state analogue (TSA) complex of PtAK3. Crystallization trials of PtAK3–TSA complex yielded three distinct crystal forms. X-ray diffraction data collected at 1.75 A, 2.30 A, and 1.60 A resolution for the Form I, II, and III crystals, respectively, revealed that each form adopts a different space group and unit-cell parameters. Matthews coefficient analysis indicated the presence of one, two, and three PtAK3 molecules per asymmetric unit in the Form I, II, and III crystals, consistent with the monomeric, dimeric, and trimeric noncrystallographic symmetry detected by self-rotation function analysis. These findings suggest that the PtAK3–TSA complex can adopt multiple oligomeric states. Such structural variability may be linked to the substrate-inhibition behavior characteristic of PtAK3. Ongoing structural analysis of each crystal form is expected to clarify how oligomeric assembly and conformational differences contribute to the unique catalytic mechanism of PtAK3.
Received: November 17, 2025
Reviewed by anonymous referee(s), and accepted: January 24, 2026
Published: February 17, 2026
発行者：高知大学理工学部 〒780-8520 高知県高知市曙町二丁目5-1
Faculty of Science and Technology, Kochi University, Kochi, 780-8520 Japan
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