| Summary: | 博士 === 國立臺灣大學 === 生化科學研究所 === 105 === Hexokinase catalyzes the reaction of glucose phosphorylation by consuming ATP, which mainly serves as the gateway enzyme in glycolysis. Beyond the roles in enzyme catalysis and glucose metabolism, the hexokinase from Arabidopsis thaliana (AtHXK) is an identified sugar sensor that detects fluctuation of glucose concentration. Furthermore, it transmits glucose signal and participates in glucose signaling to alter physiological phenomena of the plant. In this study, the crystal structures of AtHXK1 and AtHXK2 in different forms are presented: (I) AtHXK1 binary complex with glucose 6-phosphate; (II) AtHXK1 quaternary complex with glucose, Mg2+, and ADP; (III) AtHXK1 quaternary complex with 2-deoxyglucose, Mg2+, and ADP; (IV) AtHXK2 apo-form; (V) AtHXK2 binary complex with glucose; (VI) AtHXK2 quaternary complex with mannose, Mg2+, and ADP. Structure form I represents the state of enzyme-product complex of AtHXK1. Structures form II and III include well-recognized Mg2+ ion, coordinating water molecules, and ADP within active site of AtHXK1, which provide the first direct view for interacting patterns of metal ion and nucleotide to Arabidopsis hexokinase. Structure form IV is the apo-form of AtHXK2 in open state. Structure from V is the enzyme-substrate complex of AtHXK2 in closed state. Structure form VI obviously presents the interacting patterns of metal ion and nucleotide to AtHXK2, which is also in closed state. The three structures show that AtHXK2 has an obvious domain reorientation induced by glucose binding, which provides direct view for conformational change of protein structure during the catalytic process. However, ADP binding does not induce such change as glucose does. The obtained ADP quaternary complex structures of AtHXK1 and AtHXK2 shall allow the catalytic mechanism taken by Arabidopsis hexokinase being proposed, and further applied for better understanding about the functional diversities exhibited by the other four Arabidopsis hexokinase family proteins.
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