Residue Asn 204 in the glucose binding site of hexokinase IV was mutated, in two separate experiments, to either Ala or Asp. The Asn → Ala mutant had a KM nearly 50-fold greater than the wild-type enzyme, and the Asn → Asp mutant had a 140-fold greater KM value than the wild-type enzyme. These mutations impact the intermolecular interactions between the enzyme and the glucose substrate. The amide functional group of the Asn side chain can form (dipole-dipole interactions, hydrodgen bonds, London Dispersion Interactions, or Ion-Dipole Interactions) with the hydroxyl groups of the glucose substrate and can potentially function as either a (hydrogen bond donor and/or acceptor, hydrogen bond donor, or hydrogen bond acceptor) . The methyl group of Ala cannot participate in hydrogen bond formation, which explains the (increase or decrease) in glucose affinity as indicated by the higher KM for the mutant enzyme. The side chain of Asp could potentially serve as a (hydrogen bond donor and/or acceptor, hydrogen bond donor, or hydrogen bond acceptor) , but the higher KM value for this mutant indicates that the substrate binds even (less well or better) than when the hydrogen bonding is abolished at this site. This indicates that the —NH2 group of the Asn side chain functions as a hydrogen bond donor when interacting with the —OH groups of the glucose substrate and that this interaction is vitally important for substrate binding.
Residue Asn 204 in the glucose binding site of hexokinase IV was mutated, in two separate experiments, to either Ala or Asp. The Asn → Ala mutant had a KM nearly 50-fold greater than the wild-type enzyme, and the Asn → Asp mutant had a 140-fold greater KM value than the wild-type enzyme. These mutations impact the intermolecular interactions between the enzyme and the glucose substrate.
The amide
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