Biochemical and structural analysis of this loop variant and GCK variants associated with hyperinsulinemic hypoglycemia reveal two distinct mechanisms of enzyme activation. Positional randomization of the loop, coupled with genetic selection in a glucokinase-deficient bacterium, uncovers a hyperactive GCK variant with substantially reduced cooperativity. Here, using limited proteolysis, we map the site of disorder in unliganded GCK to a 30-residue active-site loop that closes upon glucose binding. Prior work has demonstrated that millisecond timescale order-disorder transitions within the enzyme’s small domain govern cooperativity. Cooperativity in human glucokinase (GCK), the body’s primary glucose sensor and a major determinant of glucose homeostatic diseases, is fundamentally different from textbook models of allostery because GCK is monomeric and contains only one glucose-binding site.
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