Hexokinase

• Two isozymes, not two states of one. 1IG8 and 3B8A are only 77% identical, so 18.58° is closure plus isozyme difference — no cleaner pair exists.
• The lobes are found, not assigned. A consensus search sorts 262 and 140 of 467 paired residues into two lobes; 65 left over are the hinge and termini.
• Superposed on the large lobe alone. That puts the whole difference on the small lobe (0.85 Å vs 5.68) and feeds the closure animation.

The first step of glycolysis, and the enzyme has to shut its mouth to take it. Hexokinase puts a phosphate from ATP onto glucose, and the reason the sugar goes anywhere at all is that the enzyme folds around it first.

• Phosphate traps the sugar; closure keeps out water. Charged glucose-6-phosphate can't cross back out, and an open site would waste ATP phosphorylating water instead — Koshland's 1958 induced fit, proved here.
• Four human hexokinases, two jobs. I–III shut off once their product builds up; glucokinase doesn't, so its rate tracks blood glucose — a sensor that triggers insulin release. Mutated, it causes MODY2.
• These files are yeast. Same closing motion as in humans.