Question

Hexokinase losses 50% of its activity at 45C in 12 minutes. But in the presence of a saturating concenctration of glucose it loses only 3% of its activity in 12 minutes.

Why?

Answer 1

Think structurally. What effect does heat have on enzyme structure, and hence function? Compare that to the effect had by presence of saturating glucose concentrations and you'll have your answer. :)

Answer 2

thanks that does not help[

Answer 3

OK, so the bonds that hold non-adjacent amino acids in a protein together are relatively weak and unstable. That means that it takes very little energy to disrupt the bonds. When the bonds are disrupted and broken, the protein no longer has the same conformation. Heating a protein to 45 C is putting vastly more heat energy into the system than the protein would ever encounter in a real biologic system. What happens to all those low energy bonds? They break. And what happens to the protein structure when all the bonds holding it together are disrupted...?

Compare that to just putting the enzyme in a concentrated but neutral solution. What effect does that have on enzyme structure? Not that much.

And think about what effect structure has on function. When the enzyme has been denatured and has all the structural integrity of a noodle, do you think it is able to catalyze its reaction?