why is enzyme activity lost at zero degrees?

Answer 1

Every enzyme has a temperature range of optimum activity. Outside that temperature range the enzyme is rendered inactive and is said to be totally inhibited. This occurs because as the temperature changes this supplies enough energy to break some of the intramolecular attractions between polar groups (Hydrogen bonding, dipole-dipole attractions) as well as the Hydrophobic forces between non-polar groups within the protein structure. When these forces are disturbed and changed, this causes a change in the secondary and tertiary levels of protein structure, and the active site is altered in its conformation beyond its ability to accomodate the substrate molecules it was intended to catalyze. Most enzymes (and there are hundreds within the human organism) within the human cells will shut down at a body temperature below a certain value which varies according to each individual. This can happen if body temperature gets too low (hypothermia) or too high (hyperthermia).

Answer 2

At zero degrees there is no molecular movement.