Temperature and pH affect enzyme activity. As with all other reactions, enzyme-catalyzed reactions ___1___ in speed with an increase in temperature. However, as the temperature increases beyond a critical point, the protein structure begins to get disrupted resulting in ___2___ and loss of enzyme function. Every enzyme has a(n) ___3___ temperature and activity tends to decrease on either side of this temperature. Most human enzymes work best at around ___4___. Some enzymes require nonprotein ___5___ such as zinc and manganese ions. Other enzymes may require organic ___6___ such as vitamin C and vitamin D. A variety of substances inhibit enzyme activity. ___7___ are so similar to the enzyme's substrate that they are able to enter the enzyme's active site and block the normal substrate from binding. This process is reversible and can be overcome by increasing the concentration of the enzyme's substrate. Another class of inhibitors does not affect an enzyme at its active site, they are called ___8___.