At very high temperatures the the structure of enzymes is damaged and most of enzymes denature. the enzyme loses it shape, the substrate can no longer fit into the active site and so the reaction rate falls.
Effect of pH:
Enzymes are also affected by changes in pH. The point where the enzyme works best at is called the optimum pH. Optimum pH value varies from one enzyme to another, pH 2 for pepsin and pH 8 for pancreatic lipase.
Extremely high or low pH value mostly results in loss of enzyme activity. Enzymes have a 3D shape which is held together by hydrogen bonds, so if the solution is too acidic or too alkaline it denatures the enzyme. Change in hydrogen ions causes the hydrogen bonds holding the enzyme's shape together to break which causes the enzyme to change it's shape which may alter the shape of the active site and because of that substrate would not fit in the active site and reaction rate will fall..
effect of substrate concentration:
substrate concentration is the concentration of the molecules an enzyme works on.
Low substrate concentration:
in a low substrate concentration there are few substrate molecules to occupy all the available active site. Because the enzyme's active site is empty waiting for the substrate to bind. But as there is a low substrate concentration it limits the rate at which the products can be formed.
Intermediate substrate concentration:
as there are more substrate molecules added the rate of reaction increases and results in more reactions because all the active sites are filled and are being used.
High substrate concentration:
increasing the substrate concentration increases the reaction rate but when excess substrate molecules are added they are unable to find any free active site because all the active site of the enzyme is occupied. At this point adding more substrate will make no difference and the rate of reaction will not speed up any further.