Temperature and pH are major factors that can change the tertiary structure of enzyme proteins, and consequently alter the shape of the enzyme. Since enzymes and substrates match like key and lock, slight alterations in the shape of the enzyme will affect the bonding to the substrate and subsequently, stop the catalytic reaction. This is known as denaturing.
Learn more about how enzymes work here.
How Temperature Affects Enzyme Activity
As temperature increases so do the rate of enzyme reactions. This increase is only up to a certain point until the elevated temperature breaks the structure of the enzyme. Once the enzyme is denatured, it cannot be repaired. As each enzyme is different in its structure and bonds between amino acids and peptides, the temperature for denaturing is specific for each enzyme.
On the other hand, lower temperatures lead to slower chemical reactions. Enzymes will eventually become inactive at freezing temperatures but will restore most of their enzyme activity when temperatures increase again.
How pH Affects Enzymes
Same as with temperature, every enzyme has a different pH optimum.
In general, enzyme activity increases with pH until the optimum pH is reached. Then, decreases again as pH continues to rise. Extreme pH levels will cause irreversible denaturing.