The activity of an enzyme is sensitive to the presence of specific chemicals,which is called:

Basically how many types of enzymes have been recognised by the International Union of Biochemistry?

Which enzyme catalyzes the transfer of a group from one substrate to another?

Enzymes are proteins. Proteins are long chains of amino acids linked to each other by peptide bonds. Amino acids have many functional groups in their structure. Many of these functional groups are ionisable. As they are weak acids and bases in chemical nature,this ionisation is influenced by the $pH$ of the solution. For many enzymes,activity is influenced by the surrounding $pH$. This is depicted in the curve below. Explain briefly.

The given graphical representation relates to enzyme activity under three conditions ($pH$,temperature,and substrate concentration). What do the given axes $X$ and $Y$ represent?
$X$-axis $-$ $Y$-axis

Fumarase enzyme belongs to which class?