Class 1 

catalyze reactions in which one molecule is oxidized, while another is reduced.

e.g. * dehyrdrogenase 

Class 2 

transfers a group (e.g methyl group, phosphate group) from one molecule to another.

e.g. * kinase 

Class 3 

cleaves C-O, C-N, C-C bonds hydrolytically

e.g. hexose bisphosphatase 

Class 4 

cleaves C-O, C-N, C-C bonds by elimination.  leaves double bonds or rings.

e.g. aldolase. dehyrdratase

Class 5

rearrange bonds within a single molecule

e.g. * isomerase, * mutase 

Class 6

join two molecules.  coupled to ATP hydrolysis

e.g. * carboxylase