What are the different types of restriction enzymes?

Restriction enzymes are generally categorized into four groups, types I, II,III and IV, which differ primarily in structure, cofactor, cleavage site and specificity.

• Type I enzymes: These enzymes cleave at sites remote from a recognition site, which require both ATP and S-adenosyl-L-methionine as cofactors to function. They are multifunctional proteins with both restriction digestion and methylase activities.
• Type II enzymes: They cleave within or at short specific distances from a recognition site. Most of the enzymes fall into this group require magnesium. They are single function protein with only restriction digestion activity.
• Type III enzymes: These enzymes cleave at sites a short distance from a recognition site. ATP is required for this type of enzymes to function. They exist as part of a complex with a modification methylase.
• Type IV enzymes: They recognize and cut modified DNA, typically methylated, hydroxymethylated and glucosyl-hydroxymethylated DNAs.