● Thousands of `color{violet}("enzymes")` have been discovered, isolated and studied.

● Most of these `color{violet}("enzymes")` have been classified into different groups based on the type of reactions they `color{violet}("catalyse.")`

● `color{violet}("Enzymes")` are divided into `color{brown}("6 classes")` each with `color{brown}("4-13 subclasses")` and named accordingly by a `color{brown}("four-digit number.")`

`color{green}(star "Oxidoreductases/dehydrogenases:")` Enzymes which `color{violet}("catalyse oxidoreduction")` between two substrates `S` and `S’` e.g.
`color{violet}" S reduced + S’ oxidised →S oxidised + S’ reduced.")`

`color{green}(star "Transferases:")` Enzymes catalysing a transfer of a group, `color{brown}(G)` (other than hydrogen) between a pair of substrate `S` and `S’` e.g.

`color{violet}("S - G + S’ → S + S’ - G)`

`color{green}(star "Hydrolases:")` Enzymes catalysing hydrolysis of ester, ether, peptide, `color{violet}("glycosidic, C-C, C-halide or P-N bonds")`.

`color{green}(star "Lyases:")` Enzymes that `color{violet}("catalyse removal")` of groups from substrates by `color{violet}("mechanisms")` other than `color{violet}("hydrolysis leaving double bonds.")`

`color{violet}(overset(overset(X)(|))C - overset(overset(Y)(|))C → X-Y + C = C)`

`color{green}(star "Isomerases:")` Includes all `color{violet}("enzymes catalysing inter-conversion")` of optical, geometric or positional isomers.

`color{green}(star "Ligases:")` Enzymes catalysing the linking together of 2 compounds, e.g., enzymes which `color{violet}("catalyse")` joining of `color{violet}("C-O, C-S, C-N, P-O")` etc. bonds.

● `color{violet}("Enzymes")` are composed of one or several `color{violet}("polypeptide chains.")`

● However, there are a number of cases in which non-protein constituents called `color{brown}("cofactors")` are bound to the the enzyme to make the `color{violet}("enzyme catalytically active.")`

● In these instances, the protein portion of the enzymes is called the `color{brown}("apoenzyme.")`

● Three kinds of cofactors may be identified: `color{brown}("prosthetic groups, co-enzymes")` and `color{brown}("metal ions.")`



`color{green}(star "Prosthetic groups:")`

● They are `color{violet}("organic compounds")` and are distinguished from other `color{violet}("cofactors")` in that they are `color{brown}("tightly bound")` to the `color{violet}("apoenzyme. ")`

● For example, in `color{brown}("peroxidase and catalase,")` which catalyze the breakdown of `color{violet}("hydrogen peroxide")` to water and oxygen, haem is the prosthetic group and it is a part of the active site of the `color{violet}("enzyme.")`

`color{green}(star "Co-enzymes:")`

● They are also `color{violet}("organic compounds")` but their association with the apoenzyme is only `color{brown}("transient")`, usually occurring during the `color{violet}("course of catalysis.")`

● Furthermore, `color{violet}("co-enzymes")` serve as co-factors in a number of different `color{violet}("enzyme catalyzed reactions")`.

● The essential`color{violet}(" chemical components")` of many `color{violet}("coenzymes")` are `color{brown}("vitamins")`, e.g., coenzyme nicotinamide adenine dinucleotide (NAD) and NADP contain the `color{violet}("vitamin niacin.")`

`color{green}(star "Metal Ions:")`

● A number of `color{violet}("enzymes")` require metal ions for their activity which form `color{brown}("coordination bonds")` with side chains at the active site and at the same time form one or more `color{violet}("coordination bonds")` with the substrate, e.g., zinc is
a `color{violet}("cofactor")` for the `color{violet}("proteolytic enzyme carboxypeptidase.")`

● Catalytic activity is lost when the `color{violet}("co-factor")` is removed from the enzyme which testifies that they play a `color{violet}("crucial role")` in the `color{violet}("catalytic activity of the enzyme.")`