Table 3. Enzyme Classification: Secondary classes of oxidoreductase enzymes in EC system.

Oxidoreductases: second EC digit	Hydrogen or electron donor
1.	Alcohol (CHOH)
2.	Aldehyde or ketone (C═O)
3.	─CH─CH─
4.	Primary amine (CHNH2 or CHNH3+)
5.	Secondary amine (CHNH)
6.	NADH or NADPH (when another redox catalyst is the acceptor)