· Some of the enzymes are simple proteins. Examples: amylase, trypsin. Many enzymes require one or more non-protein components called cofactors. If the cofactor is an organic molecule, it is known as coenzyme. The cofactor may also be a metal ion.
· Coenzymes may be defined as non-protein, low molecular weight organic compounds required for the activity of enzymes. Example: Thiamine pyrophosphate (TPP).
· The protein part of the enzyme is known as ‘apoenzyme’. The entire enzyme system consisting of the apoenzyme and coenzyme (or) prosthetic group is known as holoenzyme.
· Most of the coenzymes are linked to their apoenzymes by noncovalent forces. Example: The coenzyme ATP is attached to its apoenzyme hexokinase through weak non-bonding interactions.
· Some of the coenzymes are tightly bound to their apoenzymes through covalent bonds. These are termed as prosthetic groups. Example: The prosthetic group biotin is attached to its apoenzyme carboxylase through a covalent bond.
· The coenzymes undergo alterations during enzymatic reaction. Hence the coenzymes are regarded as second substrates (or) cosubstrates.
· Many coenzymes are the derivatives of water soluble B-complex vitamins. Examples: Niacin.
· The Coenzymes can also be organic substances other than vitamins. Example : ATP (adenosine triphosphate), CDP (Cytidine diphosphate)
· Nucleotides and their derivatives can act as coenzymes.
· Example: NAD, FMN, FAD, coenzyme-A etc.
· The specificity of an enzyme is mostly dependent on the apoenzyme and not on the coenzyme. For example, NAD+ functions as coenzyme for several enzymes like alcohol dehydrogenase and lactate dehydrogenase.
· Coenzymes functions as group transfer agents (Table 4.1).