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Chapter: 11th Biochemistry : Chapter 4 : Enzymes


Some of the enzymes are simple proteins.


·     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).

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11th Biochemistry : Chapter 4 : Enzymes : Coenzyme |

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