Coenzymes and cofactors A large number of enzymes require an additional non protein ‑component to carry out its catalytic functions called as cofactors. A coenzyme or metal ion that is covalently bound to the enzyme protein is called prosthetic group

Cofactors-two types 1)Inorganic ions such as Fe2+, Mg2+, Mn2+, Zn2+ 2)A complex organic molecule called coenzyme.

Some enzymes require both a coenzyme and one or more metal ions for their activity.

Coenzymes function as transient carriers of specific functional groups.

Vitamins as coenzymes

• Coenzymes molecules are often vitamins or are made from vitamins.

• Many coenzymes contain the nucleotide adenosine as part of their structures, such as ATP, coenzyme A and NAD+.

Coenzymes1. Thiamine 2. Riboflavin 3. Niacin 4. Pantothenic acid (5) 5. Pyridoxine (6) 6. Biotin(7) 7. Cobalamin (12) 8. Folic acid (9) and 9. Lipoic acid

Functions of coenzymes and their precursors Functions of important coenzymes and their precursors ---------------------------------------------------------------------------------------------- Coenzyme Short Chemical groups Vitamin form(s) transferred precursor ---------------------------------------------------------------------------------------------- Thiamine pyro- TPP Two carbon aldehydes Thiamine phosphate Flavin adenine FAD dinucleotide electrons Riboflavin Flavin mono nucleotide FMN Nicotinamide adenine NAD dinucleotide Hydride ion Nicotinic acid Nicotinamide adenine NADP dinucleotide Phosphate Coenzyme A CoASH acyl group Pantothenic acid Pyridoxal PP Amino group Pyridoxol phosphate (Pyridoxamine) Coenzyme B12 Cobamide hydrogen atoms and Vitamin B12

alkyl grooups Biocytin BCCP Carbon dioxide Biotin Tetrahydrofolate THF (FH4) One-carbon groups Folate Lipoamide - Electrons and acyl groups Lipoate ----------------------------------------------------------------------------------------------

Mechanism of coenzyme action How does coenzyme act?Coenzyme accelerates the enzymatic reactioni ) by helping the cleavage of the substrate ii) by also acting as acceptor for one of the cleavage products.

In an enzyme catalysed reaction, the substrate combines with the apoenzyme to form activated complex(ES) in the presence of coenzyme.

The bond in the substrate is strained, ruptured leading to products. One of the cleavage products is directly transferred to the coenzyme which has suitable receptor site in its structure. Other product remains attached to the enzyme

The other cleavage product now dissociates from the apoenzyme liberating the enzyme protein for fresh reaction. The cleavage product attached to the coenzyme is next released from the surface of the coenzyme after the completion of enzyme action. Now both apoenzyme and coenzyme are regenerated to their original form and are ready for fresh reaction. A prosthetic group also acts in a similar fashion with the difference that the prosthetic group is firmly attached to the surface of the apoenzyme.

Mechanism of coenzyme action

Thiamine pyrophosphate (TPP-B1)

The coenzyme form of thiamine is thiamine pyrophosphate (TPP) –B1.

The two important reactions in which TPP functions as coenzyme are

i) Oxidative decarboxylation of keto acids such as pyruvate ‑and ketoglutarate. ‑ ii) Transketolase reaction.--Consists of a pyrimidine + a thiazole ring ( from cysteine, glycine or tyrosine & sugar moiety)TPP synthesized in microbes and plants.

TPP biosynthesis

Riboflavin(B2)

• The flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are the two coenzyme forms of riboflavin.

• FMN and FAD serve as prosthetic groups of oxidation reduction ‑

enzymes known as flavoenzymes or flavoproteins.

Biosynthesis of FMN and FAD

Niacin (B3)

• The coenzyme forms of niacin are NAD+ and NADP+.

• as the coenzymes of a large number of oxido reductases collectively called as pyridine linked dehydrogenases.‑

Biosynthesis of Nicotinic acid

Pantothenic acid –B5 The coenzyme form of pantothenic acid is coenzyme A and is represented as CoASH. The function of coenzyme A is to serve as a carrier of acyl group in reactions. The thiol group ( SH) acts as a ‑carrier of acyl group . Ex.i. Reactions associated with fatty acid oxidation, fatty acid synthesis, pyruvate oxidation and biological acetylations. ii. It is also involved in many biosynthetic processes such as synthesis of cholesterol, terpenes, and steroids.

Biosynthesis of Coen. A

Pyridoxine/ (PLP B6)

• The coenzyme form of pyridoxine is known as pyridoxal phosphate (PP)

• The most common type of reaction requiring PP as a coenzyme is transamination.

• Enzymes catalysing such reactions are known as transaminases or aminotransferases.

Biosynthesis of Pyridoxal phosphate

Biotin (vitamin H; coenzyme R; vitamin B7).

The important function of biotin is its role as coenzyme for carboxylase ( catalyses carbon dioxide fixation or carboxylation reaction). •Ex. The carboxylation of acetyl CoA to malonyl CoA in presence of acetyl CoA carboxylase requires biotin as coenzyme.

• Propionyl carboxylase and pyruvate carboxylase are also associated with biotin. Biosynthesis in microbes, plants-complex.Animals cannot synthesize.

Biosynthesis of biotin

Folic acid (B9) The coenzymic form of folic acid is tetrahydro folic acid •Tetrahydro folic acid is associated with one carbon metabolism.

•The tetra hydrofolic acid serves as a carrier of single carbon moieties such as methyl, methylene, methenyl, formyl or formimino group.

• It is involved in the biosynthesis of purines, pyrimidines, serine, methionine and glycine. •BiosynthesisMicroorganisms, plants and animals.Starting with GTPdihydroneopterinehydroxymethyldihydropterine+aminobenzoic acid+glutamine dihydrofolic acidreduction to THF

Vitamin B12 (cobalamin)

Coenzyme forms •5 deoxyadenosyl cobalamin. ‑•methyl cobalamin. are required for the action of several enzymes.

1. Methyl malonyl CoA mutase uses 5 deoxyadenosyl cobalamin ‑as coenzyme.

2. Methyl cobalamin functions as a carrier of methyl group to homocysteine and convert it to methionine

Lipoic acid

•The oxidised and reduced forms of lipoic acid

•Lipoic acid functions as a coenzyme in pyruvate and ketoglutarate dehydrogenase multienzyme ‑complexes.

Other vitamins

• Vitamin B4: adenine a nucleobase, is synthesized by the human body.

• Vitamin B8: adenosine monophosphate or alternately myo-inositol, is synthesized by the human body

• Vitamin B10: para-aminobenzoic acid (PABA)• Vitamin B11: pteryl-hepta-glutamic acid —chick

growth factor, which is a form of folic acid. vitamin S