What are Vitamins and coenzymes?

A class of organic compounds needed in the human body for normal activities is known as vitamins. They are characterized into two parts depending upon their solubility in water. They can act as coenzymes, cofactors, or prosthetic groups for enzymes.

"Vita" in vitamins signifies the fact that these substances are essential for life (vita). The first vitamin discovered was thiamine, which is an amine. The vitamin necessities of the body are generally provided by the Recommended Dietary Allowances (RDA). Coenzymes also fall under the category of organic molecules which functions to impel the reaction by binding themselves with a particular enzyme.

Enzymes or biological catalysts are proteins possessing one or more amino acid chains in their structure. Some enzymes also contain a nonprotein entity, which is generally termed as a cofactor. Cofactors are the low molecular mass components that are essential for protein functioning.

They can be of two types, namely inorganic ions and coenzymes, which are organic compounds. These are mostly derived from vitamins. Examples of coenzymes are nicotinamide adenine dinucleotide, coenzyme A, adenosine triphosphate (ATP).

Why are cofactors and coenzymes required?

Most enzymes are proteins or nucleic acids, but catalysis is not easy with only amino acid or nucleotide functional groups. For these reactions, cofactors or coenzymes are required. So, our diet should contain vitamins to provide coenzymes that further help in catalyzing biochemical processes.

Enzymes and their functions

Enzymes are highly specific, meaning only a specific substrate with a specific structure can bind with a particular enzyme. Coenzyme A is used only in oxidative and biosynthetic reactions. The specificity of the enzyme is due to its shape. Each enzyme has a specific shape, possessing functional groups to bind with enzymes. They help in catalyzing a particular reaction.

The action of the enzymes can be explained using the lock-and-key mechanism. It is explained in the following two steps:
Step 1: Formation of the enzyme-substrate activated complex.
The substrate or reactant fits like a key into the cavities of the enzymes to form the enzyme-substrate complex. The complex-formation step is reversible and fast and thus does not contribute to the rate law.

It is represented as follows:

E+S↔ES

Where, E is enzyme and S is substrate.

Step 2: Disintegration of the complex to release product molecules
The enzyme-substrate complex then decomposes to give the product. Thereafter, the enzyme is regenerated, and it becomes available for further reaction.
The reaction is represented as follows:

ES→EP→E+P

where, ES is enzyme-substrate complex, EP is enzyme-product complex and P is product.

These steps can be summarized in the below diagram.

Coenzymes and their action

Coenzymes are loosely bound with enzymes and help them by providing suitable functional groups or changing their structural configuration. In this way, the substrate-binding of the enzyme becomes easy, and the formation of the products is faster.

Coenzymes cannot catalyze a reaction without an enzyme. Most of the coenzymes are carrier molecules and transfer things from one enzyme to another.

Apoenzyme and Holoenzyme

Apoenzyme

It represents the inactive stage of an enzyme before its binding with an organic or inorganic cofactor.

Holoenzyme

It represents the active form of an enzyme when it binds to the cofactors or prosthetic groups.

Difference between enzymes and coenzymes

 An example of a coenzyme is coenzyme A that contains pantothenic acid as its major component and operates to manufacture energy, activates the enzyme, and speeds up reactions working in the body. Flavin adenine dinucleotide (FAD) is also an example of a "redox-active coenzymes".

Water-soluble and Fat-soluble vitamins

Water-soluble vitamins tend to dissolve in water, whereas fat-soluble vitamins are similar to oils, and therefore are insoluble in water. There are different classes of vitamin B with their respective chemical names. They are as follows: thiamin (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxal (B6), biotin (B7), and cobalamin (B12).

The various water-soluble and fat-soluble vitamins are listed below.

Importance of Vitamin B12

Vitamin B12 or cobalamin is an organometallic compound existing naturally in its pure form. It is the only vitamin that contains metal in its structure.

• It absorbs iron, calcium, and vitamin A from food.
• It helps in the formation of red blood cells.
• It protects against stress and dementia.
• Its deficiency can cause anemia, nervousness, muscle tremors, etc.

Functions of water soluble vitamins

Mostly, water-soluble vitamins perform as coenzymes or help in the synthesis of coenzymes. Vitamins named thiamin (B1), riboflavin (B2), niacin (B3), pyridoxal (B6), biotin (B7), cobalamin (B12), and folic acid have their respective coenzyme whereas vitamin C, also called ascorbic acid has no coenzyme.

A table consisting of water-soluble vitamins, coenzymes, coenzyme functions, and deficiency diseases is shown below.

Characteristics of cofactors and Vitamins

Cofactor is the non-protein entity of an enzyme. The characteristics of cofactors and the general properties of vitamins are discussed below.

• All vitamins are not cofactors.
• All water-soluble vitamins can be converted to cofactors except vitamin C.
• Among fat-soluble vitamins, only vitamin K can be converted to a cofactor.
• Cofactors function as carriers of specified functional groups, e.g., acyl groups and methyl groups.

Classes of Coenzymes

As per Chemistry LibreTexts, there are four classes of coenzymes.

• Co-substrates: They get altered during the reaction and further regenerated by another enzyme.
• Prosthetic groups: They remain covalently or tightly bound with the enzyme throughout the reaction.
• Vitamin-derived coenzymes: These are the derivatives of vitamins that are not synthesized by mammals. They are obtained as nutrients.
• Metabolite coenzymes: They are manufactured from common metabolites.

Common Mistakes

Students may confuse coenzymes and cofactors. Cofactors are non-protein entities that increase the activity of enzymes. They are classified into coenzymes (organic) and inorganic ions.

• Apoenzyme is an inactive enzyme, while holoenzyme is an active enzyme-containing cofactor.
• Only water-soluble vitamins constitute coenzymes, except for vitamin C.
• Coenzymes are nonspecific.

Context and Applications

This topic is applicable in the professional exams for both undergraduate and graduate courses, especially for:

Bachelors in Chemistry

Bachelors in Biology

Masters in Chemistry

Masters in Biochemistry

Related Concepts

Biochemical reactions

Catalysts

Reaction rate

Fats and lipids

Practice Problems

Q1: Which of the following vitamins can act as a coenzyme?

(a) Vitamin B

(b) Vitamin K

(c) Vitamin A

(d) Vitamin D

Correct option: (a)

Q2: Which of the following is suitable for X?

Holoenzyme=Apoenzyme + X

(a) Enzyme

(b) Coenzyme

(c) Vitamin

(d) Proteins

Correct option: (b)

Q3: Identify the vitamin-forming coenzymes that activate specific enzymes.

(a) Vitamin K

(b) Vitamin B

(c) Vitamin E

(d) Vitamin C

Correct option: (b)

Q4: Identify another name for vitamin B12.

(a) Cobalamin

(b) Ascorbic acid

(c) Folic acid

(d) Cholecalciferol

Correct option: (a)

Q5: Inadequacy of vitamin B3 in the body can lead to:

(a) Beri-beri

(b) Night blindness

(c) Scurvy

(d) Pellagra

Correct option: (d)