Co-enzyme and Co-factor

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Co-enzyme and Co-factor

Co-enzyme: Co-enzymes are non-colloidal, heat-stable, dialyzable nonprotein organic molecules of low molecular weight that are essentially required for the activity of the enzyme. A coenzyme is associated with the specific enzyme as a prosthetic group.

The complete functional entity of an enzyme including its protein. A part and nonprotein part is called holoenzyme. The protein entity of the holoenzyme is called apoenzyme and its non-protein organic component is coenzyme.

Co-enzymes are nonprotein organic molecules. These usually have one of the vitamins as their active group.

Function: Co-enzymes act as links between metabolic pathways. These can be considered as special intracellular enzyme-substrates. These help in oxido-reduction, group transfer and isomerisation reactions and in reactions which form covalent bonds. Their functions are as follows:

1. Co-enzymes Acting as Hydrogen Acceptor: These are nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). Both these enzymes act as hydrogen acceptors.

2. Co-enzymes Acting with Decarboxylases: Pyridoxamine phosphate or pyridoxal (derivative of vitamin B₆) act as prosthetic group for a number of enzymes of amino acid metabolism.

3. Co-enzyme helping in Acetylation: Co-enzyme A accepts acetyl group from one metabolite and donates it to another. It thus helps in metabolism of fat. Co-enzyme A is formed of thioethanolamine, pantothenic and (Vitamin B), pyrophosphate and nucleotide adenylic acid.

4. Co-enzyme for Oxidative Decarboxylation: For splitting of Pyruvic acid into CO₂ and acetaldehyde in yeast cells, enzyme carboxylase requires thiamine pyrophosphate as a coenzyme. Co-enzyme is required which contains lipoic acid and thiamine Pyrophosphate.

5. Glucose-1, 6-diphosphate, glyceric acid 2, 3-diphosphate and glucose-1-phosphate-uridine nucleotide, biotin, folic acid and vitamin as coenzyme.

Cofactors

Some enzymes require small nonprotein components called cofactors for their activity. For example, the cytochromes are conjugated proteins having tightly bound prosthetic groups. The cytochromes have a metalloporphyrin complex and are involved in electron-transfer reactions.

Enzyme Kineties: The kinetic characteristics of enzymes can be illustrated by a graph plotting V_o against substrate concentration [S]. This is called 'Michaelis plot'. It is rectangular hyperbola.

Without having anything to react with, the enzyme does not show any activity in the absence of substrate. This is represented as V_o. The reactivity of enzyme or the velocity of enzyme controlled reaction increases with the availability of the substrate. While all other conditions are kept constant, the increase in concentration of substrate [S] increases the measured initial velocity V₁to a maximum value V_max and no further. Further increase in substrate concentration does not increase reaction velocity, because at this point, all available enzyme molecules will be fully employed.

Michallis-Menten Model of Enzyme Kinetics

A simple Michaelis equation represents the dependence of the initial reaction velocity V₀ on the concentration of enzyme [E] and substrate [S].

Here, K₁, K₂and K₃ are rate constants for the reactions. The rate of product is calculated from the following simple expression:

This is called Michaelis- Menten equation. Here, V_max is the maximum velocity of enzyme reaction, K_m is Michaelis constant and, [S] is substrate concentration.

Michaelis constant (K_m) is the concentration of substrate which gives half the maximum velocity. Kinetic constant or the turn-over number in the number of substrate molecules converted into product per unit time by single catalytic site when the enzyme is fully saturated with substrate.

Allosteric Enzymes: Enzymes having quaternary structure, i.e. formed of more than one polypeptide subunits) have a regulatory site or allosteric site in addition to the active or substrate site. Allostery means 'Different shape’. An outside molecule, other than the substrate, binds to the regulatory site and can enhance or diminish the reactivity of the enzyme at the substrate site.

An allosteric hypothetical enzyme has minimum two subunits.

1. Catalytic subunit has full catalytic activity and bears active site or substrate site

2. Regulatory subunit has the regulatory or allosteric site which binds to the inhibitor or activator.

Proenzymes: In living beings, many proteins are manufactured and secreted in the form of inactive precursor proteins. These are called proproteins. Conversion of a proprotein to the mature protein involves one or more successive Proteolysis clips'. As a result of such selective proteolysis the inactive proprotein is converted into active mature protein.

Enzymes are proteins. Most of the digestive enzymes are also secreted in inactive form. These inactive or precursor enzymes are called proenzymes or zymogens. For example, all the proteolytic enzymes are secreted in inactive form, such as:

1. Propepsin is called pepsinogen.

2. Protrypsin is called trypsinogen.

3. Prochymotrypsin is called chymotrypsinogen.

One basic reason why proteolytic enzymes or proteases are secreted in inactive form, is to avoid the autodissolution or autodigestion of the tissue of origin, where these enzymes are synthesised, stored and secreted.

Further more, the synthesis and secretion process of an enzyme may be slow relative to its physiological demand. Hence, an adequate amount of the required enzyme is stored in inactive or precursor form to be converted into active stage as and when required.

Isoenzymes: Isoenzymes are multimolecular forms of an enzyme. These are physically distinct, easily separable forms of an enzyme which differ in their kinetic properties but catalyse the same reaction. Chemically, the various isozymes of a group may be totally unrelated or may be similar, being formed of same subunits arranged in different combinations.

Isoenzymes of numerous dehydrogenases, oxidases, transminases, phosphatases, transphosphorylases and proteolytic enzymes are known.

Isoenzymes are oligomeric being formed of more than one dissimilar protomers or subunits. Frequently, one tissue produces one protomer predominantly and the other tissue a different protomer, producing different forms of enzymes.

Lactate dehydrogenase catalyses transfer of 2 electrons and one H⁺ from lactate to NDA⁺. Each has a molecular weight of about 35,000. These combine to form five different isoenzymes:

(H₄)

(H₃M)

(H₂M₂)

(HM₃)

(M₄)

Occurrence: Different isoenzymes have different locations. These may occur in different locations within the same cell, in different tissues of the same organism or in different living beings. For example, both malate dehydrogenase and glutamate oxaloacetate transaminase exist as cytoplasmic and mitochondrial forms.

These catalyse the same reaction but have no close structural relationship. Similarly, malate dehydrogenase from different sources are different in physical and chemical properties but catalyse the same reaction of oxidation of malate to oxaloacetate.

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