Co-enzymes and Enzyme Structure

Study Notes

Co-enzymes

Co-enzymes are non-protein parts of enzymes, also known as prosthetic groups, which are heat stable.
The protein part of the enzyme is called the apo-enzyme, which is heat labile.
When combined, the apo-enzyme and co-enzyme form the holo-enzyme.

Classification of Co-enzymes

Co-enzymes can be divided into two groups:
- Those involved in reactions catalyzed by oxidoreductases, donating or accepting hydrogen atoms or electrons.
- Those involved in reactions transferring groups other than hydrogen.

First Group of Co-enzymes

In this group, the change in the substrate is counter-balanced by the co-enzyme.
Examples include NAD+-NADPH, FAD-FADH2, and FMN-FMNH2.
NAD+ (Nicotinamide Adenine Dinucleotide) is a co-enzyme synthesized from Nicotinamide, a vitamin B complex member.
The structure of NAD+ is Nicotinamide-RiboseP-P-Ribose-Adenine.
NAD+ plays a crucial role in the reversible reaction of lactate to pyruvate, catalyzed by the enzyme lactate dehydrogenase.
During this reaction, two hydrogen atoms are removed from lactate, with one hydrogen and two electrons accepted by NAD+ to form NADH.

Second Group of Co-enzymes

These co-enzymes are involved in reactions transferring groups other than hydrogen.
Examples include co-enzymes that belong to the vitamin B complex group.
A few examples are given in Table 5.1.

Adenosine Triphosphate (ATP)

ATP is considered the energy currency in the body.
ATP was first isolated by Fiske and Subba Row in 1926, and its importance in muscle contraction was shown by Lohmann in 1929.
The second and third phosphate bonds in ATP are 'high energy' bonds.
During the oxidation of foodstuffs, energy is released and stored as chemical energy in the form of ATP.
Endergonic reactions are carried out with the help of energy released from ATP hydrolysis.

Metallo-enzymes

Metallo-enzymes are enzymes that require certain metal ions for their activity.
Examples include enzymes that require copper, iron, or zinc ions.
In some cases, the metal ion is tightly bound with the enzyme, while in others, the enzyme may be active without the metal ion but is enhanced by its presence.

Co-factors

Co-factors are a collective term that includes co-enzymes and metal ions.
Co-enzymes are organic co-factors.

Description

Learn about the different parts of enzymes, including co-enzymes, prosthetic groups, apo-enzymes, and holo-enzymes. Understand their properties and how they work together in enzyme-catalyzed reactions.