Co-enzymes and Enzyme Structure

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.

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.