The reactants or substrates require energy before they start to react. For this reaction, it takes time if enzymes are not present, but in the presence of enzyme, it reduces the activation or transition energy and favours the reaction as shown in Figure 6.3. Figure 6.3 Effect of Enzyme on Activation Energy of a Reaction (A is…
An enzyme is a catalyst, and it cannot alter the equilibrium of a chemical reaction. This means that an enzyme accelerates the forward and reverse reaction by some factors precisely. To attain this equilibrium without enzymes, it takes several hours, but with enzymes, it is attained within a second. Thus, enzymes accelerate the attachment of…
In many biochemical reactions, the energy of reactant is converted into a different form with high efficiency. Example:
The synthesis of lactose by mammary gland is a particular example. Lactose synthetase catalyses the synthesis of lactose and consists of a catalytic subunit. The catalytic subunit (refer Figure 6.1) by its own cannot synthesise lactose; it helps or catalyses the attachment of galactose to a protein–carbohydrate (glycoprotein). Figure 6.1 Structure of Catalytic and Modifier Subunit of…
The enzyme that catalyses the first step in a biosynthetic pathway is controlled by ultimate product. Example: HMG-CoA reductase.
Enzymes are controlled by reversible attachment of phosphoryl groups to specific serine and threonine residues. Example: The activities of enzymes that synthesise and degrade glycogen are coordinate in this way.
ZYMOGEN ACTIVATION Enzymes are usually synthesised in an inactive precursor form known as zymogen, and this gets activated at a physiologically appropriate time and place. Digestive enzymes are controlled in this way, which is called as proteolytic activation or zymogen activation. Example: Trypsinogen is synthesised in pancreas and activated in small intestine to form active enzyme…
Thrombin is an enzyme involved in blood coagulation is even more specific in that the side chain on the carboxyl of susceptible peptide bond must be arginine, whereas the one on the amino must be glycine.
The degree of specificity of enzymes for substrate is usually high and sometimes virtually absolute. Proteolytic enzymes, for instance, catalyse the hydrolysis of peptide bond.
Some enzymes exhibit specificity toward the cis-trans forms. Example: Fumarase, the inter-conversion of fumaric and malic acid.