The multiple forms of an enzyme catalysing the same reaction are isoenzymes or isozymes; they, however, differ in their physical and chemical properties, which include structure, electrophoretic and immunological properties, Km and Vmax values, optimum pH, relative susceptibility to inhibitors, and degree of denaturation. Example: Lactate dehydrogenase LDH inter-converts the lactate and pyruvate (Figure 6.34). Figure 6.34 Conversion of…

Ribozymes, RNA molecules, present in the enzyme can function as enzymes. Altman and coworker in 1983 found that ribonuclease P (Figure 6.33), an enzyme-cleaved precursor of tRNA, was functional due to RNA component present in the enzyme and not protein part of the enzyme. This RNA ribozymes also obey Michaelis-Menten kinetics. Figure 6.33 Ribozyme–Ribonuclease P

Enzymes have a lot of variability in the half lives of individual enzymes. Example: LDH 4 = 5–6 days; LDH 1 = 8–12 hours; amylase = 3–5 hours. In general, the key and regulatory enzymes are rapidly degraded. If not needed, they immediately disappear, and as and when required, they are quickly syn

Most of the enzymes, particularly the rate-limiting ones, are present in very low concentration. Many rate-limiting enzymes have short, half lives. This helps in efficient regulation of enzyme levels. There are two types of enzymes: Induction: The term induction is used to represent increased synthesis of enzymes which are under gene control through mediation of hormones or other…

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 as shown in Figure 6.32.

Zymogen Activation Some enzymes are synthesised in an inactive precursor from, which is activated at a physiologically appropriate time and place. Digestive enzymes exemplify this kind of control, which is called proteolytic activation or zymogen activation. Example: Trypsinogen is synthesised in pancreas and activated in small intestine to form active enzyme trypsin. Figure 6.32 Covalent modification of Phosphorylase

The various factors that influence enzyme activity in vitro are temperature and pH, which are quite constant in living cell. In biological system, regulation of enzyme activities occurs at different stages in one or more of the following ways to achieve cellular economy:

Allosteric enzymes show relationships between V0 and [S] that differ from Michaelis-Menten kinetics. They do exhibit saturation with the substrate [S] is sufficiently high, but for some allosteric enzymes, when V0 is plotted against [S], a sigmoid saturation curve occurs. Archibald Hill, in1910, first analysed an equation for the allosteric enzyme kinetics. For a protein with N…

When the effector may be is different from substrate, in which case, the effector is said to be heterotrophic (Figure 6.29). Example: Feedback inhibitors. Aspartate transcarbamoylase. Figure 6.29 Heterotrophic Effector of Aspartate Transcarbamoylare Conformational Changes in Allosteric Enzymes Most of allosteric enzymes are oligomeric in nature. It exists in two conformational statuses: t (tense or taut) form and r (relaxed) form.…

When the substrate itself serves as an effector, the effector is said to be homotropic. Most often, an allosteric functions is a positive effector. This enzymes show a sigmoidal curve (Figure 6.28) when reaction velocity plots against substrate concentration. Figure 6.28 Homotropic Effectors–sigmoidal Curve The concept of cooperative binding is analogous to the binding of