Zymogen

Lesson 4

Objective

To understand Zymogen and its activation

What Factors Influence Enzymatic Activity?

Proinsulin is an 86-residue precursor to insulin

Zymogens or proenzymes are inactive precursors of enzymes. Typically, proteolytic cleavage produces the active enzyme.

Figure 15.3 The proteolytic activation of chymotrypsinogen

Proteolytic Enzymes of the Digestive Tract

The Cascade of Activation Steps Leading to Blood Clotting

Figure 15.4 The intrinsic and extrinsic pathways converge at factor X, and the final common pathway involves the activation of thrombin and its conversion of fibrinogen into fibrin, which aggregates into ordered filamentous arrays that become crosslinked to form the clot.

Isoenzymes

• Many enzymes occur in more than one molecular form in the same species, in the same tissue or even in the same cell

• Catalyse the same reaction but different kinetic property

• Differ in their response to allosteric modulators

LDH

• Made up of 4 subunits• Mol wt. 144 kDa• H and M subunits have same no of amino acids

(334) and same mol.wt (36kDa) (but differ in amino acid composition and immunological properties

• In the tetramer, the subunits occupy equivalent positions

• Each monomer has an active centre

• The H subunit contains more acidic and fewer basic residues than the M form

• It has more strong negative charge• In electrophoresis LDH-1 containing H4

migrates fastest and M4 isoenzyme slowest

Isozymes Are Enzymes With Slightly Different Subunits

The isozymes of LDH

• Subunits come from different genes• A common precursor gene was probably

duplicated at some point in evolution• The 2 genes then continued to develop further

independently of each other through mutation and selection

All LDH isoforms catalyse the same reactionBut they differ in their Km and Vmax values for their substrate particularly PyruvateM4 has low Km for pyruvate, (high reacton rate)H4 has high Km for pyruateMuscle has high glucose conversion to lactateHeart muscle is aerobic and oxidise pyruvate to CO2 (so reaction rate slow)

Diagnostic importance of isoenzymes

• During intracellular damage, enzyme appear in blood• The type and activity of enzyme reflects the severity

of damage and site of cell injury• LDH-1 increases in blood after cardiac infarction

(LDH-5 hardly changes)• H4 and H3M increase in certain tumors, leukemia• Alkaline phosphatase is used to distinguish liver

lesions from bone lesions in metastatic carcinoma• Creatine kinase in muscle metabolism