(a) Explain enzymes, their characteristics as well as induced-fit model of mechanism of enzyme action. (20 marks) (b) List…

Enzymes are specialized biocatalysts, predominantly proteinaceous in nature (with exceptions like ribozymes), that accelerate biochemical reactions by lowering their activation energy without being consumed in the process.

Characteristics of Enzymes and Induced-Fit Model

Enzymes exhibit distinct physiological characteristics:

1. Catalytic efficiency: They display high turnover numbers (k_cat), accelerating reactions by 10^6to10¹² fold.
2. Substrate specificity: Exhibited at stereo, absolute, or group levels due to complementary active-site geometry.
3. Reversibility and regulation: Most catalyzed reactions are reversible, and catalytic rates depend on environmental factors including optimum temperature, optimum pH, substrate concentration, and presence of allosteric modulators or competitive/non-competitive inhibitors.

According to the International Union of Biochemistry and Molecular Biology (IUBMB), enzymes are classified into six systematic classes: Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, and Ligases.

Daniel Koshland (1958) proposed the Induced-Fit Model of enzyme action, advancing Emil Fischer’s rigid "Lock-and-Key" hypothesis. Koshland posited that the enzyme's active site is structurally flexible rather than pre-shaped. When the substrate approaches, weak non-covalent interactions (hydrogen bonds, electrostatic forces, hydrophobic interactions) induce a precise conformational change in the enzyme. This realigns catalytic residues (e.g., nucleophiles, acid-base groups) into an optimal geometry around the substrate, straining the substrate's critical bonds, stabilizing the transition state (ES^ddagger), and facilitating conversion into products (E + S rightleftharpoons ES → EP → E + P).

Coagulation Factors and the Blood Clotting Cascade

Blood clotting operates via a sequential enzymatic cascade involving thirteen plasma coagulation factors designated by Roman numerals:

1. Factor I: Fibrinogen (soluble precursor)
2. Factor II: Prothrombin (zymogen; Vitamin K-dependent)
3. Factor III: Tissue Factor / Thromboplastin
4. Factor IV: Calcium ions (Ca²⁺, required across multiple cleavage steps)
5. Factor V: Proaccelerin / Labile factor
6. Factor VI: Accelerin (obsolete; active form of Factor V)
7. Factor VII: Proconvertin / Stable factor (Vitamin K-dependent)
8. Factor VIII: Antihemophilic factor A
9. Factor IX: Christmas factor / Plasma thromboplastic component (Vitamin K-dependent)
10. Factor X: Stuart-Prower factor (Vitamin K-dependent)
11. Factor XI: Plasma thromboplastin antecedent (PTA)
12. Factor XII: Hageman factor (contact factor)
13. Factor XIII: Fibrin-stabilizing factor (transglutaminase)

The cascade proceeds along three interconnected pathways:

1. Extrinsic Pathway: Vascular injury exposes Factor III, which binds circulating Factor VII in the presence of Factor IV (Ca²⁺) to form the VIIa-III complex, rapidly activating Factor X.
2. Intrinsic Pathway: Damage exposing subendothelial collagen activates Factor XII → XIIa, sequentially activating Factor XI → XIa, then Factor IX → IXa. The active IXa, together with cofactor VIIIa and Ca²⁺, forms the "tenase" complex on platelet membranes, cleaving Factor X to Xa.
3. Common Pathway: Factor Xa unites with Factor Va, phospholipids, and Ca²⁺ to form the prothrombinase complex. This complex cleaves Factor II (prothrombin) into active thrombin (IIa). Thrombin subsequently cleaves Factor I (fibrinogen) into insoluble fibrin monomers. Finally, Factor XIIIa (activated by thrombin and Ca²⁺) cross-links these monomers via covalent isopeptide bonds, transforming the fragile mesh into a stabilized haemostatic clot.

Neoteny versus Paedogenesis

Neoteny is an evolutionary and developmental phenomenon (a form of heterochrony) wherein an organism decelerates or retards somatic development relative to normal gonadal maturation, resulting in sexually mature adults retaining larval or juvenile morphological characteristics.

A classic example is the Mexican axolotl (Ambystoma mexicanum), which retains larval external gills, a flattened swimming tail, and an entirely aquatic lifestyle throughout adult life due to low thyroid-stimulating hormone (TSH) release. Similarly, the mudpuppy (Necturus maculosus) exhibits permanent, obligatory neoteny. In human evolution, paedomorphic/neotenic somatic retention is evident in our large brain-to-body ratio, flattened orthognathic facial plane, and sparse body hair compared to adult non-human primates.

Neoteny differs fundamentally from paedogenesis based on the underlying heterochronic mechanism:

1. Somatosensory vs. Gonadal Timing: Neoteny represents delayed somatic maturation while reproductive organs mature at the normal chronological rate. In contrast, paedogenesis (progenesis) is characterized by accelerated, precocious gonadal maturation within a developmentally unaltered larval or juvenile body.
2. Occurrence: Neoteny is prevalent in amphibians (Ambystoma, Proteus) and chordate evolution. Paedogenesis typically occurs in invertebrates as a rapid reproductive strategy under nutrient-rich conditions, such as larval reproduction in gall midges (Miastor), the beetle Micromalthus debilis, and sporocysts/rediae of trematode flatworms (Fasciola hepatica).

Thus, while both processes culminate in paedomorphosis, neoteny slows the body's clock, whereas paedogenesis speeds the reproductive clock.