Enzymes and Bioenergetics

Enzymes: Classification and Mode of Action

Enzymes are biological catalysts that accelerate biochemical reactions both inside living organisms (in vivo) and in controlled laboratory conditions (in vitro). They are highly specific to their substrates and possess remarkable catalytic power, meaning they enhance the rate of reactions tremendously without themselves undergoing permanent changes. Most enzymes are proteins, with molecular weights ranging from about 2000 to more than one million Daltons. However, a small group of catalytic RNA molecules known as ribozymes also exhibit enzymatic activity. The enzymatic activity of proteinaceous enzymes depends heavily on their conformational structure; denaturation or alteration of this structure can affect their function. Many enzymes require cofactors to be catalytically active. These cofactors can be metal ions such as Fe²⁺, Mn²⁺, Zn²⁺, Mg²⁺ or complex organic molecules called coenzymes. When an enzyme is combined with its cofactor, it forms a holoenzyme, whereas the protein part alone is called an apoenzyme. Coenzymes are often derived from vitamins and participate transiently in catalysis by carrying specific functional groups. Examples include NAD (derived from vitamin B3) which transfers hydride ions, and coenzyme A (from vitamin B5) which transfers acyl groups. Metal ions serve as cofactors for various enzymes, such as Fe²⁺/Fe³⁺ in catalase and peroxidase, Mg²⁺ in DNA polymerase, and Zn²⁺ in carbonic anhydrase. The active site of an enzyme is a specific region where the substrate binds and the reaction occurs. This site is a small, three-dimensional pocket formed by the folding of the polypeptide chain. Binding involves various non-covalent interactions including hydrogen bonds, electrostatic forces, Van der Waals forces, and hydrophobic interactions. Two classical models explain enzyme-substrate interaction: Emil Fischer's Lock and Key model posits that the active site and substrate have complementary rigid shapes fitting exactly like a key in a lock. Daniel Koshland's Induced Fit model suggests that the active site is flexible and undergoes conformational changes upon substrate binding, enhancing catalytic efficiency. Enzymes exhibit different types of specificity: absolute specificity (acting on a single substrate), group specificity (acting on substrates with similar functional groups), stereospecificity (acting on specific stereoisomers), and geometrical specificity (distinguishing cis/trans isomers). The International Union of Biochemistry (I.U.B.) classifies enzymes into seven major classes based on the reactions they catalyze: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, and translocases. Additionally, many enzymes exist as isoenzymes or isozymes, which catalyze the same reaction but differ in amino acid composition and physicochemical properties, allowing tissue-specific regulation. Understanding the classification and mode of action of enzymes is fundamental to biotechnology, as enzymes are widely used in industrial processes, diagnostics, and therapeutics. **Table on page 2 (9×3)** | Coenzyme | Precursor vitamin | Role in the catalytic reaction | | --- | --- | --- | | Biocytin | Biotin (vitamin B7) | Transfer of CO_{2} | | Coenzyme B12 (5'-adenosylcobalamin) | Vitamin B12 | Transfer of an alkyl group | | Flavin adenine dinucleotide (FAD) | Riboflavin (vitamin B2) | Transfer of electrons | | Coenzyme A | Pantothenic acid (vitamin B5) | Transfer of acyl and alkyl group | | Nicotinamide adenine dinucleotide (NAD) | Niacin (vitamin B3) | Transfer of hydride (:H_{2}) | | Pyridoxal phosphate | Pyridoxine (vitamin B6) | Transfer of amino group | | Thiamine pyrophosphate | Thiamine (vitamin B1) | Transfer of aldehydes | | Tetrahydrofolate | Folic acid (vitamin B9) | Transfer of one carbon group | **Table on page 2 (9×2)** | Metal Ions | Enzyme name | | --- | --- | | Fe^{2+} or Fe^{3+} | Catalase, peroxidase, cytochrome oxidase | | Cu^{2+} | Cytochrome oxidase | | Mg^{2+} | DNA polymerase | | Mn^{2+} | Arginase | | K^{+} | Pyruvate kinase | | Mo^{2+} | Nitrogenase, nitrate reductase | | Zn^{2+} | Carbonic anhydrase, alcohol dehydrogenase | | Ni^{2+} | Urease | **Table on page 3 (8×3)** | Class No. | Class name | Type of reaction catalyze | | --- | --- | --- | | 1. | Oxidoreductases | Oxidation-reduction reactions (transfer of electrons) | | 2. | Transferases | Transfer of groups | | 3. | Hydrolases | Hydrolytic reactions (transfer of functional groups to water) | | 4. | Lyases | Addition or removal of groups to form double bonds | | 5. | Isomerases | Transfer of groups within molecules to yield isomeric forms | | 6. | Ligases | Condensation of two molecules coupled through ATP hydrolysis | | 7. | Translocase | Transfer of ion/molecules across the membrane |

• Enzymes are biological catalysts that speed up biochemical reactions without being consumed.
• Most enzymes are proteins; some RNA molecules (ribozymes) also have catalytic activity.
• Enzymes often require cofactors: metal ions or organic coenzymes derived from vitamins.
• The active site is a specific 3D pocket where substrate binds and catalysis occurs.
• Lock and Key and Induced Fit models explain enzyme-substrate specificity and binding.
• I.U.B. classifies enzymes into seven classes based on reaction type; isoenzymes catalyze the same reaction but differ structurally.
• 📌 Enzyme: Protein catalyst accelerating biochemical reactions.
• 📌 Cofactor: Non-protein component (metal ion or coenzyme) required for enzyme activity.
• 📌 Holoenzyme: Complete enzyme with its cofactor.

Factors Affecting Enzyme Activity

The catalytic activity of enzymes is influenced by several environmental factors that affect the enzyme's structure and its interaction with the substrate. The key factors include temperature, pH, substrate concentration, and the presence of modulators such as inhibitors or activators. 1. Temperature: Enzyme activity generally increases with temperature due to enhanced molecular motion, reaching a maximum at an optimum temperature. Beyond this temperature, activity rapidly declines due to denaturation of the enzyme's protein structure. The typical optimum temperature for most human enzymes is around 37°C (98.6°F), but some enzymes like Taq DNA polymerase from Thermus aquaticus remain active at temperatures as high as 100°C. The graph of enzyme activity versus temperature is bell-shaped. 2. pH: Each enzyme has an optimum pH at which its activity is maximal. Deviations from this pH reduce enzyme activity due to changes in ionization states of amino acid residues at the active site or overall enzyme denaturation. For example, pepsin has an acidic optimum pH of 1-2, while alkaline phosphatase has an optimum pH of 10-11. The activity versus pH curve is also bell-shaped. 3. Substrate Concentration: Increasing substrate concentration increases the rate of reaction as more substrate molecules interact with enzyme molecules. However, after a certain concentration, the enzyme becomes saturated with substrate, and further increases do not enhance the rate. At saturation, the enzyme works at its maximum velocity (Vmax). 4. Modulators: These include inhibitors and activators that bind to enzymes and alter their activity. Inhibitors decrease enzyme activity, while activators increase it. These effects are discussed in detail in the enzyme inhibition section. Understanding these factors is crucial for optimizing enzyme use in industrial and laboratory settings, ensuring maximum efficiency and stability.

• Temperature affects enzyme activity with an optimum temperature for maximal activity.
• pH influences enzyme activity; each enzyme has a unique optimum pH.
• Substrate concentration increases reaction rate up to enzyme saturation point.
• Enzyme activity decreases beyond optimum temperature or pH due to denaturation.
• Modulators such as inhibitors and activators regulate enzyme activity.
• 📌 Optimum temperature: Temperature at which enzyme activity is highest.
• 📌 Optimum pH: pH at which enzyme exhibits maximum activity.
• 📌 Saturation: Condition when all enzyme active sites are occupied by substrate.