Unit 5: Enzymes, Vitamins and Minerals - Subjective Questions

Enzymes are biological catalysts, usually proteins, that increase the rate of biochemical reactions without being consumed permanently. Some catalytic RNA molecules, called ribozymes, also function as enzymes. Key characteristics include: - Enzymes lower the activation energy of reactions but do not change the overall free-energy change or equilibrium constant. - They are highly specific for their substrates and reactions. - They are effective in very small concentrations. - Most enzymes operate under mild temperature and pH conditions. - Enzyme activity can be regulated by activators, inhibitors, covalent modification, and changes in enzyme concentration. - Enzymes form temporary enzyme-substrate complexes during catalysis.

The lock-and-key model proposes that the active site has a rigid shape exactly complementary to the substrate. It explains enzyme specificity but does not fully explain changes in enzyme shape during catalysis. The induced-fit model proposes that substrate binding causes a conformational change in the enzyme. This change positions catalytic groups correctly and forms a more effective active site. The induced-fit model is considered more accurate because it explains: - Flexible enzyme-substrate interactions. - Proper orientation of substrates. - Stabilization of the transition state. - Strain imposed on substrate bonds. - Product release after the reaction. Thus, the induced-fit model gives a more realistic description of enzyme action.

Enzyme catalysis occurs through the following steps: 1. The substrate binds to a specific region of the enzyme called the active site. 2. Weak interactions such as hydrogen bonds, ionic interactions, hydrophobic interactions, and van der Waals forces stabilize the enzyme-substrate complex. 3. The enzyme correctly orients the substrate and may alter its shape. 4. Catalytic amino acid residues participate in acid-base catalysis, covalent catalysis, or metal-ion catalysis. 5. The enzyme stabilizes the high-energy transition state, thereby lowering the activation energy. 6. The substrate is converted into product, which then leaves the active site. The enzyme remains chemically available for another catalytic cycle.

Enzyme specificity refers to the selective nature of enzyme-substrate interactions. Major types include: - Absolute specificity: An enzyme acts on only one substrate. For example, urease specifically hydrolyzes urea. - Group specificity: An enzyme acts on substrates containing a particular functional group. Trypsin acts on peptide bonds involving basic amino acids. - Bond specificity: An enzyme acts on a particular type of chemical bond. Lipases hydrolyze ester bonds in lipids. - Stereospecificity: An enzyme acts on only one stereoisomer. Many enzymes distinguish between D- and L-isomers of sugars or amino acids. Specificity depends mainly on the structure and chemical properties of the active site.

Enzymes are commonly named by adding the suffix -ase to the substrate or type of reaction, such as lactase or dehydrogenase. The systematic classification uses an EC number consisting of four parts. The first number indicates the major class, while the remaining numbers specify the subclass, sub-subclass, and individual enzyme. The major enzyme classes are: - Oxidoreductases: Catalyze oxidation-reduction reactions. - Transferases: Transfer functional groups between molecules. - Hydrolases: Break bonds by adding water. - Lyases: Remove or add groups without hydrolysis or oxidation, often forming double bonds. - Isomerases: Catalyze intramolecular rearrangements. - Ligases: Join two molecules using energy, usually from ATP. Modern classifications may also recognize translocases as a separate class for enzymes that catalyze movement of ions or molecules across membranes.

The major enzyme classes differ according to the reactions they catalyze: - Oxidoreductases: Catalyze electron or hydrogen transfer; example: lactate dehydrogenase. - Transferases: Transfer groups such as methyl, amino, or phosphate groups; example: aminotransferase. - Hydrolases: Break bonds through the addition of water; example: protease. - Lyases: Add or remove groups to form or break double bonds without hydrolysis; example: decarboxylase. - Isomerases: Convert a molecule into one of its isomers; example: phosphoglucose isomerase. - Ligases: Join two molecules and usually use ATP energy; example: DNA ligase. The classification is based primarily on the type of chemical reaction rather than the biological source of the enzyme.

Temperature affects enzyme activity in two opposing ways. At low temperatures, molecules possess less kinetic energy, so enzyme-substrate collisions are less frequent and the reaction rate is low. As temperature rises, molecular motion and collision frequency increase, causing the reaction rate to increase until an optimum temperature is reached. Above the optimum temperature, heat disrupts hydrogen bonds, ionic interactions, and other forces maintaining the enzyme structure. The enzyme may then become denatured, causing a rapid decline in activity. In humans, many enzymes have an optimum temperature close to . The effect of temperature is often represented by a curve showing a gradual increase followed by a sharp decrease.

pH affects enzyme activity by changing the ionization state of amino acid side chains at the active site and by altering the overall three-dimensional structure of the enzyme. Each enzyme has an optimum pH at which its catalytic activity is highest. For example, pepsin functions best in a strongly acidic environment, whereas many intestinal enzymes work best in a slightly alkaline environment. At pH values far from the optimum: - Substrate binding may become weaker. - Catalytic amino acids may lose the required charge. - Ionic bonds maintaining protein structure may be disturbed. - Extreme pH may cause denaturation. Therefore, organisms maintain suitable pH conditions in different cellular compartments to support enzyme function.

When substrate concentration is kept constant, increasing enzyme concentration generally increases the reaction rate because more active sites are available. The increase is approximately proportional as long as sufficient substrate is present. When enzyme concentration is constant, increasing substrate concentration initially increases the rate because more enzyme-substrate complexes form. At high substrate concentrations, nearly all active sites become occupied. The enzyme is then saturated and the reaction reaches a maximum rate, . Further addition of substrate produces little or no increase in rate. Thus, enzyme concentration is important when substrate is abundant, whereas substrate concentration has the greatest effect before enzyme saturation occurs.

For a simple reaction, the mechanism is: Under the steady-state assumption, the concentration of the enzyme-substrate complex remains approximately constant during the initial phase of the reaction. The Michaelis-Menten equation is: Here, is the initial reaction velocity, is the maximum velocity, is the substrate concentration, and is the Michaelis constant. When , the velocity is half of the maximum velocity: A low generally indicates high apparent affinity between enzyme and substrate, while a high indicates lower apparent affinity under the same conditions.

is the substrate concentration at which the reaction velocity is equal to half of . It is a characteristic constant for an enzyme-substrate pair under specified conditions. A lower usually suggests a greater apparent affinity for the substrate. is the maximum reaction velocity achieved when all enzyme active sites are saturated with substrate. It depends on the total enzyme concentration and the catalytic rate of the enzyme. These parameters help to: - Compare enzyme efficiency. - Estimate substrate affinity. - Analyze the effect of inhibitors. - Understand whether an enzyme is operating below or near saturation. The Michaelis-Menten relationship is .

The Lineweaver-Burk plot is a double-reciprocal form of the Michaelis-Menten equation: A plot of against gives a straight line. Its features are: - The -intercept is . - The -intercept is . - The slope is . The plot can be used to estimate kinetic constants and identify types of enzyme inhibition. However, it gives disproportionate weight to measurements made at low substrate concentrations, where experimental errors may be large. Therefore, nonlinear analysis of the Michaelis-Menten equation is often preferred for accurate calculations.

The main differences are: - Competitive inhibition: The inhibitor competes with the substrate for the active site. It increases the apparent , but remains unchanged. Its effect can often be reduced by increasing substrate concentration. - Noncompetitive inhibition: The inhibitor binds at a site distinct from the active site and can bind to the free enzyme or enzyme-substrate complex. Pure noncompetitive inhibition decreases , while remains unchanged. Increasing substrate concentration does not fully overcome the inhibition. - Uncompetitive inhibition: The inhibitor binds only to the enzyme-substrate complex. Both apparent and decrease. Increasing substrate concentration does not reverse the inhibition. These effects are determined from kinetic measurements and graphical analysis.

Reversible inhibition occurs when an inhibitor binds to an enzyme through weak, noncovalent interactions. The inhibitor can dissociate, allowing enzyme activity to return. Competitive, noncompetitive, and uncompetitive inhibition are common reversible forms. Irreversible inhibition occurs when an inhibitor forms a stable covalent bond with the enzyme or permanently damages an essential functional group. Enzyme activity cannot be restored simply by removing the inhibitor. Examples include heavy metals binding to sulfhydryl groups and certain toxins that permanently modify active-site residues. Irreversible inhibitors may be useful as drugs or pesticides, but they can also cause toxicity by blocking essential metabolic enzymes.

Allosteric enzymes possess regulatory sites that are separate from their active sites. Binding of an activator or inhibitor at an allosteric site changes the enzyme's conformation and alters its activity. Many allosteric enzymes contain multiple subunits and show cooperative substrate binding. In feedback inhibition, the final product of a metabolic pathway inhibits an enzyme acting early in that pathway, usually the first committed step. This mechanism: - Prevents unnecessary production of the final product. - Conserves cellular energy and raw materials. - Helps maintain metabolic balance. - Provides rapid and reversible control. Feedback inhibition is an example of negative regulation and is distinct from inhibition caused by simple substrate competition.

Many enzymes require non-protein components for catalytic activity. A cofactor is a non-protein component required by an enzyme and may be an inorganic ion or an organic molecule. A coenzyme is an organic cofactor, often derived from vitamins, that transfers electrons or chemical groups. A prosthetic group is a cofactor tightly or permanently attached to the enzyme, such as the heme group in some enzymes. Metal ions such as , , and may stabilize charges, participate in redox reactions, or help bind substrates. The protein portion without its required cofactor is an apoenzyme; the complete active enzyme is a holoenzyme.

Vitamins are organic compounds required in very small amounts for normal growth, metabolism, maintenance, and health. They generally cannot be synthesized in sufficient quantities by the body and must be obtained from the diet. They are classified as follows: - Fat-soluble vitamins: Vitamins A, D, E, and K. They are absorbed along with dietary fats, stored mainly in the liver and adipose tissue, and may accumulate to toxic levels. - Water-soluble vitamins: Vitamin C and the B-complex vitamins. They are generally not stored in large amounts, excess quantities are excreted in urine, and regular dietary intake is important. Many vitamins act as coenzymes or precursors of coenzymes in metabolic reactions.

The major fat-soluble vitamins have the following roles: - Vitamin A: Supports vision, epithelial tissues, immunity, and growth. Sources include liver, dairy products, eggs, and provitamin A carotenoids in vegetables. Deficiency may cause night blindness and xerophthalmia. - Vitamin D: Promotes calcium and phosphate absorption and supports bone mineralization. It is obtained through sunlight exposure, fish oils, eggs, and fortified foods. Deficiency causes rickets in children and osteomalacia in adults. - Vitamin E: Acts mainly as an antioxidant that protects cell membranes. Sources include vegetable oils, nuts, seeds, and whole grains. Severe deficiency may cause neurological and muscular problems. - Vitamin K: Is required for synthesis of several blood-clotting factors. Sources include green leafy vegetables and intestinal bacterial synthesis. Deficiency may lead to prolonged bleeding.

B-complex vitamins mainly function as coenzymes in energy metabolism, biosynthesis, and blood-cell formation. Examples include: - Thiamine, B1: Required for carbohydrate metabolism; deficiency causes beriberi. - Riboflavin, B2: Forms part of FAD and FMN; deficiency may cause cheilosis and inflammation of mucous membranes. - Niacin, B3: Forms NAD and NADP; deficiency causes pellagra, characterized by dermatitis, diarrhea, and dementia. - Pyridoxine, B6: Participates in amino acid metabolism and neurotransmitter synthesis; deficiency may cause anemia and neurological symptoms. - Folate, B9: Required for nucleotide synthesis and red blood cell formation; deficiency causes megaloblastic anemia. - Cobalamin, B12: Supports DNA synthesis and myelin maintenance; deficiency causes megaloblastic anemia and neurological problems. Vitamin C is required for collagen synthesis, wound healing, antioxidant protection, and iron absorption. Its deficiency causes scurvy, bleeding gums, and poor wound healing.

Minerals are inorganic elements required for body structure, fluid balance, nerve function, muscle contraction, enzyme activity, and other physiological processes. They cannot be synthesized by the body and must be obtained from food and water. Macrominerals are needed in relatively large amounts, usually more than approximately per day. Examples include calcium, phosphorus, magnesium, sodium, potassium, chloride, and sulfur. Trace elements are required in much smaller amounts. Examples include iron, zinc, copper, iodine, selenium, manganese, molybdenum, and fluoride. Both groups are essential, but excessive intake of some minerals can produce toxic effects.