Practice 5 structured short- and long-answer medical questions on Weekly Year 1: Enzymes, Vitamins and Minerals Exam - June 19, 2026 (Sections B & C). Review...
Answer: Enzyme specificity refers to the ability of an enzyme to catalyze a reaction with a particular substrate or a limited range of substrates. This is due to the unique shape and chemical properties of the enzyme's active site, which complement the substrate's shape. 1. **Absolute specificity**: The enzyme acts on only one specific substrate. For example, urease acts exclusively on urea, catalyzing its hydrolysis into ammonia and carbon dioxide. 2. **Group specificity**: The enzyme acts on a specific functional group or type of chemical bond, regardless of the rest of the molecular structure. For example, trypsin cleaves peptide bonds specifically where the carboxyl group is contributed by the basic amino acids arginine or lysine.
Answer: Enzyme activity, and thus the rate of an enzyme-catalyzed reaction, can be influenced by several factors. Two major factors include: 1. **Temperature**: Each enzyme has an optimal temperature at which its activity is maximal. Initially, increasing temperature increases kinetic energy and collision frequency between enzyme and substrate, enhancing reaction rate. However, exceeding the optimal temperature leads to denaturation of the enzyme's three-dimensional structure, particularly its active site, causing a significant and irreversible loss of activity. 2. **pH**: Similar to temperature, enzymes exhibit optimal activity within a narrow pH range. Deviations from this optimal pH can alter the ionization state of amino acid residues in the enzyme's active site or crucial for its structure, affecting substrate binding or catalytic efficiency. Extreme pH values can lead to irreversible denaturation of the enzyme.
Answer: Vitamins are classified into two main groups based on their solubility: 1. **Fat-soluble vitamins**: These vitamins are absorbed along with dietary fats and are stored in the body's fatty tissues and liver. They are not readily excreted. * **Example**: Vitamin D (Cholecalciferol). * **Key Function**: Essential for calcium and phosphate homeostasis, promoting their absorption in the intestine and regulating bone mineralization. 2. **Water-soluble vitamins**: These vitamins dissolve in water, are generally not stored in significant amounts, and any excess is typically excreted in the urine. * **Example**: Vitamin C (Ascorbic Acid). * **Key Function**: Acts as a crucial antioxidant, is essential for collagen synthesis (e.g., in connective tissue, skin, bones), and enhances the absorption of non-heme iron.
Answer: Calcium is the most abundant mineral in the human body and performs several critical physiological roles: 1. **Structural Component**: It is the primary mineral component of bones and teeth, providing structural rigidity and support. 2. **Muscle Contraction**: Calcium ions (Ca2+) are essential for initiating and regulating muscle contraction in skeletal, cardiac, and smooth muscles. 3. **Nerve Impulse Transmission**: Ca2+ plays a crucial role in neurotransmitter release at synapses, facilitating communication between nerve cells. 4. **Blood Coagulation**: It is an essential cofactor (Factor IV) in the coagulation cascade, necessary for the activation of several clotting factors. 5. **Hormone Secretion**: Involved in the release of various hormones and enzymes. One common clinical manifestation of chronic calcium deficiency is **osteoporosis** (in adults), characterized by reduced bone mineral density, leading to increased bone fragility and fracture risk. In children, it can lead to **rickets**, affecting bone development.
Answer: B vitamins are a group of water-soluble vitamins that play a critical role as coenzymes in numerous metabolic pathways, particularly those involved in energy production and biosynthesis. They are not used as energy sources themselves but are essential for enzymes to function correctly. A coenzyme is a non-protein organic molecule that binds to an enzyme (apoenzyme) to form an active enzyme complex (holoenzyme), facilitating specific biochemical reactions. Let's consider **Thiamine (Vitamin B1)**: **1. Coenzyme Form(s)**: Thiamine is converted in the body, primarily in the liver, into its active coenzyme form: **Thiamine Pyrophosphate (TPP)**, also known as Thiamine Diphosphate (TDP). This conversion is catalyzed by the enzyme thiamine pyrophosphokinase, which phosphorylates thiamine using ATP. **2. Major Metabolic Pathways where it Functions**: TPP acts as a crucial coenzyme for several key enzymes involved in carbohydrate metabolism: * **Pyruvate Dehydrogenase Complex (PDC)**: TPP is an essential coenzyme for the E1 (pyruvate decarboxylase) component of the PDC. This complex catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA, which is a critical link between glycolysis and the tricarboxylic acid (TCA) cycle (Krebs cycle). This step is central to aerobic glucose metabolism and ATP production. * **Alpha-Ketoglutarate Dehydrogenase Complex**: TPP is also a coenzyme for this complex in the TCA cycle, catalyzing the oxidative decarboxylation of alpha-ketoglutarate to succinyl-CoA. This is another vital step in energy generation. * **Transketolase**: TPP is required by transketolase, an enzyme in the pentose phosphate pathway (PPP). The PPP generates NADPH (essential for reductive biosynthesis and antioxidant defense) and ribose-5-phosphate (a precursor for nucleotide synthesis). **3. Biochemical Consequences of Severe Deficiency**: Severe thiamine deficiency (Beriberi) primarily impacts tissues with high energy demand, such as the brain, nerves, and heart, due to impaired carbohydrate metabolism: * **Impaired Glucose Metabolism**: Reduced activity of PDC and alpha-ketoglutarate dehydrogenase leads to an accumulation of their substrates (pyruvate, lactate, and alpha-ketoglutarate). This can result in lactic acidosis due to increased anaerobic metabolism. * **Reduced Acetyl-CoA Production**: The bottleneck at PDC significantly reduces the supply of acetyl-CoA to the TCA cycle, severely impairing aerobic ATP production. * **Impaired NADPH and Nucleotide Synthesis**: Decreased transketolase activity affects the PPP, reducing NADPH levels and hindering biosynthesis of fatty acids, steroids, and nucleotides. **4. Clinical Consequences of Severe Deficiency**: The biochemical impairments lead to distinct clinical syndromes: * **Dry Beriberi**: Primarily affects the nervous system. Symptoms include peripheral neuropathy (muscle weakness, paresthesia, foot drop), Wernicke-Korsakoff syndrome (acute neurological disorder characterized by confusion, ataxia, ophthalmoplegia, and chronic memory impairment). * **Wet Beriberi**: Primarily affects the cardiovascular system. Symptoms include high-output cardiac failure, tachycardia, cardiomegaly, and edema, often due to the heart's inability to meet its high energy demands. * **Infantile Beriberi**: Occurs in breastfed infants of thiamine-deficient mothers, often presenting with cardiac symptoms, aphonia, and acute heart failure. In summary, thiamine, as TPP, is indispensable for the enzymes that drive central pathways of carbohydrate metabolism. Its severe deficiency leads to a profound disruption of energy production and critical biosynthetic pathways, manifesting in severe neurological and cardiovascular dysfunction.