Which of the following statements are true regarding a competitive inhibitor? Select all that apply
Inhibitor binds to active site of the enzyme and the substrate.
Inhibition can be overcome by high [S].
Inhibitor binds to active site of the enzyme.
Substrate and inhibitor are bound to different sites on the enzyme.
Inhibitor is slowly broken down by the enzyme.
Correct Answer : B,C
Competitive inhibition occurs when a molecule similar in structure to the substrate competes for the same binding site on an enzyme. This prevents the formation of the enzyme-substrate complex, thereby reducing the rate of reaction. In Michaelis-Menten kinetics, competitive inhibitors increase the Km (Michaelis constant) because a higher concentration of substrate is needed to reach half-maximal velocity, while the Vmax remains unchanged.
Rationale:
A. This statement is incorrect because the inhibitor and the substrate cannot bind to the active site at the same time. The binding is mutually exclusive; if the inhibitor occupies the site, the substrate is blocked. Competitive inhibition is defined by this binary competition for a single molecular coordinates on the enzyme surface.
B. Inhibition can be overcome by high [S] is a hallmark of competitive inhibition. As the substrate concentration increases, the probability of a substrate molecule outcompeting an inhibitor molecule for the active site increases. At sufficiently high concentrations, the enzyme can still reach its maximum velocity (Vmax), effectively masking the presence of the inhibitor.
C. Inhibitor binds to active site of the enzyme is true and describes the fundamental mechanism of this process. The inhibitor often mimics the transition state or the molecular structure of the substrate to fit into the catalytic pocket. This direct occupation of the active site is what distinguishes it from allosteric or non-competitive inhibition.
D. This statement describes non-competitive or allosteric inhibition, where the inhibitor binds to a regulatory site different from the active site. In such cases, increasing substrate concentration does not reverse the inhibition. In competitive inhibition, the substrate and inhibitor must target the exact same site on the enzyme molecule.
E. An inhibitor is generally not a substrate and is not broken down by the enzyme. The purpose of an inhibitor is to occupy the site and remain there to prevent catalytic activity. If the enzyme could break down the inhibitor, the molecule would simply be a competing substrate, not a true clinical or biochemical inhibitor.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
Antiemetics are a diverse class of pharmacological agents designed to inhibit the complex physiological process of emesis. This process is coordinated by the chemoreceptor trigger zone (CTZ) and the vomiting center in the medulla. Antiemetics work by blocking specific receptors, such as serotonin, dopamine, or histamine, that transmit pro-emetic signals. They are essential for managing chemotherapy-induced nausea and preventing post-operative complications related to gastric expulsion.
Rationale:
A. Drug that prevents or stops vomiting is the correct definition of an antiemetic. The term is derived from "anti-" (against) and "emesis" (vomiting). Ondansetron, specifically, is a 5-HT3 receptor antagonist that blocks serotonin signals in the gut and brain. This effectively interrupts the reflex arc that leads to the forceful expulsion of gastric contents through the mouth.
B. A drug that decreases motility in the gastrointestinal tract to arrest spasm or diarrhea is known as an antispasmodic or an antidiarrheal. Agents like loperamide or dicyclomine fit this description. While some antiemetics may have mild secondary effects on gut motility, their primary therapeutic intent is to suppress the vomiting reflex rather than treating hypermotility or loose stools.
C. A drug that neutralizes stomach acid is defined as an antacid, such as calcium carbonate or aluminum hydroxide. These agents work chemically in the stomach lumen to raise the pH. They do not possess the neuroreceptor-blocking properties required to stop nausea and vomiting. Antacids are indicated for dyspepsia and heartburn, whereas antiemetics address the neurological trigger of emesis.
D. A drug that causes movement of the bowels is called a laxative or a cathartic. These agents are used to treat constipation by promoting defecation. Ondansetron and other antiemetics actually tend to have the opposite effect, often causing constipation as a common side effect. Their physiological goal is to stabilize the upper GI tract rather than stimulating the lower bowel.
Correct Answer is D
Explanation
Diclofenac is a potent analgesic and antipyretic agent utilized in a topical formulation to treat localized musculoskeletal pain and osteoarthritis. It functions by inhibiting the cyclooxygenase enzymes, thereby reducing the synthesis of inflammatory prostaglandins. This targeted delivery minimizes systemic absorption, significantly reducing the risk of gastrointestinal and cardiovascular adverse events associated with oral administration of this drug class.
Rationale:
A. Anti-infective agents are medications designed to inhibit the growth of or destroy pathogenic microorganisms such as bacteria, viruses, or fungi. Diclofenac has no antimicrobial activity and does not target cellular structures of pathogens. Its use is strictly limited to the management of inflammatory pain and stiffness in joints and soft tissues.
B. Mu-receptor agonists are opioid analgesics that bind to receptors in the central nervous system to alter the perception of pain. Diclofenac operates via a peripheral mechanism and does not interact with the opioid system. It lacks the potential for physical dependence or respiratory depression, distinguishing it from controlled substances used for severe pain.
C. Hyaluronic acid is a glycosaminoglycan found in connective tissue, often used as a viscosupplement or humectant to lubricate joints or hydrate skin. While it is sometimes co-administered or present in certain gels, it is a structural molecule rather than a pharmacological drug class. Diclofenac is a synthetic chemical compound with specific enzymatic inhibitory properties.
D. Diclofenac belongs to the Nonsteroidal Anti-Inflammatory (NSAID) class. It provides therapeutic effects by blocking COX-1 and COX-2, which prevents the conversion of arachidonic acid into pro-inflammatory prostaglandins. The topical formulation allows for high concentrations at the site of inflammation while maintaining low plasma levels, optimizing the safety profile for chronic use.
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