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USMLE ROAD MAP BIOCHEMISTRY – PART 3

Nhiều người châu Á thiếu một hình thức thấp-Km acetaldehyde dehydrogenase, có trách nhiệm để giải độc acetaldehyde được tạo ra bởi quá trình oxy hóa của ethanol trong gan. Acetaldehyd tích lũy trong máu của cá nhân đó dẫn đến đỏ bừng mặt và các hiệu ứng thần kinh biểu hiện bởi người đàn ông gốc Nhật Bản. | 36 USMLE Road Map Biochemistry 5. After such a large meal which of the following scenarios describes the relative activity levels for these two enzymes Hexokinase Glucokinase A. Not active Not active B. v Wmax Not active C. v Vmax Not active D. v Vmax v V E. v Vmax v V ANSWERS 1. The answer is D. Many Asians lack a low-Km form of acetaldehyde dehydrogenase which is responsible for detoxifying acetaldehyde generated by oxidation of ethanol in the liver. Acetaldehyde accumulation in the blood of such individuals leads to the facial flushing and neurologic effects exhibited by the man of Japanese descent. 2. The answer is B. A noncompetitive inhibitor binds to the enzyme at a site other than the substrate binding site so it has little measurable effect on the enzyme s affinity for substrate as represented by the m. However the inhibitor has the effect of decreasing the availability of active enzyme capable of catalyzing the reaction which manifests itself as a decrease in Vmax. 3. The answer is D. Organophosphates react with the active site serine residue of hydrolases such as acetylcholinesterase and form a stable phosphoester modification of that serine that inactivates the enzyme toward substrate. Inhibition of acetylcholinesterase causes overstimulation of the end organs regulated by those nerves. The symptoms manifested by this patient reflect such neurologic effects resulting from the inhalation or skin absorption of the pesticide diazinon. 4. The answer is B. The therapeutic rationale for ethylene glycol poisoning is to compete for the attention of alcohol dehydrogenase by providing a preferred substrate ethanol so that the enzyme is unavailable to catalyze oxidation of ethylene glycol to toxic metabolites. Ethanol will displace ethylene glycol by mass action for a limited time during which hemodialysis is used to remove ethylene glycol and its toxic metabolites from the patient s bloodstream. 5. The answer is D. This problem provides a practical illustration .