ENZYME INHIBITOR CASE Intended Learning Outcomes: (ILOs): By the end of this case, the student should be able to: Differentiate between reversible competitive inhibitor, reversible non competitive inhibitor & irreversible inhibitor. Conclude the mechanism of action of inhibitors by knowing its effect on Km & Vmax of the enzyme. Describe the clinical importance of enzyme inhibitors.
Mohamed Ali a 10 years old child admitted to the pediatric out-clinic of Ain Shams University Hospitals by his mother. He was complaining of repeated attacks of headache, sweating and fatigue even on mild exercise. His mother also noted that Mohamed & his younger sister suffered from learning disabilities.
On examination /his skin & mucus membrane of mouth showed severe pallor and a blue line was noticed in the gingival margin close to his teeth. The doctor noticed that the mother was pale too with the same blue line in her gingival margin like her son.
By asking the mother, it was found that the family lives in an old building with LEAD MADE WATER PIPES.
X-Ray Findings
Mohamed Lab tests showed The doctor recommended to do blood analysis for all members of the family including complete blood picture, % estimation of plasma lead concentration. Mohamed Lab tests showed Blood lead level: 20 /dL (normal range: less than 10 g/dL) Hb: 9.0 g/dL (normal range: 12-14 g/dL) Urine: showed presence of -Amino levulenic acid & Coproporphyrin (intermediate products in heme synthesis)
NOW, CAN YOU ANSWER THE FOLLOWING QUESTIONS ? So the doctor treated the child with EDTA & penicillamine to excrete the excess lead in urine. He also advised the family to change directly the water pipes of the family house. He also recommended the family to have healthy diet. NOW, CAN YOU ANSWER THE FOLLOWING QUESTIONS ?
What is the mechanism of enzyme inhibition in this case? What is the enzyme in heme synthesis which is affected by lead poisoning? The enzyme affected is ferrochelatase enzyme which when inhibited heme synthesis decreases, thus causing anemia which is microcytic hypochromic. What is the mechanism of enzyme inhibition in this case? Lead binds irreversibly to the active site of the enzyme by making covalent bond with thiole (sulfhydryle) radical of cysteine residue in the active site. - Vmax - Km unchanged
What is the effect dialysis on this inhibition? What is the effect of increasing substrate concentration on this inhibition? It cannot be reversed. What is the effect dialysis on this inhibition? It cannot be reversed
How to make accurate calculation of Vmax & Km for an enzyme? This done by making lineweaver-Burk plot for the effect of changing substrate concentration on velocity of the reaction: Calculation of Vmax by the hyperbolic curve is not accurate because, it becomes no longer linear close to Vmax Calculation of Vmax by lineweaver-Burk plot is more accurate because it is linear
Study questions: Which is true for a denatured enzyme: It looses its native conformation. Its peptide bonds are broken Enzyme solubility increases It can still binds its substrate & catalyze the reaction Initial velocity of an enzyme – catalyzed reaction is which of the following: Velocity before adding the enzyme Velocity before accumulation of product Velocity at zero substrate concentration The lowest velocity Velocity with substrate concentration equal to Km
A non competitive inhibitor of an enzyme: Increases Km with no change in Vmax Decreases Km & Vmax Increases Vmax with no change in Km Decreases Vmax with no change in Km A coenzyme is which of the followings: The protein part of an enzyme An inactive enzyme An enzyme of different electrophoretic pattern An inorganic cofactor A substance needed for the enzyme action NB: a = apoenzyme b = proenzyme or zymogen c = isoenzyme d = metal ions
An enzyme catalyzes reaction by which of the following mechanisms? Forcing the reaction against natural equilibrium Lowering the activation energy of the reaction Binding the products of the reaction to guarantee its continuity Giving the reactants energy required to make the reaction NB d: is wrong because if the reaction needs ATP, the enzyme does not replace this ATP The enzyme can lower which of the following spaces: A. a B. b C. c D. d
Michaelis [Km] constant equals which of the following? Half the substrate concentration the gives Vmax. The substrate concentration that half saturate the enzyme The initial velocity at half saturation of the enzyme Half Vmax The effect of methotrexate on dihydrofolate reductase & also the effect of malonate on succinic dehydrogenase is? Increased Km with no change in Vmax Decrease Km & decreased Vmax No change in Km & decreased Vmax Increased Km & decreased Vmax
What is meant by induction of an enzyme? Increased synthesis of the enzyme from the gene Increased kinetic energy of the enzyme Conformational change leading to activation of the enzyme Enhanced affinity between the enzyme & its substrate Activation of an inactive enzyme The effect of allopurinol on xanthine oxidase enzyme: Increase the formation off uric acid Decrease both Vmax & Km Can be reversed by dialysis. Can be reversed by increasing substrate concentration. Considered to be a suicide inhibition.
An allosteric molecule is characterized by? Can bind covalently to the enzyme Can bind to the active site of the enzyme Binds non covalently to the enzyme Is derived from a vitamin Which of the followings does not bind to the active site of the enzyme: The substrate The competitive inhibitor The coenzyme The allosteric molecule The cofactor