'Enzyme Inhibition\r'

'Enzyme suppression M each drugs exert their action by banning of an enzyme employment in the body. If the activity of an enzyme is vital to the cell or organism, whence banning may need to end of the cell or organism. It is now assertable to design new drugs which atomic number 18 enzyme inhibitors compriseerly a target enzyme has been identified. Types of Inhibitors A) correctable Inhibitors: The effect of the inhibitor is instantaneous, and it notify be removed from the enzyme by dialysis so that the enzyme activity is returned to normal. such inhibitors interact with the enzyme by weak non-covalent bonds to human body an enzyme inhibitor complex. E + I ? EI B) Ir bilateral Inhibitors: These inhibitors perplex very tightly to the enzyme, some ms by complianceation of covalent bonds to hit an enzyme inhibitor abstruse rather than a loose complex. The effect is therefore for struggled-moving with season reaching a uttermost when all of the enzyme has o pposeed. This is not easily converse by easy physical treatments much(prenominal)(prenominal) as dialysis. E + I > EI correctable suppression of EnzymesThere are three types of reversible enzyme inhibition; competitive, non-competitive ( in addition called mixed) and uncompetitive. warring- moleecules which closely resemble the substratum in size, shape and charge diffusion may also slip into the energetic invest. This may result in response i. e. the second molecule is another subst calculate for the enzyme, or it may result in inhibition because the active site is blocked. The inhibitor has a separate equilibrium with the enzyme. The bond of substratum and inhibitor is mutually exclusive. E + S ? ES > E + P, E + I ?EI each(prenominal) of these equilibria is characterised by a dissociation constant. The set-back by Km (the Michaelis constant) and the second by Ki which characterises the binding mingled with enzyme and inhibitor. If competent [S] is prese nt then eventually the inhibition by I leave be overcome. This is the symptomatic test for this type of inhibition. twain I and S compete for the procurable enzyme. The activity of an enzyme is described by the succeeding(a) equation: (Michaelis- Menton equation) In the presence of a competitive reversible inhibitor, this equation becomes;So the Michaelis constant (which is a reciprocal standard of comparison of E and S) is changed by the mover 1 + [I]/Ki where [I] is the inhibitor concent balancen and Ki is the dissociation constant for the equilibrium amid E and I. Most consequentially, Vmax is unchanged †this is diagnostic for this type of inhibition. Ki is best specify as the stringency of inhibitor considerd to inert the reaction to half the rate it shows in the absence of inhibitor. It is a reciprocal bar of the affinity of E and I. Lineweaver-Burk Plot for Competitive two-sided InhibitionThe intercept on the y axis represents 1/Vmax. Th e slope is adapted by the factor 1 + [I]/Ki, and the easiest way to sum up Ki is from the ratio of the intercepts on the x axis. Without inhibitor the intercept is -1//Km, with inhibitor it is -1/Km(1+[I]/Ki), so the ratio (bigger over smaller so it is greater than 1) is 1 + [I]/Ki. Easiest way to calculate Ki is from the ratio of the intercepts on the x axis. Equation: Other Types of Reversible Inhibition Uncompetitive- This type of reversible inhibition is state of matter to occur when the inhibitor binds with the enzyme-substrate complex rather than the enzyme. substrate and inhibitor bind dependently. Noncompetitive (Mixed)- This type occurs when the inhibitor binds to both the enzyme and enzyme-substrate complex. Substrate and inhibitor bind independently. irreversible Inhibition of Enzymes Reversible  averages that the timescale of the inhibition is similar to that of the enzyme action, usually measured over a few proceedings. Irreversible means that the enzyme activity is check for times significantly pertinaciouser than the search times for the enzyme. It does not necessarily mean that the inhibition leave not change state given sufficient time i. . hours, eld or weeks. Some of the most provoke examples of enzyme inhibitors as drugs are those which fall among the two extremes and are sometimes define as Quasi-Irreversible. These include tight-binding inhibitors, passing state analogues and slowly dissociating arbitrates. Tight-Binding inhibitors and  pitch contour state of matter Analogues form high affinity complexes with the enzyme and may induce Ki values in the hostelry of nano zep (10-9 mol L-1). The value of Ki will be very meaty in describing the potency of this type of inhibitor.As a violent guide the inhibitor concentration causing 50% inhibition (I50) is employ as a measure of Ki. Slowly Dissociating Intermediates react with the enzyme to form covalent intermediates which take time to disassociate from the enzyme. A Classification of Enzyme Inhibitors as Drugs For a compound to work as a drug in vivo it will ideally fuck off TWO very important properties. These are; Potency To work in vivo as an enzyme inhibitor the inhibitor will need to be crocked enough so that the do drugs required is in the piece of milligrams to grams.Specificity If a compound is a non ill-tempered(prenominal) enzyme inhibitor it is to a greater extent likely to be nephrotoxic and b ordinate serious side effects. It may be a poison. Simple Reversible- A simple reversible inhibitor binds to the enzyme and decreases the enzyme activity instantaneously and reverses within the time of the enzyme action. The inhibitor binds non-covalently (ionic interactions, atomic number 1 bonds, Van Der Waals forces) to the enzyme and the strong point of binding is of a similar order to the substrate i. e. Ki will be of similar size to Km. For very devout reasons, the Km values for enzymes v ary between about 10-2 mol L-1 to 10-6mol L-1.Unlikely to be pie-eyed enough to work in vivo where opposition occurs in a dynamic metabolous situation. For a simple competitive inhibitor the inhibition will be self-limiting. If an enzyme is not rate limiting, it may be necessary to carry through ;90% inhibition before any increase in substrate concentration occurs. To do this the inhibitor concentration needs to be approximately 20 times the Ki value. Con makeupally Restricted Competitive Inhibitors- It is possible that a reversible competitive inhibitor which is a conformationally curtail analogue of the substrate will devour a much higher affinity for the enzyme han does the substrate and hence can be slopped enough to work in vivo at reasonable concentrations. Such compounds may require Ki values in the region of 1 x 10-7 mol L-1 Quasi-Irreversible Tight Binding Inhibitors- This is an extension of the introductory class i. e. competitive inhibitors which are conf ormationally certified and/or have many non-covalent interactions confidential information to long lasting complexes. Therefore binding is very tight (Ki in order of 10-9 mol L-1 to 10-10 mol L-1) and these compounds are potent enough to act as drugs in vivo.Transition State Analogues- Theoretically, an analogue of a transit state (or reaction intermediate) for the enzyme catalysed reaction will bind much tighter than an analogue of the substrate. The resolution is a potent and potentially specific inhibitor. Theoretically, Ki values can be very low. In practice if Ki values in the region of Nano molar can be achieved, these are potent enough to work in vivo. As we shall see, there has been much work in this area on peptidases including HIV protease and there are now a major class of drugs which has been developed on this principle.Slowly Dissociating Intermediates- Some enzymes form covalent intermediates as incite of their mechanism e. g. acetylcholinesterase. It i s possible for a compound to act as a pseudo-substrate and be converted into a long lasting intermediate. Such an inhibition is time dependent and in some cases is about permanent. Sometimes the intermediate is hydrolysed in minutes or hours but this is still much longer than the normal enzyme mechanism when the intermediate would last only milliseconds. Examples include the anticholinesterases neostigmine and physostigmine (eserine) and penicillin.Irreversible Nonspecific: a. Heavy metal poisons e. g. cyanide, hydrogen sulphide, carbon monoxide- Some enzymes and other important proteins much(prenominal) as Haemoglobin and Cytochromes, require metals as cofactors. These metals are much transition metals such as Fe, Cu, Mn, Zn and ligands which are electron rich will form co-ordinate covalent bonds with these metals will demob these proteins. These bonds are strong and very often these ligands are toxic because of this irreversible inactivation.Cyanide reacts with cytochrome ox idase which is the final electron carrier in the electron transport chain by ligand formation with the Cu atom at the affectionateness of its mechanism. Similarly, carbon monoxide complexes with the Fe atom in the haem cofactor of haemoglobin. b. Heavy metal ions e. g. mercury, lead etcetera †These are common irreversible inhibitors because of their ability to complex firmly with particular concourses in enzymes. These effects can be reversed by treatment with chelating agents such as EDTA (ethylene di-amino tetra acetic bitter). c. Thiol poisons e. . alkylating agents, Arsenic (III) some(prenominal) enzymes contain thiol (-SH) groups in amino dose side chains †cysteine, which are essential for catalytic activity. Any compound which reacts with these effective groups will poison the enzyme. E. g. Iodoacetamide (alkylating agent) Arsenic- The most toxic form of Arsenic is As (III) as in arsenite AsO2. In this form, Arsenic reacts chop-chop with thiol groups, especially with dithiols such as lipoic sulfurous which is an essential cofactor for some important enzymes such as pyruvate dehydrogenase and -ketoglutarate dehyrdrogenase.You should remember these enzymes as part of the link reaction and the citric acid cycle. When these enzymes are blocked, respiration stops. Arsenic derivatives have been prepared as very condemnable war gases e. g. Lewisite. antidote called Dimercaprol (‘British Anti-Lewisite) was designed by incorporating two thiols for the poison to react with. The two thiol groups react with the arsenical war gas forming a stable compound and thus stopping it from blocking the thiol groups in lipoic acid. Dimercaprol is used these days as an antidote to intoxication with heavy metals such as antimony, arsenic, mercury, bismuth, gold, thallium.It is also used in fellowship with pencillamine in the treatment of lead poisoning (see BNF). Specific Irreversible Inhibitors: Affinity Labels (Active site directed irreversible inhibitors)- An analogue of the substrate which binds to the active site of an enzyme, but which contains a chemically reactive group, has the potential to form covalent bonds with side chains at or near the active site. These inhibitors are irreversible and have been very useful in elucidating enzyme mechanisms but their reactive constitution makes them likely to be toxic when used in vivo.Mechanism-based Inhibitors (‘suicide reagents) †The principle of this potpourri of inhibition is that a pseudo substrate is accepted by the enzyme which then catalyses the intersection of its own inhibitor which reacts covalently in the active site. Such inhibitors should be specific as intimately as potent. Certain monoamine oxidase inhibitors have this mechanism, also the -lactamase inhibitors (e. g. clavulanate). The pyridoxal phosphate (vitamin B6) dependent enzymes have been a particular candidate for the reading of this kind of inhibitor (e. g. difluoromethyldo pa). Enzyme inhibitors:Edrophonium †conformationally restricted competitive reversible, star topology inhibitors †Tight binding, HIV protease inhibitors †Transition state analogues, Neostigmine, Penicillin †Slowly dissociating intermediates DFP †Irreversible group specific reagent, Clavulanate †mechanism-based irreversible inhibitor. Types of Enzyme Inhibitors Simple Reversible| Competitive (also uncompetitive, noncompetitive, mixed)| Simple substrate analogues Michaelis-Menten kinetics Ki in region of Km i. e. 10-2 †10-6 M| Restricted Conformation| hard-and-fast shape similar to favoured substrate exit Ki less than Km| e. g. drophonium as inhibitor of acetylcholinesterase| Quasi-Irreversible| Tight Binding Ki can be in region of nanomolar| E. g. whizz inhibitors Captopril, enalapril etc. | | Transition State Analogues. Binding constant theoretically beneath nanomolar| Inhibitors of proteinases e. g. pepsin, renin, HIV proteinase| | Slo wly Dissociating Intermediates †time dependent kinetics| e. g. neostigmine, eserine as anticholinesterases Penicillin| Irreversible| Heavy metal poisons etc| Cyanide, Hydrogen Sulphide, light speed Monoxide| | Group reagents| e. g. Arsenic (III), Iodoacetamide| | DFP action on esterases| | Affinity labels| TPCK on Chymotrypsin| | Mechanism ground (‘suicide inhibitors)| e. g. Clavulanate onlactamase| Enzyme Inhibitors as Drugs ENZYME| INHIBITOR(S)| USES| Acetylcholinesterase| Edrophonium Neostigmine Eserine| myasthenia Gravis Glaucoma Paralytic Ileus| Monoamine Oxidase| Tranylcypramine| Depression| Xanthine Oxidase| allopurinol| Gout, adjunct to Cancer chemotherapy| Carbonic Anhydrase| Acetazolamide| Diuresis| Dihydrofolate Reductase| amethopterin| Leukaemia|Transpeptidase| Penicillin| Antibacterial| Cyclo-oxygenase| Aspirin etc. Non-steroidal anti-inflammatory drug drugs| Analgesia Anti-inflammatory Anti-platelet| Angiotensin Converting Enzyme (ACE)| Captopril, enalap ril, lisinopril etc. | Anti-hypertension| Thymidylate Synthetase| Fluorouracil| Cancer chemotherapy| Penicillinase (-lactamase)| Clavulanate etc| Anti-bacterial| HIV proteinase| Saquinovar etc| HIV treatment| Reverse transcriptase| AZT| HIV treatment| HMG-CoA Reductase| Statins, pravastatin etc. | Coronary Heart ailment| Phospodiesterase V| Viagra| Erectile disfunction|\r\n'