Enzymes PDF - Almaaqal University

Almaaqal University Enzymes Dr/ Wael Sobhy Darwish Biochemistry PhD Lec-3  Mechanism of enzyme action  Enzymes are very specific and it was suggested by Fischer in 1890 that this was because the enzyme had...

Almaaqal University Enzymes Dr/ Wael Sobhy Darwish Biochemistry PhD Lec-3  Mechanism of enzyme action  Enzymes are very specific and it was suggested by Fischer in 1890 that this was because the enzyme had a particular shape into which the substrate or substrates fit exactly.  Theories of Enzyme Action 1- The Key and Lock (Fisher model) Theory: The active site of the enzyme is complementary in conformation to the substrate, so that enzyme and substrate recognize each other. - This theory postulates that active site has fixed shape. 2- The Induced Fit Theory (Khoshland model) It is a flexible model. An enzyme-substrate complex forms when the enzyme’s active site binds with the substrate like a key fitting a lock. The substrate molecule does not fit exactly in the active site. This induces a change in the enzymes shape to make a closer fit.  Rate of enzyme reactions The rate of enzyme reaction is measured by the amount of substrate changed or amount of product formed during a period of time. The rate is determined by measuring the slope of the tangent to the curve in the initial stage of the reaction. The steeper the slope, the greater is the rate. If enzyme activity is measured over a period of time, the rate of reaction usually falls, most commonly as a result of a fall in the substrate concentration.  Temperature:  Raising temperature generally speeds up a reaction, and lowering temperature slows down a reaction. However, extreme high temperatures can cause an enzyme to lose its shape (denature) and stop working.  pH pH: Each enzyme has an optimum pH range. Changing the pH outside of this range will slow enzyme activity. Extreme pH values can cause enzymes to denature.  ENZYME CONCENTRATION Enzyme concentration: Increasing enzyme concentration will speed up the reaction, as long as there is substrate available to bind to. Once all of the substrate is bound, the reaction will no longer speed up, since there will be nothing for additional enzymes to bind to.  SUBSTRATE CONCENTRATION Increasing substrate concentration also increases the rate of reaction to a certain point. Once all of the enzymes have bound, any substrate increase will have no effect on the rate of reaction, as the available enzymes will be saturated and working at their maximum rate.  Denaturation: If the hydrogen bonds within an enzyme are broken, the enzyme may unfold or take on a different shape. The enzyme is denatured. A denatured enzyme will not function properly because the shape of the active site has changed. If the denaturation is not severe, the enzyme may regain its original shape and become functional. -Causes of denaturation: – Heat – Changes in pH (Acids and bases). Heavy-metal ions (lead, arsenic, mercury). – Organic Solvents (Alcohol, acetone). – UV radiation.  Enzyme Inhibitors Enzyme inhibitors are chemicals that bind to an enzyme and prevent the formation of an enzyme-substrate complex, thereby preventing the formation of products. The Inhibitors: poisons, like cyanide, antibiotics, anti- metabolites and some drugs. Enzyme Inhibitors Reversible Irreversible Competitive Non-competitive Uncompetitive  Irreversible inhibitors: 1. The inhibitor occupying the active sites by forming covalent bonds or they may physically block the active sites. 2. The inhibitor destroying the globular structure.  Reversible inhibitors Reversible inhibitors attach to enzymes via non-covalent interactions like hydrogen bonds, hydrophobic contacts, and ionic bonds. When attached to an enzyme, reversible inhibitors do not undergo chemical reactions and can be easily eliminated by dilution or dialysis.  Competitive Inhibition The inhibitor binds at the active site and directly prevents the substrate from binding The inhibitor is structurally and chemically similar to the substrate The competitive inhibitor blocks the active site and thus prevents substrate binding As the inhibitor is in competition with the substrate, its effects can be reduced by increasing substrate concentration.  Noncompetitive Inhibition Non-competitive inhibition involves a molecule binding to a site other than the active site (an allosteric site) leading to a conformational change to the enzyme’s active site and indirectly prevents the substrate from binding. As the inhibitor is not in direct competition with the substrate, increasing substrate levels cannot mitigate the inhibitor’s effect.  Uncompetitive inhibition, Uncompetitive inhibition, also known as anti-competitive inhibition, takes place when an enzyme inhibitor binds only to the complex formed between the enzyme and the substrate (the E-S complex).  Category Irreversible Inhibitor Reversible Inhibitor There is covalent bonding Enzymes have a loose Bonding between enzymes and noncovalent bond with an inhibitors inhibitor They permanently block They temporarily block Affect enzyme activity enzyme activity Competitive Inhibitor Group-specific reagent Non-competitive inhibitor Types Reactive Substrate Analogs Uncompetitive Inhibitor Suicide Inhibitors Mixed Inhibitor Enzyme activity can not be Enzyme activity can be Enzyme activity restored restored Their activity can be reduced Substrate concentration has Substrate concentration by increasing substrate no effect on their activity concentration