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CHEM 19-2 - Factors Influencing Enzyme Activity(cont 2...

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Factors Influencing Enzyme Activity (cont.) 2) Enzyme Concentration – When [E] is substantially lower than [S], the reaction rate is directly proportional to [E]. The cellular concentration of the enzyme can be increased or decreased according to the organism’s need. Environmental Effects: 3) Temperature – A higher temp. generally means faster reaction rate (a rise in 10°C roughly doubles the reaction rate). However, with enzymes (which are often proteins), higher temperatures can cause denaturation. The optimum range is 25-50°C, as over that, disruption of the secondary, tertiary, and quaternary structures often occurs, and the active site(s) is altered such that binding of substrate can’t happen. At either 0°C or 100°C, enzyme-catalyzed reaction rates are usually essentially zero. Practical applications include why we sterilize surgical tools by placing them in boiling water (the enzymes of any bacteria on the tools are denatured) or refrigerate or freeze our food to preserve it from spoilage (to slow down enzyme activity). 4) pH – Enzymes are sensitive to pH changes, and extreme values can cause denaturation. The enzyme shows maximum activity (often in a tight pH range) at its optimum pH, usually ~7 (most bodily fluids are 6-8). However, some enzymes show optimum activity at extreme pH values, such as pepsin (digestive enzyme in stomach) whose optimum pH is 1.5-2.0. Enzyme Inhibition – Irreversible and Reversible An enzyme inhibitor is a substance that decreases the rate of an enzyme- catalyzed reaction. Examples are many poisons and many drugs, such as ACE (angiotensin-converting enzyme) inhibitors used to treat hypertension, such as captopril or ramipril. See p. 3 of handout for description of their operation.
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Irreversible inhibitor – It inactivates the enzyme by forming covalent bonds to a certain group at the active site , such that the substrate cannot bind to the enzyme. The action of cyanide (CN - ) and carbon monoxide (CO) as poisons depend on their ability to inhibit enzymes essential in the utilization of O 2 in cellular respiration. A toxic heavy metal like Hg +2 or Pb +2 can irreversibly bind to the S in Cys residues. Metalloenzymes
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