Ian Anderson (2013)Saint Ignatius College Geelong05. Enzymes.
What are enzymes?Enzymes are proteins (globular).Act as organic catalysts. Enzymes speed up the rate of a chemical reactionwithout themselves being used up (consumed) duringthe reaction.Are essential to the functioning of all cells. Without enzymes, metabolism would be too slow &insufficient energy would be available to maintain life.
How do enzymes work?Enzymes being catalysts work by lowering theactivation energy (activation energy = the amount ofenergy required to ‘kick start’ the chemical reaction).Source: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEnzym.html
How do enzymes work?Enzymes don’tChange the direction of a reaction, orAlter the amount of product formed (or substrateused).Enzymes speed up the rate at which a chemical reactionoccurs without being used up during the reaction.
How do enzymes work?Enzymes are very specific in their action, althoughspecificity varies from enzyme to enzyme.There is a specific region of the enzyme (the activesite) where the compound being acted on (thesubstrate molecule ) binds, allowing the enzyme tocatalyse the reaction.When a substrate binds to an enzyme’s active site, anenzyme-substrate complex is formed.While attached to the substrate, the enzyme causes aweakening of certain chemical bonds in the substratemolecule thus allowing the reaction to proceed morereadily.
How do enzymes work?Two models – Lock and key & Induced Fit.Lock and key model. The configuration of the active site is complimentary tothe shape of the substrate molecule.Source:http://neurobio.drexel.edu/GalloWeb/loudon_enzymes.htm
How do enzymes work?Induced fit model. The shape of the active site changes when the substrateattaches to it. This ensures a good fit.Source:http://neurobio.drexel.edu/GalloWeb/loudon_enzymes.htm
How do enzymes work?Catabolic reactions. Involve the breakdown of larger molecules into smallermolecules, with the release of energy (exergonic). e.g. respiration and digestionAnabolic reactions. Smaller molecules are joined together to form largerones. Requires the input of energy (endergonic). e.g. protein synthesis, photosynthesis
How do enzymes work?Sometimes an enzyme’s active site is not a perfect fit andthe enzyme remains inactive until it can bind withother molecules or ions that change the shape of theactive site (allowing them to achieve the perfect fit andcapture substrate molecules). Two classes of substanceplay this role:Cofactors (small inorganic substances such as Zn & Mgions), &Coenzymes (non protein, organic substances such asvitamins).
Examples of enzymes.There are over 2000 know enzymes.Enzymes are typically named for their substrates Maltase is the enzyme that acts on the disaccharidemaltose. Lipases act on lipids. Amylase acts on amylose.Exceptions to the naming rule include pepsin &trypsin.
Factors affecting enzyme activity.Enzymes are very sensitive to the conditions in whichthey work. They usually have a narrow range ofconditions under which they operate properly.Factors affecting enzyme activity Temperature. pH. Enzyme concentration. Substrate concentration. Enzyme inhibitors
Factors affecting enzyme activity.Temperature Enzymes usually work best in the temperature of theenvironment in which they are found in. e.g. most human enzymes have an optimumtemperature of ~37ºC (= body temperature). At high temp enzymes are permanently denatured. At low temp enzyme activity slows down, however nopermanent damage is done to the enzyme.
Factors affecting enzyme activity.pH Most human enzymes have an optimum pH between 6and 8. e.g. Trypsin has an optimum pH of 8.0. Enzymes that are secreted into the stomach have a muchlower optimum pH. e.g. Pepsin has an optimum pH of1.5. If pH is too high or low, enzyme can become denatured.
Factors affecting enzyme activity.Enzyme concentrationRate of reactioncontinues to increasewith an increase inenzyme concentration(assuming nonlimiting amount ofsubstrate)Source:http://www.rsc.org/education/teachers/learnnet/cfb/enzymes.htm
Factors affecting enzyme activity.Substrate concentrationRate of reactionincreases up to a point.After this rate levels offbecause all enzymemolecules are workingat their maximumcapacity (i.e. activesites are saturated). Source:http://resources.edb.gov.hk/biology/english/health/enzyme/enzyme_factors_subconc.html