LEARNING GOALS Describe the role of enzymes and why they are essential to living organisms List the factors that may affect the rate of an enzyme catalysed reaction Describe the importance of the structure of an enzyme to its functioning
ENZYMESEnzymes are molecules that act as catalyststo speed up biological reactions.The compound on which an enzyme acts isthe substrate.Enzymes can break a single structure intosmaller components or join two or moresubstrate molecules together.Most enzymes are proteins. Many fruits contain enzymes that are used in commercial processes. Pineapple (Ananas comosus, right) contains the enzyme papain which is used in meat tenderization processes and also medically as an anti-inflammatory agent.
ENZYME EXAMPLES Enzyme Role Stomach enzyme used to break protein down into Pepsin peptides. Works at very acidic pH (1.5). Digestive enzymes which act Proteases on proteins in the digestive system A family of enzymes which Amylases assist in the breakdown of carbohydrates A family of enzymes which3D molecular structures for the Lipases enzymes pepsin (top) and breakdown lipids hyaluronidase (bottom).
ENZYME EXAMPLES One of the fastest enzymes in the body is catalase. Catalase breaks down hydrogen peroxide, a waste product of cell metabolism, into water and oxygen. Accumulation of hydrogen peroxide is toxic so this enzyme performs an important job in the body.
ENZYME POWER! All reactants need to have a certain energy before they will react. This is like an energy barrier that it has to overcome before a reaction will occur. It is called the activation energy. Enzymes are organic catalysts. All catalysts lower the energy barrier, allowing the reactants (substrates) to react faster forming the products. Enzymes do not participate in the reaction.
FinishENZYMES Without enzyme: The activation energy required is high. Direction of reaction Reactant With enzyme: The activation energy required is lower. High energy Product Low energy Start
ENZYMES Enzymes have a specific region where the substrate binds and where catalysis occurs. This is called the active site. Enzymes are substrate-specific, although specificity varies from enzyme to enzyme. When a substrate binds to an enzyme’s active site, an enzyme- Space filling model of the yeast enzyme hexokinase. Its active substrate complex is formed. site lies in the groove (arrowed)
molecules. It has three active sites (arrowed). Ribonuclease S, that breaks up RNA This model (above) is an enzyme called Enzyme molecule: The complexity of the active site is what makes each enzyme so specific (i.e. precise in terms of theActive site: substrate it acts on).The active site contains both bindingand catalytic regions. The substrateis drawn to the enzyme’s surface andthe substrate molecule(s) arepositioned in a way to promote areaction: either joining two molecules the cleft of the enzyme.together or splitting up a larger one. acts on. They are drawn into chemicals that an enzyme Substrate molecules are the Substrate molecule: ENZYME ACTIVE SITES
LOCK AND KEY MODEL The lock and key model of enzyme action, proposed earlier this century, proposed that the substrate was simply drawn into a closely matching cleft on the enzyme molecule. Products Substrate Symbolic representation of the lock and key model of enzyme action. 1. A substrate is drawn into the active sites of the enzyme. Enzyme 2. The substrate shape must be compatible with the enzymes active site in order to fit and be reacted upon. 3. The enzyme modifies the substrate. In this instance the substrate is broken down, releasing two products.
INDUCED FIT MODEL More recent studies have Two substrate molecules are revealed that the process is drawn into the much more likely to involve cleft of the enzyme. an induced fit. The enzyme The enzyme or the reactants changes shape, (substrate) change their shape forcing the substrate slightly. molecules to combine. The reactants become bound to enzymes by weak chemical bonds. This binding can weaken bonds The resulting end within the reactants product is released by the enzyme themselves, allowing the which returns to its reaction to proceed more normal shape, ready readily. to undergo more reactions.
CHANGING THE ACTIVE SITE Changes to the shape of the active site will result in a loss of function. Enzymes are sensitive to various factors such as temperature & pH. When an enzyme has lost its characteristic 3D shape, it is said to be denatured. Some enzymes can regain their shape while in others, the changes are irreversible.
THE EFFECT OF TEMPERATURE ONENZYME ACTION Speeds up all reactions, but the rate of Optimum denaturation of enzymes Temperature also increases at higher for enzyme Too hot for temperatures. Enzyme to work High temperatures break the disulphide bonds holding the tertiary Too cold structure of the enzymefor Enzyme together thus changing the to work shape of the enzyme. This destroys the active sites & therefore makes the enzyme non – functional.
THE EFFECT OF TEMPERATURE ON ENZYME ACTION The curve in the blue represents an enzyme isolated from an organism living in the artic. These cold dwelling organisms are called psychrophiles. The curve in red represents an enzyme isolated from the digestive tract of humans. The curve in green represents an enzyme isolated from a thermophile bacteria found growing in geothermal sea beds.
THE EFFECT OF PH ON ENZYMEACTION Like all proteins, enzymes are denatured by extremes of pH (acidity/alkalinity). The green curve is for pepsin that digests proteins in the stomach. The red curve represents the activity of arginase that breaks down arginine to ornithine & urea in the liver.
THE EFFECT OF ENZYMECONCENTRATION ON ENZYMEACTION Assuming that the amount of substrate is not limiting, an increase in enzyme concentration causes an increase in the reaction rate.
THE EFFECT OF SUBSTRATECONCENTRATION ON ENZYMEACTION Assuming that the amount of enzyme is constant, an increase in substrate concentration causes a diminishing increase in the reaction rate. A maximum rate is obtained at a certain concentration of substrate when all enzymes are occupied substrate (the rate cannot increase any further).
THE EFFECT OF COFACTORS ONENZYME ACTION Cofactors are substances that are essential to the catalytic activity of some enzymes. Cofactors may alter the shape of enzymes slightly to make the active sites functional or to complete the reactive site. Enzyme cofactors include coenzymes (organic molecules) or activating ions (eg. Na+, K+..) Vitamins are often coenzymes (eg. Vit B1, Vit B6…)
THE NATURE OF ENZYME INHIBITORS Enzyme inhibitors may or may not act reversibly: Reversible: the inhibitor is temporarily bound to the enzyme, thereby preventing its function (used as a mechanism to control enzyme activity). Irreversible:the inhibitor may bind permanently to the enzyme causing it to be permanently deactivated.
THE NATURE OF ENZYMEINHIBITORS Reversible Enzymes work in one of two ways: Competitive inhibitors: the inhibitor competes with the substrate for the active site, thereby blocking it and preventing attachment of the substrate. Non-competitive: the inhibitor binds to the enzyme (but not at the active site) and alters its shape. It markedly slows down the reaction rate by making the enzyme less able to perform its function (allosteric inhibition).
SUMMARY: ENZYMES1. Enzymes work very rapidly and help to speed up biological reactions.2. Enzymes can be used multiple times (however they do degrade eventually).3. Enzymes can work in both directions of a chemical reaction.4. Enzymes have optimal temperatures and pH that they will operate. Beyond these optimum ranges they will either not work or become denatured (unfolded/breakdown).5. Enzymes are usually specific to one particular substrate.