Candidates should be able to: (a) define enzymes as proteins which function as biological catalysts (b) explain enzyme action in terms of the ‘lock and key’ hypothesis (c) investigate and describe the effect of temperature and of pH on enzyme activity. WHAT ARE ENZYMES o An enzyme is a biological or organic catalyst. o It is a substance which can alter or speed up a chemical reaction, without itself being chemically changed at the end of the reaction. SOURCES o Enzymes may be extracted from any living organism. Sources include bacteria, fungi, yeast, plants and animals. APPLICATIONS o DETERGENT: We use enzymes in our everyday life for example in detergents containing proteases and lipases to remove stubborn stains. o FRUIT JUICES: Pectinase is added to digest pectin
(the “glue” that holds plant cells together) thus reducing wastage, enhance clarity etc. o MEAT TENDERIZER: Papain is extracted
from papaya and used as a meat tenderizer. o CLEAN UP OIL SPILLS: Certain enzymes
produced by microbes can digest harmful oil and convert them into harmless carbon dioxide and water.
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CHAPTER 4: ENZYMES o ANIMAL FEED: Enzymes are added to animal feed to
increase digestibility. WHAT ARE ENZYMES MADE OF? o Enzymes are made of proteins. o Proteins are made up of Amino acids. WHY MUST FOOD BE DIGESTED o Undigested foods are too large to be absorbed by our body. o Digestion is the process by which large, insoluble food molecules are broken down into small, soluble and diffusible molecules. USES OF ENZYMES o BREAKDOWN Enzymes are used to breakdown complex substances to simpler substances, for example the oxidation of glucose to release energy and form carbon dioxide and water. o SYNTHESIS Other enzymes are also used to synthesize complex substances from simpler ones, for example the build up of proteins from amino acids. CLASSIFICATION Enzymes are classified according to the chemical reactions they catalyze. ENZYMES CARBOHYDRASE CELLULASE PROTEASE LIPASE AMYLASE CHARACTERISTICS 1. ALTER OR SPEED
SUBSTRATE carbohydrate cellulose protein lipids starch UP THE RATE OF REACTION
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CHAPTER 4: ENZYMES Enzymes alter or speed up the rate of a chemical reaction. 2. POTENT Enzymes are very potent. They remain unchanged at the end of the chemical reaction and can be reused. A small amount of enzyme can bring about a large amount of chemical reaction. 3. SPECIFIC Enzymes are very specific in their action i.e. protease acts only on proteins, lipase acts only on lipids and not starch. 4. REVERSIBLE REACTIONS Enzymatic reactions are reversible: A+B C+D HOW DO ENZYMES WORK? THE LOCK AND KEY HYPOTHESIS o The substrate is the substance on which the enzymes act e.g. proteins, lipids. o The specificity of an enzyme is due to its shape (surface configuration). o The substrate binds at the active site of the enzyme. o It is believed that the enzyme molecule alters its shape slightly when the substrate molecule fits in, so as to facilitate the reaction. FACTORS AFFECTING ENZYME ACTIVITY 1. TEMPERATURE o The best temperature at which the enzyme work is called the optimum temperature. o At very low temperatures, the enzyme is inactive. o As the temperature increases, the enzyme activity increases (usually, the enzyme is twice as active for every 10 °C rise in temperature until the optimum temperature is reached).
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CHAPTER 4: ENZYMES o However, beyond the optimum temperature, enzyme
Enzyme activity
activity decreases until the enzyme is completely denatured (i.e. no enzyme activity).
most active
Optimum temperature
denature o Denaturation of enzymes: d - High temperature, various chemicals such as inactiv acids and alkalis can denature enzymes. Temperature e - When the enzyme is denatured, the shape is changed, the three-dimensional structure of the enzyme is lost and the active site is altered. - Extreme heat can bring about an irreversible destruction of enzymes. 2. pH o Enzymes are affected by the acidity or alkalinity of the solutions in which they work. o Different enzymes have different optimum pH. Some enzymes work best in slightly acidic solutions (e.g. pepsin and rennin in the stomach) while others require slightly alkaline solutions (e.g. intestinal enzymes). o Extreme changes in pH denature the enzymes.
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Enzyme activity
CHAPTER 4: ENZYMES
most active
Optimum pH
Enzyme activity
3. SUBSTRATE denature& ENZYME CONCENTRATION denature o At a d substrate concentration of A, the enzyme d activity increases as substrate concentration increases up till a point X. pH o A further increase in substrate concentration does not increase the rate any further because all the enzyme molecules are used up (saturated). Enzyme concentration is the limiting factor. o The only way to increase enzyme activity is to increase the enzyme concentration to B. o But when it reaches point Y again, enzyme concentration becomes the limiting factor again.
Y
X
Enzyme concentration B Enzyme concentration A
LIMITING FACTOR Substrate concentration
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CHAPTER 4: ENZYMES o Any factor that directly affects the rate of a
chemical reaction when its quantity is changed is called a limiting factor. o The value of the limiting factor must be increased so as to increase the rate of reaction. COENZYMES o Some enzymes require another compound called a coenzyme to be bound to them before they can catalyse reactions. o Coenzymes are non-protein organic compounds, most are vitamins (B complex vitamins are essential components of many coenzymes)
