Aspects of the vitamin, food and drugs GCSE chemistry are on the "Extra Organic Chemistry" page.

Living cells use chemical reactions to produce new materials. Living things produce catalysts called enzymes which allow chemical reactions to occur quite quickly at ordinary temperatures and pressures. Enzymes are powerful 'biochemical catalysts' and are widely used in the food industry and are being used more and more to manufacture many other chemicals. These biological catalysts promote most of the reactions in living tissue. The names of enzymes end in ...ase e.g. amylase, protease, invertase, isomerase etc.

• Cells contain protein molecules that act as biological or biochemical catalysts, they are known as ENZYMES.

• The chemical reactions brought about by living cells are quite fast in conditions that are warm rather than hot.

• This is because the cells use these enzyme catalysts. The 'key and lock' mechanism is explained later on. 

• Enzymes are protein molecules which are usually damaged by temperatures above about 45º C. Although not damaged by lower temperatures, the reactions may be too slow to be of any use. (see rates notes at the end of this section)

• Different enzymes work best at different pH values.

• The enzymes in yeast cells (living organism's) convert sugar into ethanol ('alcohol') and carbon dioxide in the brewing and drinks industry. A similar process is used to convert sugar cane into ethanol and distilled to use as biofuel.

  • e.g. glucose ==> ethanol and carbon dioxide in water and the absence of air.

  • C₆H₁₂O_6(aq) ==> 2C₂H₅OH_(aq) + CO_2(g)

  • This process occurs efficiently between 25 to 55^oC and is called fermentation and is used to produce the alcohol in beer and wine. The carbon dioxide dissolved in the final alcoholic drink produces the fizz!

  • Note on raising agents in cooking: It is this reaction producing bubbles of carbon dioxide which make dough mixtures rise in the kitchen or food industry when yeast is used in baking bread or cake making etc.

    • An alternative to yeast is to use sodium hydrogencarbonate ('sodium bicarbonate' or 'baking soda') in baking. The rising action is also due to carbon dioxide gas formed from its reaction with an acid (e.g. tartaric acid), and nothing to do with enzymes:

      • self-raising baking powder = carbonate base + a solid organic acid, giving

      • sodium hydrogencarbonate + acid ==> sodium salt of acid + water + carbon dioxide

  • A simple laboratory test for carbon dioxide is that it forms a milky precipitate with limewater.

  • However other enzymes in living material can also catalyse oxidation with the oxygen in air. When alcoholic drinks turn sour it is due to the alcohol being oxidised to the weak organic acid ethanoic acid, commonly know as 'vinegar'!

• Enzymes are involved in the following processes in the home

  • bread dough raising (see above)

  • biological detergents may contain protein-digesting protease enzymes and fat-digesting enzymes lipase enzymes.

• In industry, enzymes are used to bring about reactions at normal temperatures and pressures that would otherwise require more expensive and more energy demanding equipment e.g.

  • Proteases break down proteins and are used to 'pre-digest' the protein in some baby foods.

  • Carbohydrases are used to convert starch syrup into sugar syrup.

  • Invertase is used to make the sugar for soft chocolates.

  • Isomerase* is used to convert glucose syrup into fructose syrup, which is much sweeter and therefore can be used in smaller quantities e.g. in slimming foods.

    • (* The name comes from the word 'isomers' which means molecules of the same molecular formula but different structures. Glucose and fructose both have the molecular formula C₆H₁₂O₆).

  • Pectinase breaks down insoluble pectin polysaccharides and so is used in clarify fruit juices.

  • Amylases break down carbohydrates and Lipases break down fats.

  • Enzymes are used in genetic engineering and penicillin production.

  • The dairy industry uses enzymes made by microorganisms (bacteria) to produce yoghurt and cheese from milk.

    • The bacteria enzymes convert the sugar in milk (lactose) into lactic acid.

  • Enzymes in biological detergents help break down staining food materials.

• Successful industrial processes often depend on the immobilization of the enzyme:

  • Methods of immobilisation:

    • It can be trapped in a silica gel lattice or collagen matrix or cellulose fibres.

    • Encapsulating in beads of alginate or polymer microspheres can also be used immobilize enzymes.

      • In both cases the use of a matrix, mesh or small particles means the solvent the containing reactant molecules can readily flow by the enzyme giving a good rate of reaction - its effectively giving a good 'surface area' of contact between reactants and enzymes.

  • Advantages of immobilisation:

    • Stabilising the enzyme keeps it functioning for a longer period because it can be easily recovered for further use.

    • To immobilise the enzyme allows a continuous process, this means a continuous input of raw materials and output of product, so can run 24 hours a day for many weeks or months efficiently.

    • A batch process means loading the reactor vessel with reactants, extract the products, clean out the reactor/fermentor, re-load with reactants etc. etc. i.e. less efficient, costs time and so is less economic!

    • There is less contamination of the product with the enzyme because of ease of separation

This would be the graph for an enzyme with an optimum pH of 7.0

What is the optimum pH of an enzyme?

pH effect: The structure of the protein enzyme can depends on how acid or alkaline the reaction medium is, that is, it is pH dependent. If it is too acid or too alkaline, the structure of the protein is changed and it is 'denatured' and becomes less effective. In the optimum pH range, the enzyme catalysis is at its most efficient.

If the enzyme does not have the correct 'lock' structure, it cannot function efficiently by accepting the 'key' substrate molecule. Most enzymes have an optimum pH of between 4 and 9, and quite frequently near the neutral point of 7.

However, the enzyme pepsin has a peak at pH2 and can operate in the very acid (hydrochloric) conditions of the stomach to help breakdown proteins for digestion in the small intestine.

Temperature: The structure of the protein enzyme can depend the temperature. If the enzyme does not have the correct 'lock' structure, it cannot function efficiently.  The shape of the graph is due to two factors. (1) The initial rise in rate of reaction is what you normally expect for any chemical reaction. The increase in temperature increases the average kinetic energy (KE) of the molecules to increase the chance of the product forming from the higher KE 'fruitful collisions. (2) However as the temperature rises further, the increasing thermal vibration of the enzyme molecule causes its structure to break down (denature) and so the 'lock' is damaged so the enzyme is less efficient (see key-lock below). This may be due to the failure of weak intermolecular forces or actual ionic/covalent bonds, but the 3D molecular structure  of the enzyme is changed so that the substrate molecule cannot 'dock in' to be changed into products. The optimum temperature for the fastest rate of reaction is often around 30-40^oC (note our body temperature is about 37^oC, no coincidence!). Eventually at high temperatures the enzyme ceases to function.

• KEY in sequence: E = free enzyme, S = free substrate reactant molecule, E-S = enzyme-reactant complex, E-P = enzyme-product complex, E = free enzyme, P = free product

• The enzyme is a complex protein molecule, but there is a particular site where the reactant molecule 'docks in' by random collision. The enzyme is sometimes referred to as the 'lock' and the initial reactant substrate molecule as the 'key', hence this is called the 'key and lock' mechanism. This is also explains why enzymes are very specific - you need the right molecular key for a particular molecular lock.

• Once the 'reactant-enzyme complex' is formed the enzyme function changes the reactant molecule into the new product molecule.

• The 'enzyme-new molecule complex' breaks down to free the new product molecule and the enzyme who's reactive site can now be re-used by another reactant molecule. 

  • Note 1. Compared to the un-catalysed reaction, the enzyme provides a 'chemical change route' with a much lower activation energy, and so this greatly increases the rate of reaction as more molecules have enough kinetic energy to react at the same temperature.

  • Note 2. The products are shown as two molecules, because there are quite often two products for each step of the breakdown of a bigger molecule into smaller molecules e.g. protein to 'smaller protein' + amino acid, or starch to 'smaller starch' plus a glucose molecule etc.  But there can be just one product molecule e.g. when isomerase changes glucose into fructose. There can also be two substrate reactant molecules being combined to form a bigger molecule or a long natural polymer molecule like starch being broken down to small sugar molecules. In other words there are lots of possibilities!

  • Note 3. Many drugs work by blocking the sites normally used by enzymes. The molecular key (the drug) goes onto the reactive enzyme site, but stays there, so inhibiting enzyme activity which promotes harmful chemical-organism effects in the body. The harmful effect might be the production of toxic chemicals from a bacteria or the reproduction of a harmful organism etc.

  • Note 4. "Rates of Reaction Notes" fully explains all the factors affecting the rate of any chemical reaction, including explaining experimental methods and reaction profile diagrams and activation energy.

  • Note 5. Different reactions need different enzymes, and also if enzymes, which bring about the same chemical change, are quite likely to have different optimum rate pH's or temperatures. this phenomena is known as the specificity of enzymes is related to the unique structure of each enzyme and its 'reactivity' limited to interaction with particular substrate molecules.

• -

Extra Advanced chemistry notes on Enzyme structure on the stereochemistry page

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