3. The chemistry of life

3.6 Enzymes

Enzymes: are globular proteins that work as catalysts – they speed up chemical reactions without being altered themselves.Active site: a special region on the surface of the enzyme where substrates bind.Substrate: the substances that enzymes convert into products.

Lock and Key Hypothesis• The substrate and the active site match each other in two

ways: structurally and chemically

a) Large globular protein enzymeb) Active Site where the substrate combines to the enzymec) Substrate which fits the active sited) Activated complex. The substrate is weakened to allow the reaction.e)Unchanged enzyme/ re-used at low concentrationsf) Product of the reaction

3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.

• Explain means to give a detailed account of causes, reasons or mechanisms.

• Effect of temperature on the rate of an enzyme catalysed reaction:

• (a) As the temperature increases enzyme stability decreases. The kinetic energy of the enzyme atoms increases causing vibrations in the enzyme molecule that lead to the hydrogen bonds to breaking, shape changes in the active site.

3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.

• Explain means to give a detailed account of causes, reasons or mechanisms.

• Effect of temperature on the rate of an enzyme catalysed reaction:

• (b) As the temperature increases the kinetic energy of the substrate and enzyme molecules also increases. Therefore more collisions of the substrate with the active site and the formation of activated complex's and product. The rate of reaction is increasing.

3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.

• Explain means to give a detailed account of causes, reasons or mechanisms.

• Effect of temperature on the rate of an enzyme catalysed reaction:

• (c) The optimal temperature (X) is the highest rate of reaction. Compromise between decreasing enzyme stability and kinetic energy of the reactants.

3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.

• Explain means to give a detailed account of causes, reasons or mechanisms.

• Effect of temperature on the rate of an enzyme catalysed reaction:

• (d) Higher temperature increases the kinetic energy of the enzyme atoms so much that they break bonds, change shape of the active site.

3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.

• pH also affects the rate of reaction of an enzyme catalysed reaction.

• At the optimal pH (a) or (b) the maximum rate of reaction is achieved.

• Above or below the optimal pH the rate decreases.

• The change in rate is because bonds are made and broken which change the shape of the active site and therefore decrease the rate of reaction.

• The two enzyme shown in the image illustrate the fact that different enzymes can have very different optimal pH.

• e.g. Blue curve = pepsin (a)= pH3, Red curve =salivary amylase (b)= pH 7.2

• (a) As the substrate concentration is increased the rate of reaction increases.

• There are more collisions between the substrate and the enzyme such that more activated complex's are formed and therefore product per unit time.

• (b) Further increases in substrate also increase the rate but proportionately less than previously.

• The number of occupied active site is increasing and there is competition for the active site.

• (c) The rate is constant.

3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.

Denaturation• Definition: Denaturation is a structural change in a

protein that results in the loss (usually permanent) of its biological properties.

Enzymes are globular proteins, affected by temperature and pH. If the shape of the active site is changed considerably, the enzyme will not work.High temperatures denature the enzyme because the extra energy increases vibration, breaking intra-molecular bonds.pH causes denaturation because it breaks hydrogen bonds.

Lactose-free milk• Production of lactose-free milk is an example of an industrial

process depending on biological methods (biotechnology). These methods are of huge and increasing economic importance.

• Lactose is a disaccharide (glucose + Galactose) milk sugar • Around 90% of all humans show some kind of lactose

intolerance. • People who are lactose intolerant can drink milk if it is lactose

free.• Lactase is an enzyme extracted from yeast that can digest the

milk sugar to glucose and galactose.

Lactose-free milk

• Process:• Milk is treated with

industrially-processed lactase (obtained from Aspergillus niger)

• Lactase is immobilized in alginate beads to avoid enzymes in the final product.

• Milk is re-circulated until all lactose has been converted

7.6 ENZYMESAHL

Metabolic pathways

• Chemical changes in living things often occur with a number of intermediate stages.

• Each stage has its own enzyme.• Catabolic pathways breakdown molecules• Anabolic pathways build up molecules• Metabolic pathways consist of chains and cycles

of enzyme-catalysed reactions.• The product of one reaction is the reactant of

the next reaction.

Metabolic pathways

–Linear chain pathways

• Cycle pathways

Example: Blood clottingPlatelet/cell damage

Enzyme inhibitors• Enzyme inhibitors are molecules that interact

in some way with the enzyme to prevent it from working in the normal manner.

• There are a variety of types of inhibitors including: nonspecific, irreversible, reversible - competitive and noncompetitive.

• Poisons and drugs are examples of enzyme inhibitors.

A competitive inhibitor is any compound which closely resembles the chemical structure and molecular geometry of the substrate. A noncompetitive inhibitor is a substance that interacts with

the enzyme, but usually not at the active site.

Example of competitive inhibition

• Ethanol is metabolized in the body by oxidation to acetaldehyde, which is in turn further oxidized to acetic acid by aldehyde oxidase enzymes. Normally, the second reaction is rapid so that acetaldehyde does not accumulate in the body.

Ethanol Oxidation

Acetaldehyde Aldehyde

oxidase

Acetic acid

• A drug, disulfiram (Antabuse) inhibits the aldehyde oxidase which causes the accumulation of acetaldehyde with subsequent unpleasant side-effects of nausea and vomiting.

• This drug is sometimes used to help people overcome the drinking habit.

Example of competitive inhibitionEthanol

Oxidation

Acetaldehyde Aldehyde

oxidase

Acetic acid

Example of non-competitive inhibition

• ACE inhibitors help control blood pressure.

• When blood pressure drops, the vessels are constricted to reduce the amount of blood.

• In people with hypertension or heart failure, this reaction can make their problem worse.

• ACE inhibitors are medications that inhibit Angiotensin Converting Enzyme – they prevent increased blood pressure.

End-product inhibition• It prevents a build-up of products• Role of allosteric site: now it is the place

where the product can bind, causing a conformational change in the enzyme’s active site, temporarily inhibiting it.