Physics Made Easy

Enzymes

Enzymes are globular proteins with catalytic properties, i.e. they increase the rate of a reaction by lowering its Activation Energy (the minimum energy needed to initiate a reaction). The reagents or substrates, fit into the active site of the enzyme where they are held together to react and form a product more easily. The Chemistry section covers catalysis in more detail (or will do once it exists).

Lock and Key hypothesis of enzyme action: in this hypothesis, the active site of the enzyme is exactly complementary to the substrate, and the reaction proceeds as below-

locknkey.jpg

The substrate is the “key” which fits exactly into the “lock” of the enzyme.

Induced fit theory of enzyme action: this is a modified version of the lock and key theory. Here, the active site of the enzyme is not quite complementary to the substrate. However, when the substrate enters the active site, its shape is altered so that the substrate now fits exactly.

Enzyme groups:

  1. Oxidoreductases catalyse all oxidation and reduction reactions (i.e. reactions that involve the transfer of oxygen and hydrogen). Dehydrogenase and oxidase are both oxidoreductases.
  2. Transferases catalyse the transfer of chemical/functional groups between substances, e.g. transaminase and phosphorylase.
  3. Hydrolases catalyse hydrolysis reactions e.g. lipases, carbohydrases and proteases.

Catabolic reactions: any reaction where large molecules are broken down into smaller ones.

Anabolic reactions: any reaction where smaller molecules are built up to make large ones.

Together, anabolic and catabolic reactions are classed as metabolic reactions (hence metabolism).

Tip to remember which way round anabolic and catabolic go. Remember, the anode is positive and anabolic is building up (a+) whilst the cathode is negative and catabolic is breaking down (c-).

Effect of temperature on enzyme activity: the working temperature of enzymes is 0-60oC, with optimum temperature at ~40oC (body temperature).

enzymetemp.jpg

If the temperature is too high, there is enough energy to break down the tertiary structure of the enzyme (the weak hydrogen bonds that determine the shape of the molecule are broken). This alters the shape of the active site so that the substrate no longer fits; the enzyme is said to be denatured.

If the temperature is too low, the substrate molecules simple do not have enough energy to move into the active site and be catalysed.

Effect of pH on enzyme activity: the optimum pH for each enzyme is different depending on where in the body it is supposed to work (pepsin works best in acid conditions and is found in the stomach; trypsin works best in weakly alkaline conditions). The graph has the same form as the one for temperature.

pH depends on the concentration of H+ and OH ions (see Chemistry section for more details on pH). If the concentration of H+ or OH is too high, the presence ions will alter the tertiary structure of the protein by breaking the weak hydrogen bonds that determine the shape of the molecule, once again altering the shape of the active site so that the substrate no longer fits.

Effect of substrate concentration on enzyme activity:

substrateconc.jpg

Effect of enzyme concentration on enzyme activity: graph similar to above; as long as excess substrate is present, the reaction rate increases with enzyme concentration. If there is no excess, no further increase is possible (there is simply no more substrate available to be catalysed no matter how much more enzyme is added).

Typically, the concentration of substrate will be about 1000 times the concentration of enzyme.

Non-competitive inhibition: this is where an inhibitor molecule binds to the enzyme away from the active site so that the substrate can no longer fit and the enzyme is inactivated. (effectively the concentration of enzyme is reduced). Cyanide is a non-competitive inhibitor; it inhibits cytochrome oxidase, preventing the transfer of electrons in respiration.

noncompinhib.jpg

Competitive inhibition: in this case, the inhibitor molecule is of a similar shape to that of the substrate, so substrate and inhibitor compete for the active site. When the inhibitor occupies active sites, it decreases the probability that enzyme-substrate complexes will form. Malonate is a competitive inhibitor that inhibits succinic dehydrogenase (competes with succinate molecules).

compinhib.jpg

End product inhibition: imagine a series of enzyme catalysed reactions

A -> B -> C -> D

If excess D (end-product) is formed, it non-competitively inhibits the enzyme that converts A to B (must be a non-competitive inhibitor otherwise the system wouldn’t work at high concentrations of A). This effectively stops the production of B, and thus C and D. As no more D is being made, the excess D will eventually be used up. When this happens, the inhibition on the A ® B reaction is lifted, and the system starts up again. This is an example of negative feedback and is useful in ensuring that endless quantities of unnecessary end product are not produced.

Enzyme cofactors are non-proteins whose presence is essential for the functioning of some enzymes. There are three main types:

  1. Activators are inorganic/mineral ions that combine with the enzyme or substrate, probably to make the formation of the enzyme-substrate complex easier. Salivary amylase requires chloride ions whilst thrombokinase needs Ca2+ ions as activators.
  2. Co-enzymes are organic molecules. NAD is a co-enzyme which picks up excess hydrogen ions, thus maintaining pH for the dehydrogenase enzyme.
  3. Prosthetic groups are organic molecules that are physically bound to an enzyme, e.g. the haem molecule is bound to catalase.

Enzymes in biotechnology: enzymes can be used for the production and detection of certain compounds because, unlike inorganic catalysts, they are specific and do not produce harmful by-products. They can also work at low temperatures and pressures as well as a range of pH values (more cost effective than an inorganic catalyst which might need a high temperature in order to work).

Biosensors: enzymes can be used to detect specific compounds. The example used here is the analysis of glucose:

β-D-glucose + O2 -> gluconic acid + H2O2

DH2 + H2O2 -> 2 H2O + D (coloured compound)

The first reaction is catalysed by glucose oxidase, the second by peroxidase. Obviously, the appearance of the coloured compound D indicates that glucose is present in the original sample.

Thermostability: some enzymes are altered so that they will be stable at higher temperatures. For example, subtilisin is modified for use in detergent so that it will work effectively at 60oC instead of being denatured (important if you want to do a 60oC wash!)

Immobilisation of enzymes: the enzyme is attached to an inert material, e.g. a membrane or ceramic/polymer gel. Reactants can now be passed over the enzyme almost continuously, whilst contamination of products is prevented. Cost-effectiveness is increased as the enzyme can be quickly an easily recovered.

Production of fructose by enzymic hydrolysis of starch: demonstrates one of the many industrial uses of enzymes

  1. First, a-amylase is used to convert starch into dextrin
  2. Saccharifying enzymes are now used to break dextrin down into glucose.

Finally, immobilised glucose isomerase is used to convert glucose to fructose. The product, High Fructose Corn Syrup (HFCS) is used in the States; in Europe its production is restricted to protect the income of sugar beet farmers.

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