2.4 Enzymes

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2.4.1 Enzymes- biological catalysts

Key words:

Enzyme: a biological catalyst that speeds up metabolic reactions. A catalyst is a chemical that speeds up the rate of a reaction and remains unchanged a resoluble at the end of the reaction, it lowers the activation energy of a reaction so less energy is required for the reaction to take place.

Turnover number: the number of reactions that an enzyme molecule can catalyse per second.

Active site: indented area on the surface of an enzyme molecule, with a shape that is complementary tot eh shape of the substrate molecule, the tertiary shape of the active site is very important.

Substrate: molecule that is altered by an enzyme-catalysed reaction.

Product: molecule produced from substrate molecules, by an enzyme-catalysed reaction.

Enzyme-substrate complex: complex formed by temporary binding of an enzyme and substrate molecules during an enzyme-catalysed reaction.

Intracellular enzymes (inside the cell):

Catalyses each metabolic pathway in the cell. If metabolised are broken down into smaller molecules and release energy, it is a catabolic reaction. If energy is used to synthesise larger molecules from smaller ones, the reaction is anabolic.

Extracellular enzymes (outside the cell):

They are secreted from the cells for example for the digestive system: amylase and trypsin.

2.4.2 Cofactors

Cofactor:

A substance that has to be present to ensure that an enzyme-catalysed reaction takes place at the appropriate rate.

Prosthetic groups- it is permanently bound by covalent bonds to an enzyme molecule

Co-substrates- attach to the substrate to form the correct shape to fit in the active site

Coenzymes- small organic non-protein molecules that bind temporary to the active site of enzyme molecules, they are chemically changed during the reaction.

2.4.3 The mechanism of enzyme action

The lock-and-key hypothesis:

The active site is complementary to the active site. The substrate and enzyme molecules have kinetic energy and move randomly. If a substrate molecule successfully collides with an enzyme molecule an ES complex forms. The substrate molecules are either broken down or built up to form an enzyme-product complex. The product molecules then leave the active site.

The induced-fit hypothesis:

It's the hypothesis that when the substrate molecules fit into the enzymes active site, the active site slightly changes shape to mould around the substrate molecule. The active site still has a complementary shape but the subtle changes of the side groups (R-groups) of the amino acids give more precise conformation., so they bind more effectively.

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2.4.4 The effect of temperature on enzyme activity

The higher the temperature the more kinetic energy given to the enzyme and substrate molecules so more ES-complexes form, however two high temperatures above the optimum. It may break some of the weak hydrogen and ionic bonds that hold the active sites tertiary structure, so the active sites shape begins to change. The rate of reaction decreases as no substrates can fit into the active site and it eventually becomes denatured.

2.4.5 The effect of pH on enzyme activity

The lower the pH the more excess hydrogen ions on the enzyme, this will interfere with the active sites tertiary structure. Change the hydrogen and ionic bonds and alters the charge ons the active site as they are positive ions so will cluster around negative R-groups.This causes the active site to become denatured.

-Small changes of pH either side of the optimum pH slows the rate of reaction. However not all enzymes have the same optimum pH (pepsin- pH 2) (trypsin- pH 7.5)

2.4.6 The effect of substrate concentration on the rate of enzyme-catalysed reactions

As a substrate is added its concentration increases, the rate of reaction also increases. As more substrate is added more ES-complexes forms more products are formed. Substation concentration is a limiting factor and will reach its maximum rate as all the enzyme molecules will be filled up.

2.4.7 The effect of enzyme concentration on the rate of reaction

The more enzyme molecules, the more active sites become available and the more successful collisions between the enzyme and substrate occur. However enzyme concentration is a limiting factor if there is a fixed number of substrate molecules,as there would be no substrate molecules to fit in all the active sites, so will reach maximum rate.

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2.4.8 Enzyme inhibitors

Key words;

Inhibitor: a substance that reduces or stops a reaction

Competitive inhibition: inhibition of an enzyme, where the inhibitor molecule has a similar shape to that of the substrate molecule and competes with substrate for the enzyme's active site. It blocks the active site and preventions formation of ES-complexes.

Non-competitive inhibition: inhibition of an enzyme, where the competitor molecule attaches to the allosteric site of the enzyme molecule. This changes the shape of the active site, which prevents ES complexes forming. Controlling metabolic sequences.