2.10: Enzymes

What are enzymes?

• biological catalysts

• proteins

• speed up reactions by reducing the activation energy

• are not altered or used up during a reaction so can be reused

Structure of enzymes

• all enzymes are globular proteins

• they are soluble in water (due to the presence of many hydrophilic groups which face outwards)

• they have a specific tertiary (3d) structure

• the active site, made of just a few amino acids, is involved in catalysis

• the rest of the amino acids maintain the shape of the enzyme and active site

Enzymes play a crucial role in…

regulating metabolism

What are the two categories of reaction (which enzymes catalyse)?

anabolic

catabolic

Anabolic reactions

• the ‘building’ up of large molecules

• large molecules are synthesised from their components

e.g. fatty acids are synthesised in cells by the fatty acid synthetase enzyme complex

Catabolic reactions

• large molecules are broken down into smaller molecules

• these reactions usually release energy

e.g. the enzyme amylase catalyses the hydrolysis of starch into maltose

Where can enzymes function?

inside cells (intracellular)

outside cells (extracellular)

Intracellular enzyme example

• catalase

• decomposes hydrogen peroxide, a harmful byproduct of aerobic metabolism, into water and oxygen

How do extracellular enzymes function outside the cell, and what are they usually used for?

they are secreted by cells to function outside the cell

often used in digestion, where large nutrient molecules need to be broken down

Extracellular enzyme example

• amylase

• produced by salivary glands and pancreas

• catalyses the hydrolysis of starch into maltose

Active site and specificity of enzymes

the active site of an enzyme has a specific shape to fit a specific substrate

the shape of the active site (and therefore the specificity of the enzyme) is determined by the complex tertiary structure of the protein

• the shape of an active site is complementary to the substrate it catalyses

• they bind to form an enzyme-substrate complex

• the enzyme catalyses the reaction and the products are formed in an enzyme-product complex

• the enzyme then releases the products and is left unchanged

Induced fit model of enzyme action

• the enzyme and its active site, and sometimes the substrate can change shape slightly as the substrate molecule enters the enzyme

• these are called conformational changes

What factors affect enzymes?

• temperature

• pH

• enzyme concentration

• substrate concentration

How do temperature and pH affect enzymes?

enzymes work best at an optimum pH and temperature

at extreme pH and temperatures, the shape of the active site changes and enzymes become denatured

this means the enzyme can no longer bind to the substrate

Affect of substrate concentration on enzymes

• initially increasing substrate concentration increases the rate of reaction as there is an increased chance of forming enzyme-substrate complexes

• the rate of reaction plateaus when all active sites are bound to substrates

• at this point the enzyme concentration becomes the limiting factor

Effect of enzyme concentration on rate of reaction

• initially, enzyme concentration increases the rate of reaction as there are more active sites available to form enzyme-substrate complexes

• once all the substrates have bound to active sites, increasing enzyme concentration will have no affect as substrate availability becomes the limiting factor