Enzyme Structure

• Enzymes are globular proteins
• which catalyse ONE reaction
• i.e. they are SPECIFIC


• enzyme binds to SUBSTRATE
• at the ACTIVE SITE


• active site is a small region 3 -12 amino acid residues on the surface of the enzyme
• active site is COMPLEMENTARY to the shape of the substrate


• GCSE level = Lock and Key Mechanism
• Active site is exactly complementary to substrate


• A Level = Induced Fit
• active site is approximately complementary to substrate
• undergoes CONFORMATIONAL CHANGE to produce exactly complementary


• In both cases an ENZYME-SUBSTRATE COMPLEX is formed

Activation Energy

• In living organisms reactions require an input of energy to start them = ACTIVATION ENERGY
• enzymes reduce the activation energy
• analogous to rolling rock up a hill

Effect of Temperature

• increasing temp = increasing energy in system so speeds up reaction
• increasing temp increases kinetic energy of enzyme and substrate
• so more collisions
• so more enzyme-substrate complexes formed
• so faster rate of reaction
 
• at higher temps bonds in enzyme are broken resulting in denaturation
• so drop in activity
 
• result is an optimum temperature
• enough energy to produce high reaction rate
• but below the temp at which denaturation occurs
• typically 35-40⁰C
• but some enzymes work at higher and lower temps

pH

• enzymes have an OPTIMUM pH
• either side of the optimum changes in pH affect the ionization of R groups
• resulting in conformational change
• so reduced efficiency of ESC formation
• at the extremes - denaturation

Concentration

• if substrate excess - rate increases LINEARLY with inc in conc of enzyme
• i.e. enzyme conc is rate limiting
• will plateau when enzyme conc becomes so high that substrate no longer in excess
• i.e. substrate conc becomes rate limiting
• if enzyme is in excess - rate increases with inc in substrate conc
• i.e. substrate conc is rate limiting

Inhibition

• inhibitors can be competitive or non-competitive
• reversible or irreversible


• competitive inhibitors are complementary to all or part of the active site and bind to it
• they reduce the rate by reducing the number of ESCs formed
• also known as ACTIVE SITE-DIRECTED INHIBITION


• non-competitive inhibitors bind to a site on the enzyme other than the active site
• causing a conformational change in the active site
• reducing the efficiency of ESC formation
• this is NON-ACTIVE SITE-DIRECTED INHIBITION


• reversible inhibitors do not bind permanently to the enzyme their action may be temporary


• irreversible inhibitors bind tightly and permanently to the enzyme
• so the enzyme becomes non-functional

Commercial Use of Enzymes

• enzymes can be manufactured on an industrial scale and harvested - from bacteria and fungi
• pectinases in food manufacture
• plant cells are surrounded by a wall made of pectin (a polysaccharide)
• pectinases degrade the pectin thus destroying the wall
• so the contents of the cell are more likely to leak out
• pectinases are found in fungi and bacteria - they are involved in the decomposition of fruit
• industrial pectinase is derived from Aspergillus and Penicillium
• used to increase the yield of juice by adding to crushed fruit
• proteases in biological detergents
• proteases (aka peptidases or proteinases) hydrolyse peptide bonds
• either endopeptidases ar exopeptidases
• derived from fungi - Aspergillus, Mucor and bacteria - Bacillus spp
• added to washing powder to break down protein stains on clothing
• Note low temp washes here


• In both these cases the enzyme is lost at the end of the process so more needs to manufactured - costly
 

Immobilised Enzymes

• bind the enzyme to a solid medium
• The medium is inert and insoluble
• means the enzyme can be easily removed at the end of the process and re-used - financial benefit
• immobilized enzymes are more stable - do not denature so readily so more efficient


• media include cellulose and polyacrylamide
• in lab - alginate beads can be easily made


• one commercial use is lactase
• many people are lactose-intolerant
• they do not break it down to glucose and galactose
• so it is fermented by bacteria in the intestine to produce gas
• symptoms are nausea, abdominal pain and diarrhoea
• passing milk through lactase treatment reduces the amount of lactose present
• lactase is derived from Aspergillus and the yeast Kluyveromyces spp