1. Topic 3.6 - Enzymes
3.6.1 Define enzyme and active site.
• An enzyme is a globular protein acting as a biological catalyst (speeding up reaction rates by
lowering activation energy).
• An active site is the special region on the surface of an enzyme to which substrate(s) bind
(producing an enzyme/substrate complex).
3.6.2 Explain enzyme-substrate specificity.
• Each globular enzyme includes an active site with a specific, three-dimensional shape which is
complementary to the shape of the substrate
• The globular enzyme active site also includes a specific set of charges which are complementary
to the charges of the substrate
• Thus, through complementarity of shape and charge, the substrate is attracted to, and fits precisely
into, the active site
• The precise interactions between enzyme active site and substrate are essential for the catalytic
properties of enzymes to function; the complementarity is often referred to as analogous to the fit
between a lock and a key
• Enzymes vary in specificity from being exclusive to a single substrate to being generalised to
accept any molecule of a certain type
3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity.
Reasons why enzymes are altered by environmental conditions
• each enzyme has a highly specifically shaped active site which is complementary to the shape of
its substrate
• the enzyme active site is a product of its tertiary, or three-dimensional structure, which is in turn
produced by a variety of bonds: covalent, ionic, and hydrogen bonds, as well as hydrophobic
interactions
• each enzyme active site best fits its substrate at a set of optimum conditions
• deviation from optimum conditions alter the bonds which produce the tertiary structure of the
enzyme, thus altering the shape of the active site and its complementary fit to its substrate
pH
• both acids and alkalis denature enzymes
• stomach pepsin is optimised at pH
2
• pancreatic lipase is optimised at pH
8
Temperature
• at lower temperatures, all chemical reactions proceed more slowly, with a general rule of doubling
reaction rates with each 10ºC
increase
• at higher temperatures, the excessive energy breaks bonds that would otherwise create the shape
of the active site; this denaturing the enzyme
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2. Substrate concentration
• at low to medium substrate concentrations, enzyme activity is directly proportional to substrate
concentration; this is because random collisions between substrate and active site happen more
frequently with higher substrate concentrations
• at high substrate concentrations, all the active sites of the enzymes are fully occupied, so raising
the substrate concentration has no effect
3.6.4 Define denaturation.
• Denaturation is a structural change in a protein that results in a loss of its biological properties.
3.6.5 Explain the use of lactase in the production of lactose-free milk.
• Lactose is a disaccharide - it is the major sugar found in milk.
• Lactose intolerance is the inability to digest significant amounts of lactose.
• Lactose intolerance is caused by a shortage of the enzyme lactase, which is produced by the cells
that line the small intestine.
• Lactase breaks down milk sugar into two simpler forms of sugar called glucose and galactose,
which are then absorbed into the bloodstream.
• People who do not have enough lactase to digest the amount of lactose they consume may feel
very uncomfortable when they digest milk products. Common symptoms, which range from mild
to severe, include nausea, cramps, bloating, gas, and diarrhea. The condition is common among
people of northern Asian descent.
• Young children and infants with lactase deficiency should not consume lactose-containing
formulas or foods until they are able to tolerate lactose digestion.
• Lactose-free milk is produced by passing milk through a column of beads containing immobilised
lactase
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