2.4 Enzymes

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2.4 Enzymes
Biological Catalysts
Enzymes are biological catalysts that speed up metabolic reactions and remain unchanged at the
end of the reaction. The number of reactions that an enzyme can catalyse per second is known as
the turnover number
Active site the indented area on the surface of the enzyme which has a complementary shape to
the substrate molecule
Metabolic reactions ­ chemical reactions inside living cells/organisms
Catalysts can speed up reactions as they can speed up reactions without altering the pH or
temperature and instead function at temperatures and pressures that sustain life. They are more
specific than chemical catalysts because of their complementary shape to the substrate so rarely
produce unwanted byproducts.
Enzyme Structure
Enzymes are proteins and have protein structure (primary, secondary, tertiary,
quaternary) and therefore are coded for by genes
Some use cofactors
Metabolic disorders are the result of deficient enzymes in metabolic pathways
Enzymes also catalyse the formation of an organism's structural components e.g. collagen
in bone and cartilage in blood vessel walls
Active site contains about 6 ­ 10 amino acids and the tertiary structure is essential as the
shape of the active site must be complementary to the substrate molecule
Active sites are changed by temperature and pH as this alters the bonds that hold together
the bonds of the tertiary structure
Intracellular ­ inside the cell
Metabolic pathways contain a series of consecutive reactions each catalysed by a specific
The reactants and intermediates act as substrates and are known as metabolites
Catabolic ­ metabolites broken down, Anabolic ­ larger metabolites synthesised from
smaller ones
E.g. respiration and photosynthesis
Catalase breaks down hydrogen peroxide, has four polypeptide chains and a haem group,
is the fastest acting enzyme with the highest turnover number known, found in
peroxisomes in eukaryotes, white blood cells use it to kill microbes
Extracellular ­ outside the cell

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Amylase is produced in salivary glands to digest polysaccharides
Trypsin is made in the pancreas and digests proteins in the small intestine
Prosthetic groups ­ a cofactor that is permanently bound by covalent bonds to an enzyme
E.g. Zinc ion bound to the active side of carbonic anhydrase
Some cofactors act as cosubstrates and some change the distribution of charge on the enzyme's
surface and make temporary bonds to aid the substrate in binding to the enzyme.…read more

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B12 Cobalamin Pernicious anaemia (progressive and fatal)
Folic acid Tetrahydrofolate Megablastic anaemia ( large irregular erythrocytes)
Nicotinamide, B3 NAD NADP Pellagra (diarrhoea, dermatitis and dementia)
Pantothenate, B6 Coenzyme A Elevated bloodplasma triglyceride levels
Thiamine, B 1 Thiamine Beriberi (confusion, irregular heartbeat, muscular
pyrophosphate weakness, paralysis & heart failure)…read more

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Lockandkey hypothesis
Inducedfit hypothesis
Enzymes lower the activation energy for a reaction.…read more

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Q 10 ­ temperature coefficient = rate of reaction at (T + 10)
C/ rate of reaction at To C
Effect of temperature on enzymes
Extra heat energy causes the molecules to vibrate which increases the rate of successful
collisions between molecules and increases the force with which they collide
Both enzyme and the substrate move faster, therefore the rate of formation of ES
complexes increases and the rate of reaction increases.…read more

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Buffers ­ Can donate or accept protons, resist changes to pH
The tertiary structure is alters as the positive
charges on the protons break the hydrogen
bonds that hold the amino acid in an it's
helix shape. The ionic charges are also
altered and so the active site loses its shape
and can no longer fit the substrate molecule.
Pepsin Salivary Amylase Trypsin
Enzymes work at different pH's as they work in different areas and extra/intracellularly.…read more

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Effect of Enzyme Concentration
If enzyme concentration is increased the enzyme and substrate swap roles as limiting factors.…read more

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It blocks the active site and prevents forming. The active site of the enzyme no
enzymesubstrate complexes. longer has the complementary shape to its
End product inhibition ­ when enzyme catalysed reactions are regulated by the final product
molecule acting as an inhibitor for the first enzyme in the sequence. This is an example of negative
feedback. In metabolic pathways this is noncompetitive and reversible
Cyanide Snake Venom
KCN inhibits aerobic respiration and Green mamba snake venom contains
catalase an inhibitor of acetylcholinesterase.…read more

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It is also used as a blood thinner Calcium ions increase muscle previously had a myocardial
and to reduce stroke risk. contraction and strengthens infarction.
the heartbeat.
Protease inhibitor Nucleoside reverse transcriptase inhibitors
These are used to treat viral infections as they These are used to treat patients who are
prevent its replication within host cells by HIVpositive.
inhibiting protease enzymes so the viral coats They inhibit enzymes involved in DNA using
cannot be made. This is competitive inhibition. viral RNA as a template.
E.g.…read more


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