Enzymes
Keywords
Active site: Indent on the surface of an enzyme with a complementary shape to the substrate
Catalyst: Chemical that speeds up the rate of a reaction and remains unchanged/reusable at the end of the reaction
Extracellular: Outside the cell
Intracellular: Inside the cell
Metabolism: Chemical reaction that takes place inside living cells or organisms
Product: Molecule produced by an enzyme-cataylsed reaction
Substrate: Altered molecule from the enzyme-catalysed reaction
Active site
Tertiary structure of the active site is complementary to the shape of the substrate
Each enzyme is highly specific to its function
Shape of the active site can be altered by changes in temperature or pH as these affect the bonds that hold proteins in their tertiary structure
Catalase
Consists of 4 polypeptide chains
Contains a haem group with iron
Fastest acting enzyme/highest turnover number or about 60 million per second
In eurkaryotes, it is found inside peroxisomes
When white blood cells ingest pathogens they use catalase to help kill the microbes
Amylase
Produced in the salivary glands to digest polysaccharide starch to maltose
Made in the pancreas and acts to catalyse the same reaction in the lumen of the small intestine
Trypsin
Produced in the pancreas
Acts in the lumen of the small intestine
Digests proteins into small peptides by hydrolysing peptide bonds
Cofactor: Substance that has to be present to ensure enzyme-catalysed reactions occur at the right rate
Enzyme-substrate complex: Complex formed by temporary binding of enzyme and substrate during an enzyme-catalysed reaction
Prosthetic group: Cofactor that is permanently bound, by covalent bonds, to an enzyme molecule
Cofactors
Some act as co-substrates
Substrate and cofactor together form the correct shape to bind to the active site of the enzyme
Some change the charge of distribution
Changing the charge of distribution on the enzyme's active site makes the temporary bonds in the enzyme-substrate complex easier to form
Co-enzymes
Co-enzymes: Non-protein cofactors that bind temporarily to the active site of the enzyme either just before or at the same time as the substrate, but are chemically changed during the reaction
Small, organic, non-protein cofactors
Bind temporarily to the active site either just before or at the same time as the substrate
Is chemically changed during the reaction so must be recycled to their original state
Enzyme-product complex: Enzyme molecule with product molecules in its active site, joined temporarily by non-covalent forces
Lock & Key: Tertiary structure of the enzyme's active site is complementary to the substrate molecule
Substrate molecule fits into the enzyme's active site and temporarily forms hydrogen bonds, holding the two together
The substrate molecule is broken down into smaller product molecule that leave the active site
Bonds form between substrate molecules, forming an enzyme-product complex
The larger product leaves the active site
Induced Fit
Presence of the substrate molecule in the enzyme's active site induces a shape change, making the shapes more complementary/precise
Q10: Temperature coefficient calculated by rate of reaction at (T+10C)/rate of reaction at TC
Temperature
Both types of molecules will gain kinetic energy/move faster
Increase the rate of successful collisions
Rate of ES complexes increases, rate of reaction increases up to a point
May break some weak bonds, such as the hydrogen and ionic bonds that hold the tertiary structure of the active site
As the active site changes shape, it is no longer complementary to the substrate
Reaction can't occur as the enzyme has denatured
Concentration
Concentration: Number of molecules per unit of volume
Substrate
More ES complexes can form
More product is formed
As it is increased further the rate will no longer increase as substrate concentration is no longer the limiting factor
Enzyme degredation
Cells continuously degrade old enzyme molecules to their component amino acids and synthesising enzyme molecules from amino acids
Leads to elimination of abnormally shaped proteins that might otherwise accumulate/harm the cell
Leads to regulation of metabolism in the cell by eliminating any superfluous enzymes
pH
Buffers can donate/accept hydrogen ions so can resist changes in pH levels
Excess hydrogen ions will interfere with the hydrogen/ionic bonds so the active site will change shape, decreasing the rate of reaction
Will also alter the charges on the active site as more protons will cluster around negatively charged groups on the active site
Inhibitors
Competitive
Competitive inhibition: Blocks the active site and prevents formation of ES complexes
Inhibitor: Substances that reduce or stop a reaction
Non-competitive inhibition: Changes the shape of the active site, stopping ES complexes from forming
Compete directly with substrate molecules, forming an enzyme-inhibitor complex
Not changed by the enzyme
Prevents substrate from joining to the active site
Reduces number of free active site
Most are reversible
Called an inactivator if irreversible
Non-competitive
Attach to the allosteric site and disrupt the tertiary structure of the active site
Prevents ES complexes from forming as shape is no longer complementary
Some are reversible, some irreversible
End product
Product molecules may remain tightly bound to the active site (negative feedback)
Inhibition examples
Cyanide
Snake venom
Aspirin
ATPase inhibitors
Protease inhibitors
Nucleoside reverse transcriptase inhibitors