ENZYMES AND METABOLISM
- Chemical reaction – a process that changes/transforms one set of chemicals into another. Chemical reactions always involve changes in the chemical bonds that join atoms in compounds.
- Reactants – elements that begin/enter the chemical reaction
- Products – elements that are produced by chemical reaction
- Energy may be either released or absorbed during the chemical reaction.
- Exothermic reaction/Exergonic – Energy released; reactants have more energy than products.
- Endothermic reaction/Endergonic – Energy absorbed; reactants have less energy than products
- Metabolism; potential energy of molecules = chemical energy
- Cells use energy for:
- Chemical work (rearrange/build/break substances)
- Mechanical work (move flagella)
- Electrochemical work (move charged substances into or out of cytoplasm)
- Cellular metabolism – the sum of all chemical reactions in the cell
- Catabolism – breakdown of molecules to simple molecules
(AB > A+B)
- Anabolism – synthesis of complex molecules from simple molecules
(A+B > AB)
- Enzymes – a catalyst that speeds up the rate of chemical reaction by lowering the activation energy.
PROPERTIES OF ENZYME
- Large globular protein with active sites – Where substrate bounds and reaction catalyzed
- Biological catalyst – speed up reactions
- Highly specific – catalyze only specific reaction.
- Not changed or damaged by the reactions they catalyzed
- Effective in small amounts – can be used repeatedly
- Affected by external factors – temperature and pH
MECHANISM OF ENZYME ACTION
- Lock and key hypothesis (Fisher, 1890)
- Substrate (key) and rigid active site (lock) are complementary
- Substrate fits into rigid active site forming enzyme-substrate complex.
- Then, products leave the active site of enzyme and enzyme regenerated.
- The induced fit hypothesis
- Modified version and widely accepted
- Active site doesn’t fit exactly and flexible
- When enzyme collides with substrate, it will induce slight change of enzyme’s active site to become complementary with substrate.
- Close fit cause stress and distortion of chemical bonds of substrate. Bonds are broken and new bond formed, easier to change substrate to product. Thus, activation energy is lowered.
FACTORS AFFECTING ENZYME ACTIVITY
- Rate of an enzyme reaction measured by; – the amount of substrate change/ the amount of product formed in a period of time.
- Enzyme concentration
- Rate of reaction is directly proportional to concentration of enzyme
- Amount of substrate limiting the graph tails off.
- Substrate concentration
- Rate of reaction is directly proportional to concentration of substrate
- The graph remains at max point when all the active sites are occupied.
- pH
- Optimum pH – Maximum rate of reaction occurs
- Different enzymes have different optimum ph.
- Outside range of optimum pH, hydrogen ion or hydroxide ion increase – alter acidic or basic amino acids – disrupt ionic bonds – alter enzyme shape (denaturation)
- Temperature
- Rate of reaction is directly proportional to temperature
- When heat increase – kinetic energy in molecules increase – the collision between enzyme and substrate increases.
- Different enzymes have different optimum temperature.
- As the temperature higher than the optimum point, H bonds and hydrophobic interactions disrupted – denaturation – enzyme activity decreases.
- At low temp. enzyme inactivate but not denatured.
- Cofactors
- Types of enzymes – contain protein only and contain non protein
- Contain non-protein enzymes consist of protein part (apoenzyme) and non-protein part (cofactor).
- Cofactor – Non-protein substance required by some enzymes to bind with them before they can catalyze the reaction.
- Enzyme-cofactor complex – holoenzyme
COFACTORS
- Inorganic ions (enzyme activators)
- Attach temporarily to enzyme – change shape of active site so reaction can take place.
- E.g., Ca2+, Zn2+, Mg2+, Fe2+. Cl-
- Cl- increase activity of salivary amylase
- Prosthetic groups
- Bind tightly and permanently to apoenzyme.
- Assist in catalytic function of enzyme
- E.g., organic molecules – FAD (flavine adenine dinucleotide), heme
- FAD consist of riboflavin (vitamin B2) function to accept hydrogen
- Coenzymes
- Bind loosely and temporarily
- Small organic molecules
- Readily detach and help to transfer chemical groups, atoms, electrons from one enzyme to other
- E.g., NAD (nicotinamide adenine nucleotide), coenzyme A, ATP
- NAD derived from niacin and function as hydrogen acceptor
INHIBITORS
- Inhibition is a normal part of the regulation of enzyme activity within cells
- Enzyme inhibitors – a variety of small molecules which can reduce the rate of enzyme-controlled reaction.
- Inhibition – competitive (reversible), non-competitive (reversible) and non-competitive (irreversible)
- Competitive inhibition
- Inhibitor’s structure is similar to substrate – inhibitor compete with substrate for the active site
- When the inhibitor enters the active site and binds for temporarily (Enzyme-inhibitor complex) – reduces the rate of reaction
- By increasing the concentration of substrate – rate of reaction increase.
- Non-competitive inhibition (reversible)
- Inhibitor’s structure is different to substrate
- Inhibitor binds at allosteric site – altering enzyme structure – altering the active site structure – substrate cannot enter.
- Also known as allosteric inhibition. A common way of regulating metabolic pathway in cells.
- Increasing concentration of substrate cannot reduce the inhibition.
- End product inhibition – negative feedback by allosteric inhibition of the end-product.
- E.g., regulation of metabolic pathway – control of ATP production.
- When conc. ATP is high, ATP acts as an allosteric inhibitor – biochemicals reaction inhibited – ATP conc. Decrease – ATP leaves the allosteric site.
- Non-competitive inhibition (irreversible)
- Inhibitor binds tightly and permanently – form covalent bond to active site or other parts of enzyme
- Inhibitors bind to enzyme – change the structure – may cause enzyme precipitation.
- E.g., pesticides, nerve gas, heavy metal ions, Indio acetic acid
ENZYME CLASSIFICATION
- Oxidoreductases – reaction catalyzed is redox reaction (oxidation & reduction) by transfer of H, O or electrons. E.g., glucose oxidase
- Hydrolases – hydrolysis of substrate by addition of water e.g., sucrase
- Transferases – transfer of a specific group of atoms from one molecule to another e.g., glycogen phosphorylase.
- Isomerases – rearrangement of atoms within a molecule converting one isomer to another
- Lyases – breaking of chemical bonds without addition of water e.g., pyruvate decarboxylase
- Ligases – formation of new chemical bonds and uses ATP as energy source e.g., aminoacyl-tRNA synthetase.
- Ribozymes – RNA molecule that capable of acting as catalyst
