Enzyme Function and Structure

• Enzymes are essential, macromolecules that help speed up chemical reactions in living things
• Enzyme names typically end in "-ase"
• Enzymes are proteins made of amino acid monomers, contain nitrogen, and involve peptide bonds, and are organic carbon compounds

Protein Catalysts

• Enzymes function as protein catalysts
• Enzymes speed up chemical reactions by lowering the activation energy, so less energy is needed for the reaction to occur
• Enzymes are very specific, catalyzing only one specific chemical reaction
• There are many enzymes working at all times
• Enzymes are not changed after the reaction and can be recycled

Enzyme and Substrate Structure

• A substrate is a molecule that interacts with the enzyme
• Active site is location where the enzyme and substrate bind (sit down) together
• Enzymes and substrates fit together like a "lock and key"
• The enzyme modifies its active site to accommodate the substrate in a process called induced fit
• After an enzyme and substrate bind, it is called an enzyme-substrate complex
• The substrate may break apart or bond together to form a product after binding with the enzyme

Factors Affecting Enzyme Action

• Temperature affects enzyme action; the rate of an enzyme increases with higher temperatures, but enzymes can denature (destroy) if the temperature is too high
• pH affects enzyme function; enzymes work efficiently at a specific pH
• Some examples include:
  • Blood: pH of 7 (neutral)
  • Stomach: pH of 2 (acid)
  • Intestine: pH of 8 (base)
• Hormones can signal a cell to start or stop an enzyme from working
• An inhibitor may prevent or slow the enzyme rate by blocking the active site
• The velocity of an enzyme is directly proportional to the concentration of enzyme and the concentration of substrate up to its maximum velocity

Enzyme Inhibitors

• Competitive inhibitors compete with the substrate for binding to the active site and normally resemble the 3D structure
• Non-competitive inhibitors bind to a site other than the active site and deform the enzyme
• Uncompetitive inhibitors bind only to the substrate-enzyme complex
• The allosteric site allows a molecule to stimulate or inhibit enzyme activity, but is not the enzyme's active site where substrates bind

Cofactors and Coenzymes

• Cofactors are non-protein molecules that support a biochemical reaction
• There are two types of cofactors:
  • organic molecules called coenzymes
  • inorganic ions such as zinc or copper ions
• Vitamins enable enzymes to catalyze processes and production of essential proteins
• B vitamins act as a coenzyme
• Minerals are essential to catalytic processess
• Folic Acid (Vitamin B9) is important for expectant mothers
• Thiamine (Vitamin B1) is converted into thiamine pyrophosphate, required to metabolize carbohydrates and amino acids
• Iron-Sulphur Clusters are essential for biological processes involving electron transfers

Biological Enzymes

• Lipase breaks down fats (lipids)
• Protease breaks down proteins
• Cellulase breaks down fiber (cellulose)
• Amylase breaks down starch (amylose)
• Lactase breaks down dairy products (lactose)
• Sucrase breaks down sugar (sucrose)
• Maltase breaks down grains (maltose)

Enzyme Definitions

• Enzymes are specific protein catalysts that accelerate the rate of chemical reactions
• Enzyme structure is not changed by entering reactions
• Enzymes do not affect the equilibrium constant of reactions
• The rate of a chemical reaction is the change in the amount (moles, grams) of starting materials (substrates) or products per unit time
• A substrate is the substance upon which the enzyme acts

Classes of Enzymes

• There are 6 classes of enzymes: oxidoreductase, transferase, hydrolase, lyases, isomerases, and ligases
• Oxidoreductase catalyzes an oxidation-reduction reaction between two substrates
  • Oxidoreductases are classified according to the substrate oxidized and to the mechanism of oxidation
  • The mechanism of oxidation is either by the removal of hydrogen (dehydrogenase) or by the addition of oxygen (oxidases)
  • Examples include alcohol dehydrogenase, lactate dehydrogenase, and cytochrome oxidase
• Transferase catalyzes the transfer of a group other than hydrogen from one substrate to another
  • Enzymes are classified according to the group transferred: phosphotransferases, transaminases, transketolases, transacylase, transformylases, and transmethylases
  • Synthase enzymes are transferase enzymes
  • Examples include phosphotransferases (kinases) and acyltransferases (syntheses)
• Hydrolase catalyzes hydrolysis by adding water (A·B HOH → AH + BOH), peptidase is an example
• Lyases catalyze the addition of carbon dioxide, water, and ammonia across double bonds, decarboxylase is an example
• Isomerases catalyze the interconversion of one isomer into another, examples include isomerases, mutases, and epimerases, and phosphohexose isomerase
• Ligases for synthetases catalyze the joining of two substrates using the energy from ATP or GTP, glutamine synthetase is an example