Enzymes: Catalysis and Inhibition

• Enzymes are protein catalysts

• Catalysts accelerate chemical reactions without being consumed

• Catalysts stay intact and are reusable

• Molecules require activation energy to react

• Heat provides this energy, but it can denature and destroy proteins in biological systems

• Enzymes bring reacting molecules closer to assist reactions without heat

• Enzymes lower activation energy to ease transition to the activation complex

• A substrate is the reactant an enzyme acts on, which binds to the enzyme's active site (pocket or groove)

• This forms an enzyme-substrate complex

• Molecular shapes change during the reaction, allowing molecules to break free

• Only specific substrates bind to specific active sites in what is known as the Induced-fit model

• The enzyme can work repeatedly

• Some enzymes need cofactors (inorganic minerals) or coenzymes (organic vitamins) to function

Enzyme Inhibition

• Competitive inhibitors resemble the substrate, flood the active site, and block enzyme function

• With more "regular" substrate, the enzyme can resume normal function

• Noncompetitive inhibitors bind to a different site, altering the enzyme's shape and affecting its function

Allosteric Regulation

• Cells control enzyme activity by:

• Inhibiting the enzyme's action

• Restricting enzyme production

• Enzymes have allosteric sites other than active sites

• An allosteric activator binds to allosteric sites to stabilize the active form

• An allosteric inhibitor binds to allosteric sites to stabilize the inactive form

• Feedback inhibition regulates enzyme levels during sequential reactions

• The end product can act as an allosteric inhibitor, slowing down the initial enzymes in the sequence

• The process controls production and prevents it from getting out of control e.g. negative feedback

Examples of Enzymes

• Enzyme names usually end in "-ase"
• Types include:
  • Proteases
  • Carbohydrases
  • Lipases (digestive)
• Other examples:
  • Lactase: digests lactose sugar
  • Helicase: unravels DNA
  • DNA polymerase: makes DNA
  • Amylase: breaks down sugars
  • Alcohol dehydrogenase: breaks down alcohol

Industrial Uses

• Yeasts convert glucose to ethanol (alcohol) and carbon dioxide in brewing, baking, and winemaking
• Carbon dioxide gas creates air pockets in baked goods
• Corn, wheat, and barley are digested into glucose by enzymes
• Uses include cleaning, and making dairy products, leather, and paper

Metabolism

• Metabolism is the sum of anabolic (building up) and catabolic (breaking down) reactions in living things

• Kinetic energy is energy of motion

• Potential energy is stored energy

• Reactions need activation energy to form an activated complex

• Energy is either released or absorbed in a chemical reaction

• Reactant molecules go through a transition state before changing

• Chemicals store energy

• Exergonic reactions release stored energy if reactants have more energy than products

• Exothermic reactions release heat energy

• Endergonic reactions absorb energy if products have more energy than reactants

• Endothermic reactions absorb heat

• ΔH in diagrams represents enthalpy, i.e., the amount of energy in chemical substances

• It is negative when energy is released and positive when energy is absorbed, according to the equation ∆H = (Hfinal - Hinitial).

• Metabolic reactions are reversible in living systems with the help of enzymes

Redox Reactions

• Redox reactions involve electron transfer

• Oxidation is the loss of electrons

• Reduction is the gain of electrons

• Oxidation (losing) is always accompanied by reduction (gaining)

• Reducing agents provide electrons to reduce something

• Oxidizing agents take electrons and oxidize something

• LEO goes GER: Losing Electrons is Oxidation; Gaining Electrons is Reduction

• Electrons transfer from molecule to molecule along a "chain"

• Each recipient molecule has a stronger pull

• Hot potato analogy

• Oxidation involves gaining oxygen or losing hydrogen

• Reduction involves gaining hydrogen or losing oxygen