bio chapter 4

• Biological catalysts are large biological molecules called enzymes
• A catalyst speeds up a chemical reaction without being chemically changed itself at the end
• Cells can rapidly break down carbohydrates, fats, or proteins using catalysts called enzymes without significantly raising body temperature
• Enzymes are biological catalysts made of protein molecules
• Enzymes catalyze the rate of chemical reactions without being chemically changed
• Enzymes provide an alternative pathway with lower activation energy to start a chemical reaction, for example, in the breakdown of glucose

Enzymes and Activation Energy

• Enzymes speed up the rate of chemical reaction by lowering the activation energy
• Some food molecules are large and must be broken down by enzymes into smaller substances to diffuse through the cell membrane

Digestive Enzymes

• Enzymes involved in digestion are known as digestive enzymes
• Examples of digestive enzymes:
  • Amylase digests starch to maltose
  • Maltase digests maltose to glucose
  • Protease digests proteins to polypeptides, which are then digested to amino acids
  • Lipase digests fats to fatty acids and glycerol
• Enzymes either build up or break down complex substances

Enzyme-Catalyzed Reactions

• Enzyme-catalyzed reactions can be classified into:
  • Reactions that build up complex substances (anabolic reactions)
  • Reactions that break down complex substances (catabolic reactions)

Anabolic Reactions

• Anabolic reactions build up or synthesize complex substances from simpler ones
• For example, amino acids taken into cells may be used to build up proteins, catalyzed by special enzymes in the cytoplasm
• In photosynthesis, glucose is synthesized from carbon dioxide and water and controlled by enzymes

Catabolic Reactions

• Catabolic reactions break down complex substances to simple substances
• For example, large food molecules are broken down into smaller molecules by digestive enzymes
• In cell respiration, glucose is broken down to release energy and form carbon dioxide and water through a series of chemical reactions, each catalyzed by a different enzyme
• Both plant and animal cells produce catalase to break down hydrogen peroxide into water and oxygen, removing its toxic effect
• Catalase is abundant in blood and the liver of mammals
• Enzymes catalyze reactions in an organism and are produced when needed
• Enzymes are classified according to the chemical reactions they catalyze:
  • Carbohydrases digest carbohydrates
  • Proteases digest proteins
  • Lipases digest fats (lipids)

Enzyme Characteristics

• Enzymes are highly specific in their actions
• Each chemical reaction is catalyzed by a unique enzyme
• The 'lock-and-key' hypothesis describes that an enzyme is the lock, and the substrate is the key
• The substances on which enzymes act are substrates
• Enzyme reactions depend on the presence of active sites
• Active sites are grooves or 'pockets' on the surface of an enzyme molecule which the substrate molecule with the matching shape can fit
• When the substrate binds to the active site of the enzyme, an enzyme-substrate complex is formed
• It is a temporary molecule when substances bind to the enzyme
• Reactions take place at the active site to convert the substrate molecule(s) into product molecules
• The product molecule(s) separate(s) from the enzyme
• The enzyme molecule remains unchanged and is free to combine again with more substrate molecules

Enzyme Structure and Function

• Enzymes have a 3-dimensional shape with a depression called the active site
• Only a substrate with a complementary 3-D shape can fit into the active site, forming an enzyme-substrate complex
• While the substrate is attached to the active site, a chemical reaction occurs, and the substrate is converted to the products
• The products leave the active site, and the enzyme remains unchanged and can catalyze another reaction
• Enzymes have active sites complimentary to the shape of their substrates
• High temperature, acids, and alkalis can affect the shape of an enzyme and, consequently, its function
• Enzymes are used in minute amounts and remain unchanged at the end of reactions
• Enzymes are effective molecules, where the same enzyme can be used repeatedly and catalyze a large amount of substrate

Factors Affecting Enzyme Activity

• Enzymes are affected by temperature
• At low temperatures, enzymes are less active due to low kinetic energy
• As the temperature increases, the rate of enzyme reaction increases and effective collision is increased
• The optimum temperature is where the enzyme is most active
• Most human enzymes have an optimum temperature of about 40°C-45°C
• Increasing the temperature above the optimum temperature causes a rapid decrease in the rate of enzyme reaction, leading to denaturation
• The active site of the enzyme begins to lose its shape and is no longer complementary to the shape of substrate molecules, causing enzymes to lose their shape or denature
• Most enzymes are denatured at high tempuratures

Denaturation

• Denaturation is the change in the 3-D structure of an enzyme or soluble protein caused by heat or chemicals (acids/alkalis)
• Denaturation results in the loss or alteration of the enzyme's active site
• The substrate can no longer fit into the enzyme's active site, and no reaction will occur
• Enzymes are also affected by pH
• Some enzymes work best in acidic solutions, and others work best in alkaline solutions
• When enzymes are placed in pH conditions that vary from the optimum, they start to denature
• As the solution becomes acidic or alkaline from the optimum pH, its activity decreases
• At certain pH levels, the enzyme is completely denatured