Enzymes: Biological Catalysts

• Enzymes are biological catalysts, typically proteins, that accelerate chemical reactions without being consumed or altering the reaction products.
• A typical living cell contains about 4000 different enzymes.
• Enzymes are essential for controlling cellular activity in all living organisms.
• A missing or defective enzyme can have disastrous results.
• Lipase speeds up the hydrolysis of lipid triglycerides.
• Sucrase speeds up the hydrolysis of sucrose into glucose and fructose.
• These reactions are important for a cell's energy needs and survival.
• Enzymes catalyze specific cellular reactions due to their unique three-dimensional shape.
• Enzymes lower the energy barrier of a reaction by binding to a specific reactant, called a substrate, allowing the reaction to proceed faster.
• In a reaction, an enzyme combines briefly with the substrate and remains unchanged after releasing the products.
• Catalase breaks down hydrogen peroxide into water and oxygen.
• Each enzyme type catalyzes reactions for one molecule type or a group of related molecules.
• Enzyme specificity is why a typical cell needs about 4000 enzymes to function properly.
• The substrate interacts with a small region on the much larger enzyme called the active site.
• The active site is a pocket or groove that forms when the enzyme folds into its correct three-dimensional shape.
• The lock-and-key hypothesis was proposed to explain enzyme and substrate interactions.
• The induced-fit model explains the enzyme-substrate relationship, where the enzyme changes shape upon substrate binding.

Induced-Fit Hypothesis

• Enzymes are flexible, not rigid like locks.
• Enzymes change shape or conformation before substrate binding, making the active site precise for binding.
• An enzyme binds to one or more substrates, forming an enzyme-substrate complex.
• Enzymes convert substrates into products and can bind to other substrate molecules repeatedly.
• The rate at which enzymes catalyze reactions varies.
• Typical rates range from 100 to 10 million substrate molecules per second.

Cofactors and Coenzymes

• The term cofactor refers to a non-protein group that binds to an enzyme and is essential for catalytic activity.
• Cofactors are often metals like iron, copper, zinc, and manganese.
• Even small amounts of these metals are essential for the catalytic activity of enzymes.
• An enzyme requires a magnesium cofactor to function properly in the chemical pathway within mitochondria for energy production.
• The term coenzymes are what organic cofactors are called, and often come from water-soluble vitamins perform similar roles to cofactors.
• Many coenzymes shuttle molecules from one enzyme to another.
• Nicotinamide adenine dinucleotide, or NAD+, derived from vitamin B3 (niacin), is a coenzyme that acts as an electron carrier.

Factors Affecting Enzyme Activity

• Enzyme activity can be altered by conditions like enzyme and substrate concentration, temperature, and pH.
• Control mechanisms adjust reaction rates to meet the needs of the cell's chemical products.
• The concentration of both the enzyme and the substrate affects the rate of catalysis.
• The rate of reaction is proportional to the enzyme concentration if there is excess substrate.
• Increasing the substrate concentration increases the rate of reaction up to the saturation level when the enzyme amount is constant.
• The reaction rate levels off when enzyme molecules are saturated with the substrate.
• Enzyme inhibitors lower the rate at which an enzyme catalyzes a reaction by binding to the enzyme and decreasing its activity.
• Some inhibitors bind to the active site, while others bind to sites elsewhere on the enzyme structure.
• Competitive inhibition occurs when inhibitors with shapes similar to the normal substrate compete for access to the active site.
• Noncompetitive inhibition involves specific molecules inhibiting enzyme activity without competing for the active site.
• Noncompetitive inhibitors bind to an enzyme at a location other than the active site, changing the enzyme's shape and reducing its ability to bind efficiently.
• In reversible inhibition, the binding of the inhibitor to the enzyme is weak and readily reversible so enzyme activity returns to normal.
• Irreversible inhibition includes inhibitors that bind so strongly to enzymes through covalent bonds, disabling the enzyme, so the cell must synthesize more of the enzyme to overcome it.
• Cyanide is toxic as it binds to and inhibits cytochrome oxidase, an enzyme that catalyzes a key step in cellular respiration.
• Penicillin, an antibiotic, inhibits the synthesis of peptidoglycan, a key component of the bacterial cell wall.
• Penicillin binds to the active site of transpeptidase, destroying the molecule and inhibiting the formation of peptide bonds between amino acids.
• Allosteric regulation is where a protein's function at one site is affected by a molecule binding to a separate site.
• Allosteric regulation may inhibit or stimulate enzyme activity.
• Binding of an allosteric activator molecule stabilizes the enzyme in a shape that causes its active site to have a high affinity for its substrate.
• Binding of an allosteric inhibitor stabilizes an inactive form of the enzyme.
• Feedback inhibition acts as a regulator where, if the product is scarce, the inhibition is reduced, and the reaction rate increases, by one of the products of a pathway.
• Feedback inhibition prevents cellular resources from being wasted in the synthesis of molecules at intermediate steps in the pathway.
• Isoleucine, the end product, is an allosteric inhibitor of the first enzyme in the pathway, threonine deaminase; in the biochemical pathway that makes the amino acid isoleucine from threonine.
• If the cell makes more isoleucine than it needs, isoleucine combines reversibly with threonine deaminase at the allosteric site and threonine deaminase is converted to the low-affinity state, which inhibits its ability to combine with threonine, the substrate for the first reaction in the pathway.
• If isoleucine levels drop, the allosteric site of threonine deaminase is vacated, and threonine deaminase converts to the high-affinity state and isoleucine production increases.

pH and Temperature Effects

• Changes in pH and temperature strongly affect the activity of most enzymes.
• Enzymes reach maximal activity within a narrow range of temperatures and pH values.
• Enzyme activity drops off at levels outside these ranges.
• Each enzyme typically has an optimal pH where it operates at its highest efficiency.
• As pH increases or decreases away from the optimal value, the catalyzed reaction rate decreases.
• Most enzymes have a pH optimum near the pH of their cellular contents, around pH 7.
• Enzymes secreted from cells may have more variable pH optima.
• Pepsin, a protein-digesting enzyme, is secreted into the stomach and has a pH optimum of 1.5.
• Trypsin, breaks down protein and has a pH optimum of about 8 for proper function in the mildly alkaline contents of the intestine.
• The effects of temperature changes on enzyme activity reflect two processes.
• As temperature rises, the rate of an enzyme catalyzed chemical reaction increases up until a certain point.
• The second is that as temperature rises, the kinetic motions of the amino acid chains of an enzyme increase; as this happens hydrogen bonds and other forces break down, causing the loss of function.
• The rate of the catalyzed reaction doubles for every 10 °C increase in temperature in the range of 0 to about 40 °C.
• Above 40 °C, the increasing kinetic motion begins to unravel or denature an enzyme.
• For most enzymes, the peak in activity lies between 40 °C and 50 °C.
• Many animals living in frigid regions have enzymes with much lower temperature optima than average.
• Enzymes of single-celled archae are resistant to denaturation and remain active at temperatures of 85 °C or more.

Applications of Enzymes

• Milk and other dairy products are recognized as highly nutritious food sources.
• Many people suffer from lactose intolerance, an inability to properly break down the milk disaccharide lactose.
• Digestive cells secrete an enzyme called lactase to absorb lactose that catalyzes the breakdown of lactose in the monosaccharides glucose and galactose.
• People with lactose intolerance do not produce enough lactase, and therefore lactose is consumed then by bacteria leading to nausea, cramps, etc.
• Lactase enzymes can be consumed when eating dairy.
• Another dairy product, cheese, relies on the enzyme chymosin for its production.
• Chymosin was originally obtained from the stomach of calves, but is now genetically engineered.
• Bacteria are added to aid in the curdling process in milk as well to produce lactic acid as a byproduct that lowers the pH, denaturing the milk proteins.
• Chymosin is then added to hydrolyze the most abundant milk protein, casein, that causes the milk to coagulate into semisolid cheese curds.
• Fat-hydrolyzing enzymes produce cheeses with stronger flavors, such as Italian cheese Romano.
• The starch-producing industry is one of the largest users of enzymes.
• Enzymes break down starch into glucose syrup in the starch-producing industry.
• Glucose syrup sweetens many foods, medicines, and vitamins.
• The cleaning industry also relies on starch and fatty stain breakdown for cleaning purposes on clothes, etc.