Enzymes

1. Enzymes Topics 3.6, 7.6

2. Assessment Statements 3.6.1 Define enzyme and active site. 3.6.2 Explain enzyme-substrate specificity. 3.6.3 Explain the effects of temperature, pH and substrate concentration on enzyme activity. 3.6.4 Define denaturation. 3.6.5 Explain the use of lactase in the production of lactose-free milk.

3. Enzymes are organic molecules which act as catalysts • Enzymes are proteins which have taken on a very specific 3-D shape • Somewhere within the enzyme is an area (active site) designed to match a specific molecule (substrate) • Active site of enzyme matches the substrate the way a key fits into a lock, only one key will fit

5. As catalysts, enzymes influence the rate of reactions • A set of reactants in the presence of an enzyme will form product(s) at a faster rate than without the enzyme • Role of an enzyme in a reaction is to lower the energy level (activation energy) needed to start the reaction • Enzymes are not reactants and not used up in the reaction

6. Factors affecting enzyme-catalyzedreactions • Temperature • Reactions with or without enzymes will increase their reaction rate as temperature (and thus molecular motion) increases • Reactions which use enzymes do have an upper limit • Upper limit is based on the temperature at which the enzyme (as a protein) begins to lose its three-dimensional shape due to intramolecular bonds being stressed and broken • This causes a structural change in a protein that results in the loss of its biological properties and is said to be denatured

8. Effect of pH • Active site of an enzyme includes many amino acids some of which have areas that are either positively or negatively charged • + and – of a substrate must match the opposite charge when the substrate is in the active site of an enzyme in order for the enzyme to have catalytic action • When a solution has become too acidic, the large # of H+ can bond with the negative charges of the enzyme or substrate and not allow proper charge matching the two • Same as basic, but with large # of OH- • Will result in an enzyme becoming: • less efficient • completely inactive in extreme situations • denatured

9. No one pH that is best for all enzymes • Many within human body are most active when in a neutral environment • Exception: pepsin is active in stomach which has a highly acidic environment

10. Effect of substrate concentration • If there is a constant amt. of enzyme, as the conc. of a substrate increases, the rate of reaction will increase as well • Has a limit due to the fact that enzymes have a maximum rate at which they can work

11. Use of lactase to help solve the problem of lactose intolerance • Almost all humans on Earth are born with the ability to digest lactose because we have the ability to produce enzyme known as lactase • Lactase digests disaccharide lactose into two monosaccharides which are more readily absorbed into the bloodstream • By adulthood we no longer produce any significant amount of lactase • So how are we able to drink milk without the symptoms as cramping, excessive gas, and diarrhea?

12. Milk and milk produces can be treated with lactase before consumption • Lactose intolerance has been shown to have an extremely high incidence in some ethnic groups and be relatively low in others (natural variation in a population)

13. Enzyme Review

14. Assessment Statements • 7.6.1 State that metabolic pathways consist of chains and cycles of enzyme-catalyzed reactions • 7.6.2 Describe the induced-fit model • 7.6.3 Explain that enzymes lower the activation energy of the chemical reactions that they catalyze • 7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of each • 7.6.5 Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites

15. Metabolism • Def.- the sum of all the chemical reactions that occur in you as a living organism

16. Metabolic pathways • Almost all metabolic reactions in organisms are catalyzed by enzymes • Many of these reactions occur in specific sequences and are called metabolic pathways • Substrate A → substrate B → final product • Each arrow represents a specific enzyme that causes one substrate to be changed to another until the final product of the pathway is formed • Some pathways consist of cycles of reactions instead of chains of reactions • Others involve both cycles and chains (photosynthesis and cellular respiration)

18. Induced-fit model of enzyme action • As substrate “fits” into the active site, there is a conformational change of the active site, thus providing an induced fit • Same way hand fitting into glove changes shape of the glove • Conformational changes and induced fit are due to changes in the R-groups of the amino acids at the active site of the enzyme as they interact with the substrate or substrates

19. Mechanism of enzyme action • The surface of the substrate contacts the active site of the enzyme • The enzyme changes shape to accommodate the substrate • A temporary complex called the enzyme-substrate complex forms • Activation energy is lowered and the substrate is altered by the rearrangement of existing atoms • The transformed substrate – the product – is released from the active site

20. The unchanged enzyme is then free to combine with other substrate molecules

21. Activation energy (AE) • Def. - Energy necessary to destabilize the existing chemical bonds in the substrate of an enzyme-substrate catalyzed reaction • Enzymes cause chemical reactions to occur faster because they reduce the amount of energy needed to bring about a chemical reaction

22. Competitive inhibition • A molecule called a competitive inhibitor, competes directly for the active site of an enzyme • Result is that the substrate then has fewer encounters with the active site and the chemical reaction rate is decreased • Competitive inhibitor must have a structure similar to the substrate to be able to function in this way

23. Example • Use of sulfanilamide (a sulfa drug) to kill the bacteria during an infection • Folic acid is essential as a coenzyme to bacteria in production of nucleic acids • We take in folic acid through our diet • It is also produced in bacterial cells by enzyme action on para-aminobenzoic acid (PABA) • The sulfanilamide competes with the PABA and blocks the enzyme • May be reversible (inhibition may be overcome by increasing the substrate concentration) • May be irreversible

24. Non-competitive inhibition (allosteric inhibition) • Involves an inhibitor that does not compete for the enzyme’s active site • The inhibitor interacts with another site (allosteric site) on the enzyme • The allosteric site causes a change in the shape of the enzyme’s active site, making it non-functional

25. Example • Strychnine, a convulsant poison, acting as an allosteric inhibitor of glycine • Glycine is a major post-synaptic inhibitory neurotransmitter in mammalian spinal cord and brain stem • Strychnine’s binding lowers the affinity of the glycine receptor for glycine. • Strychnine thus inhibits the action of an inhibitory transmitter, causing convulsions.

26. End-product inhibition • Prevents the cell from wasting chemical resources and energy by making more of a substance than it needs • When the end-product of the metabolic pathway is present in a sufficient quantity, the assembly line shuts down by inhibiting the action of the enzyme in the first step • As the existing end-product is used up by the cell, the first enzyme is reactivated • The enzyme that is inhibited and reactivated is an allosteric enzyme