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-catalyzed reactions
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
video

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
Anabolic reactions
Catabolic reactions
Build complex molecules
Break down complex molecules
Are endergonic
Are exergonic
Are biosynthetic
Are degradative
Example: photosynthesis
Example: cellular respiration

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

27. Commercial Uses Pectinase Endonuclease DNA Protease Acts on pectin
Produces smaller, more soluble carbohydrates
Used in fruit juice clarification to increase yield
Endonuclease DNA
Acts on DNA
Produces DNA fragments
Used in genetic engineering
Protease
Acts on proteins
Produces amino acids
Found in washing powders for stain removal