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Chapter 19 - Enzymes and Vitamins_Summary
Overview Chapter 19 introduces the topic of enzymes and shows how vitamins function as
coenzymes necessary for optimum enzyme activity. Applications show how enzymes are used in
medical diagnostics and that some enzyme inhibitors are used as drugs. Enzymes allow much
finer control of chemical reactions than nonbiological catalysts.
Chapter Objectives
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To understand how enzymes are similar to traditional catalysts and how they act in chemical
reactions
To be able to describe substrate, product, and enzyme interaction
To understand the effects of temperature, pH, and enzyme and substrate concentration on rates
of reaction
To be able to discuss the two major classes of vitamins, dietary necessity, and deficiencies
Lecture Outline
19.1 Catalysis by Enzymes
• Enzymes are biochemical catalysts.
-Most enzymes are proteins.
• An enzymatic reaction converts substrate(s) to product(s).
• Enzymes have an active site, a region where the substrate binds to the enzyme.
• Enzymes exhibit both group and optical specificity.
• Catalytic activity may be measured in several ways, including turnover number, the number
of substrate molecules converted to product by one molecule of enzyme in a given unit time.
19.2 Enzyme Cofactors
• Some enzymes require cofactors for optimum activity.
-Cofactors may be small metallic ions or large organic molecules called coenzymes.
-Cofactors often stabilize tertiary structure of enzymes or anchor substrates to the enzyme.
19.3 Enzyme Classification
• Enzymes are divided into six main classes depending upon the type of reaction they catalyze.
• Oxidoreductases catalyze redox reactions.
-Oxidation or reduction can usually be recognized by a loss or gain of hydrogen or oxygen by
the substrate.
-Oxidoreductases usually require coenzymes such as NAD+ or FAD as electron acceptors.
• Transferases catalyze transfer of a functional group from one molecule to another.
• Hydrolases use a molecule of water to break a large molecule into two smaller molecules.
• Isomerases catalyze isomerizations. They move a functional group or atom to a new location
or rearrangement on the same molecule.
-Isomerases work on a single substrate and form a single product.
-Isomerases may simply interchange optical isomers of a molecule.
• Lyases catalyze addition or removal of a small molecule.
Adapted from GOB Chemistry; McMurry et. al.; 5th ed; Prentice Hall 2007
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-In many cases, removal of the small molecule results in formation of a double bond.
Enzymes have names ending in -ase.
-Most names of enzymes name the substrate on which the enzyme acts and also indicate what
action the enzyme carries out.
-Many enzymatic reactions are reversible.
Application (FYI)
Biocatalysis: Food and Chemicals
• Biocatalysis uses enzymes as catalysts with important industrial applications such as flavoring
tea, preparing tasty foods, and preparing components of plastics or pharmaceuticals.
19.4 How Enzymes Work
• Enzyme action was traditionally described as the lock and key model, in which the enzyme
was pictured as a lock and the substrate visualized as a key which fit the lock.
• Modern enzyme theory suggests that a better concept is the induced-fit model.
-The enzyme is pictured as changing shape as the substrate binds so that the two fit together
more readily.
• Enzyme and substrate molecules are believed to form an enzyme-substrate complex.
-The enzyme-substrate complex holds the substrate molecule in an unstable shape, lowering
the activation energy barrier between substrate and product.
• Following release of the product, the enzyme resumes its original shape.
• Although enzyme-substrate interactions are complex, they actually follow definite reactions.
• Enzymes act as catalysts because of their ability to exert:
-Proximity effects, in which the substrate and catalytic site are brought together
-Orientation effects, in which substrates are held at the optimum distance and position for
reaction
-Catalytic effects in which acidic, basic, or other types of groups required for catalysis are
provided
-Energy effects, in which the energy barrier to reaction is lowered
19.5 Effect of Concentration on Enzyme Activity
• Reactions occur when molecules collide. Enzymes help molecules to collide at optimum
angles.
-As substrate concentration increases, the rate of collisions between substrate and enzyme
molecules increases, so reaction velocity increases. (Reaction rate is directly proportional to
substrate concentration.)
-Since the number of enzyme molecules is usually small, the reaction velocity reaches a
maximum (Vmax).
-When V = Vmax, the enzyme is saturated with substrate and can not react any faster.
19.6 Effect of Temperature and pH on Enzymes
Like most chemical reactions, velocity of enzymatic reactions increases with temperature.
-Enzymes typically exhibit an optimum temperature.
-Most enzymes denature if heated far beyond the temperature optimum.
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Adapted from GOB Chemistry; McMurry et. al.; 5th ed; Prentice Hall 2007
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Carboxylic acid and amine groups of proteins lose and gain protons depending upon the pH of
the solution.
-As acid and amine groups lose or gain protons, charges of these groups change, resulting in
changes in tertiary or quaternary structure.
-Each enzyme has an optimum pH that favors reactivity.
Application (FYI)
Enzymes in Medical Diagnosis
• Normal tissues contain predictable levels (concentrations) of enzymes.
-As a result of illness or injury, levels of enzymes may vary from normal.
-Analysis of amounts of enzymes in body fluids can give clinicians an indication of the
physical condition of a patient without invasive procedures.
19.8 Enzyme Regulation: Inhibition
• Competitive inhibition usually occurs when an inhibitor that resembles the normal substrate
binds to the active site of the enzyme and prevents substrate binding.
-In reversible competitive inhibition, larger amounts of substrate can displace the inhibitor,
and Vmax can be attained at sufficiently high substrate concentration.
• Irreversible inhibition can occur if an inhibitor binds permanently to the enzyme and prevents
substrate binding.
-Examples of irreversible inhibitors include heavy metal salts such as lead or mercury
compounds, some phosphate-based insecticides such as parathion, and nerve gases such as
sarin.
-Parathion and sarin block cholinesterase, which normally breaks down acetylcholine.
-Inhibition of acetylcholinesterase can cause a number of symptoms. Death may occur due to
paralysis of muscles (such as diaphragm muscle fibers) involved in respiration.
Application (FYI)
Enzyme Inhibitors as Drugs
• If the action of a chemical compound on the body is known, pharmacologists can design drugs
specifically to mimic the compound. These often act much more effectively than the natural
compound.
-Knowledge of both the substrate and the receptor site enabled chemists to design captopril,
an inhibitor of ACE, angiotensin-converting enzyme.
-ACE helps control blood pressure in patients with high blood pressure.
• Enzyme inhibitors are used in treatment of many diseases. Some are being tested for treatment
of AIDS.
19.10 Vitamins
• Vitamins are small organic molecules taken in with food and required in trace amounts for
optimal enzyme activity.
-Vitamins are either coenzymes or precursors to coenzymes.
-Vitamins are often classed as water-soluble or fat-soluble.
• Water-soluble vitamins such as vitamin C seldom reach toxic levels in tissues, as excess
Adapted from GOB Chemistry; McMurry et. al.; 5th ed; Prentice Hall 2007
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amounts are usually excreted in urine.
Fat-soluble vitamins A, D, E, and K persist in the body stored in fat deposits in tissues.
-Some fat-soluble vitamins, such as vitamin A, may reach toxic levels in livers of carnivores.
HW Assignment (EOC)
19.20 - 19.22, 19.27, 19.28-19.35, 19.45, 19.56, 19.65 -19.68
Adapted from GOB Chemistry; McMurry et. al.; 5th ed; Prentice Hall 2007
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