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Outline
19.1
19.2
19.3
19.4
19.5
19.6
Catalysis by Enzymes
Enzyme Cofactors
Enzyme Classification
How Enzymes Work
Effect of Concentration on Enzyme Activity
Effect of Temperature and pH on Enzyme
Activity
19.7 Enzyme Regulation: Feedback and Allosteric
Control
19.8 Enzyme Regulation: Inhibition
19.9 Enzyme Regulation: Covalent Modification and
Genetic Control
19.10 Vitamins and Minerals
© 2013 Pearson Education, Inc.
Goals
1. What are enzymes?
Be able to describe the chemical nature of enzymes and their function in
biochemical reactions.
2. How do enzymes work, and why are they so specific?
Be able to provide an overview of what happens as one or more substrates
and an enzyme come together so that the catalyzed reaction can occur, and
be able to list the properties of enzymes that make their specificity possible.
3. What effects do temperature, pH, enzyme concentration, and substrate
concentration have on enzyme activity?
Be able to describe the changes in enzyme activity that result when
temperature, pH, enzyme concentration, or substrate concentration change.
4. How is enzyme activity regulated?
Be able to define and identify feedback control, allosteric control, reversible
and irreversible inhibition, inhibition by covalent modification, and genetic
control of enzymes.
5. What are vitamins and minerals?
Be able to describe the two major classes of vitamins, the reasons vitamins
are necessary in our diets, and the general results of excesses or
deficiencies. Be able to identify essential minerals, explain why minerals are
necessary in the diet, and explain the results of deficiencies.
© 2013 Pearson Education, Inc.
19.1 Catalysis by Enzymes
• An enzyme is a protein or other molecule that
acts as a catalyst for a biological reaction.
• Enzymes, with few exceptions, are water-soluble
globular proteins.
• An active site is a pocket in an enzyme with the
specific shape and chemical makeup necessary
to bind a substrate.
• A substrate is a reactant in an enzymecatalyzed reaction.
• Specificity of the enzyme is the limitation of the
activity of an enzyme to a specific substrate,
specific reaction, or specific type of reaction.
© 2013 Pearson Education, Inc.
19.1 Catalysis by Enzymes
• Catalase is almost completely specific for one
reaction: the decomposition of hydrogen
peroxide.
• Thrombin catalyzes hydrolysis of a peptide bond
following an arginine, and primarily acts on
fibrinogen, a protein essential to blood clotting.
• Carboxypeptidase A removes many different Cterminal amino acid residues from protein chains
during digestion.
• Papain catalyzes the hydrolysis of peptide bonds
in many locations.
© 2013 Pearson Education, Inc.
19.1 Catalysis by Enzymes
• Enzymes are also
specific with respect to
stereochemistry.
• If a substrate is chiral,
an enzyme usually
catalyzes the reaction of
only one of the pair of
enantiomers.
• Only one enantiomer fits
the active site in such a
way that the reaction
can occur.
© 2013 Pearson Education, Inc.
19.1 Catalysis by Enzymes
• The catalytic activity of an enzyme is
measured by its turnover number, the
maximum number of substrate molecules
acted upon by one molecule of enzyme per
unit time.
• Turnover numbers range from 10 to
10,000,000 molecules per second.
• Catalase is one of the fastest; it can turn over
10 million molecules per second, which is the
fastest reaction rate attainable, because it is
the rate at which molecules collide.
© 2013 Pearson Education, Inc.
19.2 Enzyme Cofactors
• An enzyme may require a metal ion, a
coenzyme, or both.
– Cofactors can be tightly held or loosely
bound, so that they can enter and leave the
active site.
• A cofactor is a nonprotein part of an enzyme
that is essential to the enzyme’s catalytic
activity (e.g., a metal ion or a coenzyme).
• A coenzyme is an organic molecule that acts
as an enzyme cofactor.
© 2013 Pearson Education, Inc.
19.2 Enzyme Cofactors
• By combining with cofactors, enzymes
acquire chemically reactive groups not
available in side chains.
• The requirement that many enzymes have
for metal ion cofactors explains our dietary
need for trace minerals.
• Vitamins are also a necessity for humans;
we cannot synthesize them, yet they are
critical building blocks for coenzymes.
© 2013 Pearson Education, Inc.
19.2 Enzyme Cofactors
• By combining with cofactors, enzymes
acquire chemically reactive groups not
available in side chains.
• The requirement for metal ion cofactors
explains our dietary need for trace
minerals. The ions form coordinate
covalent bonds with nitrogen or oxygen.
• Vitamins are also a necessity for humans;
we cannot synthesize them, yet they are
critical building blocks for coenzymes.
© 2013 Pearson Education, Inc.
19.3 Enzyme Classification
© 2013 Pearson Education, Inc.
19.3 Enzyme Classification
• Oxidoreductases catalyze oxidation–reduction
reactions of substrate molecules, most
commonly addition or removal of oxygen or
hydrogen. Because oxidation and reduction
must occur together, these enzymes require
coenzymes that are reduced or oxidized as the
substrate is oxidized or reduced.
© 2013 Pearson Education, Inc.
19.3 Enzyme Classification
• Transferases catalyze transfer of a group from
one molecule to another.
– Transaminases transfer an amino group between
substrates.
– Kinases transfer a phosphate group from adenosine
triphosphate (ATP) to produce adenosine
diphosphate (ADP) and a phosphorylated product.
© 2013 Pearson Education, Inc.
19.3 Enzyme Classification
• Hydrolases catalyze the hydrolysis of
substrates, the breaking of bonds with addition
of water. These enzymes are particularly
important during digestion, and provide amino
acids for protein synthesis and glucose for use in
energy generating pathways.
© 2013 Pearson Education, Inc.
19.3 Enzyme Classification
• Isomerases catalyze the isomerization
(rearrangement of atoms) of a substrate in
reactions that have but one substrate and one
product. In some metabolic pathways a molecule
must be rearranged—isomerized—for the next
step of the pathway to occur.
© 2013 Pearson Education, Inc.
19.3 Enzyme Classification
• Lyases (from the Greek lein, meaning “to
break”) catalyze the addition of a molecule such
as H2O, CO2 or NH3 to a double bond or the
reverse reaction in which a molecule is
eliminated to create a double bond.
© 2013 Pearson Education, Inc.
19.3 Enzyme Classification
• Ligases (from the Latin ligare, meaning “to tie
together”) catalyze the bonding together of two
substrate molecules. Because such reactions
are generally not favorable, they require the
simultaneous release of energy by a hydrolysis
reaction, usually by the conversion of ATP to
ADP. Ligases are involved in synthesis of
biological polymers such as proteins and DNA.
© 2013 Pearson Education, Inc.
19.4 How Enzymes Work
• The explanation for enzyme specificity is found
in the active site. Exactly the right environment
for the reaction is provided within the active site.
• Two models represent the interaction between
substrates and enzymes:
– In the lock-and-key model, the substrate is described
as fitting into the active site as a key fits into a lock.
– In the induced-fit model, the enzyme has a flexible
active site that changes shape.
© 2013 Pearson Education, Inc.
19.4 How Enzymes Work
© 2013 Pearson Education, Inc.
19.4 How Enzymes Work
• Enzyme-catalyzed reactions begin with migration
of the substrate into the active site to form an
enzyme–substrate complex.
• Enzymes act as catalysts because of their ability to:
– Bring substrate(s) and catalytic sites together
(proximity effect).
– Hold substrate(s) at the exact distance and in the
exact orientation necessary for reaction (orientation
effect).
– Provide acidic, basic, or other types of groups
required for catalysis (catalytic effect).
– Lower the energy barrier by inducing strain in bonds
in the substrate molecule (energy effect).
© 2013 Pearson Education, Inc.
19.4 How Enzymes Work
© 2013 Pearson Education, Inc.
19.5 Effect of Concentration on Enzyme Activity
Substrate Concentration
– If the substrate concentration is low, not all
the enzyme molecules are in use. The rate
increases with the concentration of substrate
as more enzyme molecules are put to work.
– As the substrate
concentration
continues to increase,
the increase in the
rate levels off as more
and more active sites
are occupied.
© 2013 Pearson Education, Inc.
19.5 Effect of Concentration on Enzyme Activity
Substrate Concentration
– Once the enzyme is saturated, increasing
substrate concentration has no effect.
– The rate when the enzyme is saturated is
determined by the efficiency of the enzyme,
the pH, and the temperature.
– Enzyme and substrate molecules moving at
random in solution can collide with each other
at a rate of about 108 collisions per mole per
liter per second.
– A few enzymes actually operate with close to
this efficiency.
© 2013 Pearson Education, Inc.
19.5 Effect of Concentration on Enzyme Activity
Enzyme Concentration
– It is possible for the concentration of an active
enzyme to vary according to metabolic needs. So
long as the concentration of substrate does not
become a limitation, the reaction rate varies
directly with the enzyme concentration.
© 2013 Pearson Education, Inc.
19.6 Effect of Temperature and pH on Enzyme Activity
Effect of Temperature on Enzyme Activity
– Rates of enzyme-catalyzed reactions do not
increase continuously with rising temperature.
– Rates reach a maximum and then decrease.
Enzymes denature when non-covalent attractions
between protein side chains are disrupted,
destroying the active site.
© 2013 Pearson Education, Inc.
19.6 Effect of Temperature and pH on Enzyme Activity
Effect of pH on Enzyme Activity
– The catalytic activity of many enzymes
depends on pH and usually has a well-defined
optimum point at the normal, buffered pH of
the enzyme’s environment.
– Most enzymes have their maximum activity
between the pH values of 5 to 9. Eventually,
extremes of pH will denature a protein.
© 2013 Pearson Education, Inc.
19.6 Effect of Temperature and pH on Enzyme Activity
Extremozymes: Enzymes from the Edge
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Most mammalian enzymes display optimum activity around 40 °C near pH
7.0 at 1 atmosphere of pressure.
Extremozymes are enzymes from extremophiles, organisms that live in
conditions hostile to mammalian cells.
Commercially-useful enzymes have been developed from bacteria that have
optimum growth temperatures as high as 106 °C and as low as 4 °C, and
that grow in pH as low as 0.7 and as high as 10.
Enzymes from thermophiles (heat lovers) are used to break down starch
and cellulose, and Taq polymerase is used in forensics.
Enzymes from cold-environment microorganisms (psychrophiles) are used
in products such as cold-water-wash laundry detergents.
Thermophiles synthesize special proteins called chaperonins, which
recognize and refold heat-denatured proteins. In addition, proteins from
thermophiles have tightly folded, highly nonpolar cores, and ionic surfaces.
Psychrophiles have more polar, flexible proteins than thermophiles. This
structure is necessary to maintain activity at low temperatures.
Thermophilic enzymes are also used in oil drilling.
© 2013 Pearson Education, Inc.
19.6 Effect of Temperature and pH on Enzyme Activity
Enzymes in Medical Diagnosis
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Measurement of blood levels of enzymes is a valuable diagnostic tool.
Higher-than-normal activities indicate the following conditions:
– Aspartate transaminase (AST) Damage to heart or liver
– Alanine transaminase (ALT)
Damage to heart or liver
– Lactate dehydrogenase (LH)
Damage to heart, liver, or red blood cells
– Alkaline phosphatase (ALP)
Damage to bone and liver cells
– Glutamyl transferase (GGT)
Damage to liver cells; alcoholism
Activity is measured in international units. Results are reported in units per
liter (U/L).
Among the most useful enzyme assays are those done to diagnose heart
attacks.
© 2013 Pearson Education, Inc.
19.7 Enzyme Regulation: Feedback and Allosteric Control
• A variety of strategies are utilized to adjust
the rates of enzyme-catalyzed reactions.
– Any process that initiates or increases the
action of an enzyme is an activation.
– Any process that slows or stops the action of
an enzyme is an inhibition.
• Several strategies usually operate
together.
© 2013 Pearson Education, Inc.
19.7 Enzyme Regulation: Feedback and Allosteric Control
• Feedback control occurs when the result
of a process feeds information back to
affect the beginning of the process.
• If D inhibits enzyme 1, the amount of A
and B synthesized decreases when no
more D is needed.
• When more D is needed, enzyme 1 will no
longer be inhibited.
© 2013 Pearson Education, Inc.
19.7 Enzyme Regulation: Feedback and Allosteric Control
• In allosteric control, the binding of a molecule
(an allosteric regulator or effector) at one site on
a protein affects the binding of another molecule
at a different site.
• Allosteric control can be either positive or
negative.
© 2013 Pearson Education, Inc.
19.8 Enzyme Regulation: Inhibition
Reversible Uncompetitive Inhibition
– The inhibitor does not compete with the
substrate for the active site. It binds to the
enzyme-substrate complex so that the
reaction occurs less efficiently, or not at all.
Reversible Competitive Inhibition
– A competitive inhibitor binds reversibly to an
active site through noncovalent interactions,
but undergoes no reaction, preventing the
substrate from entering the active site.
© 2013 Pearson Education, Inc.
19.8 Enzyme Regulation: Inhibition
Irreversible Inhibition
– The enzyme’s reaction cannot occur because
the substrate cannot connect with the active
site. Many irreversible inhibitors are poisons
as a result of their ability to completely shut
down the active site.
– Heavy metal ions, such as mercury and lead,
are irreversible inhibitors that form covalent
bonds to the sulfur atoms in the —SH groups
of cysteine residues.
© 2013 Pearson Education, Inc.
19.8 Enzyme Regulation: Inhibition
Enzyme Inhibitors as Drugs
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Angiotensin II, an octapeptide, is a potent pressor—it elevates blood
pressure, in part by causing contraction of blood vessels. Angiotensin I, a
decapeptide, is an inactive precursor of angiotensin II. To become active, two
amino acid residues—His and Leu—must be cut off the end of angiotensin I,
a reaction catalyzed by angiotensin-converting enzyme (ACE).
A zinc(II) ion is present in the ACE active site. The extract of venom from a
South American pit viper is a mild ACE inhibitor and contains a pentapeptide
with a proline residue at the carboxyl-terminal end.
The first ACE inhibitor on the market, captopril, was developed by
experimenting with modifications of the proline structure.
© 2013 Pearson Education, Inc.
19.8 Enzyme Regulation: Inhibition
Enzyme Inhibitors as Drugs
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Two important AIDS-fighting drugs are enzyme inhibitors. The first, AZT
(azidothymidine, also called zidovudine), resembles a molecule essential to
reproduction of the AIDS-causing human immunodeficiency virus (HIV).
AZT is accepted by an HIV enzyme as a substrate and prevents the virus
from producing duplicate copies of itself.
The most successful AIDS drug thus far inhibits a protease, an enzyme that
cuts a long protein chain into smaller pieces needed by the HIV.
Protease inhibitors cause dramatic decreases in virus population and
symptoms. The success is only achieved by taking a “cocktail” of several
drugs including AZT, which requires precise adherence to a schedule of
taking 20 pills a day.
© 2013 Pearson Education, Inc.
19.9 Enzyme Regulation: Covalent Modification and Genetic Control
Covalent Modification
– Some enzymes are synthesized in inactive form.
– Activation of zymogens or proenzymes, requires a
chemical reaction that splits off part of the molecule.
– Examples of zymogens include trypsinogen,
chymostrypsinogen, and proelastase, precursors of
enzymes that digest proteins in the small intestine.
These enzymes must be inactive when they are
synthesized so that they do not immediately digest
the pancreas.
© 2013 Pearson Education, Inc.
19.9 Enzyme Regulation: Covalent Modification and Genetic Control
Covalent Modification
– Another mode of covalent modification is the
reversible addition of phosphoryl groups to serine,
tyrosine, or threonine residues.
– Kinase enzymes catalyze the addition of a phosphoryl
group supplied by ATP (phosphorylation).
– Phosphatase enzymes catalyze the removal of the
phosphoryl group (dephosphorylation).
© 2013 Pearson Education, Inc.
19.9 Enzyme Regulation: Covalent Modification and Genetic Control
Genetic Control
– The synthesis of enzymes, like that of all
proteins, is regulated by genes .
– The genetic control strategy is especially
useful for enzymes needed only at certain
stages of development. Mechanisms
controlled by hormones can accelerate or
decelerate enzyme synthesis.
© 2013 Pearson Education, Inc.
19.9 Enzyme Regulation: Covalent Modification and Genetic Control
Mechanisms of Enzyme Control
•
Feedback control is exerted on an earlier reactant by a later product in a
reaction pathway and is made possible by allosteric control. The feedback
molecule binds to a specific enzyme early in the pathway in a way that
alters the shape and, therefore, the efficiency of the enzyme.
•
Inhibition, which is either reversible or irreversible. Reversible inhibition
can involve both the substrate and the active site (uncompetitive
inhibition) or only the active site (competitive inhibition) by molecules
that often mimic substrate structure. Irreversible inhibition occurs because
of covalent bonding of the inhibitor to the enzyme. Competitive inhibition is a
strategy often utilized in medications, and irreversible inhibition is a mode of
action of many poisons.
•
Production of inactive enzymes (zymogens), which must be activated by
cleaving a portion of the molecule.
•
Covalent modification of an enzyme by addition and removal of a
phosphoryl group, with the phosphoryl group supplied by ATP.
•
Genetic control, whereby the amount of enzyme available is regulated by
limiting its synthesis.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
• Vitamin: An organic molecule, essential in
trace amounts that must be obtained in the
diet because it is not synthesized in the body.
• Water-Soluble Vitamins
– Water-soluble vitamins are found in the aqueous
environment inside cells.
– Water-soluble vitamins contain —OH, —COOH or
other polar groups that impart water solubility.
– They range from simple molecules like vitamin C to
quite large and complex structures like vitamin B12.
– Most vitamins are components of larger coenzymes,
but vitamin C and biotin are biologically active without
any change in structure.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
• Fat-Soluble Vitamins
– The fat-soluble vitamins A, D, E, and K are stored in
the body’s fat deposits.
– The clinical effects of deficiencies of these vitamins
are well documented, but the molecular mechanisms
by which they act are not nearly as well understood
as those of the water-soluble vitamins.
– None has been identified as a coenzyme.
– The hazards of overdosing on fat-soluble vitamins are
greater than the hazards of overdosing on watersoluble vitamins because the fat-soluble vitamins
accumulate in body fats.
– Excesses of the water-soluble vitamins are more
likely to be excreted in the urine.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
• Vitamin A is essential for night vision, healthy
eyes, and normal development of epithelial
tissue. It has three active forms: retinol, retinal,
and retinoic acid. It is produced in the body by
cleavage of b-carotene.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
• Vitamin D is related in structure to cholesterol.
– It is synthesized when ultraviolet light from the
sun strikes a cholesterol derivative in the skin.
– In the kidney, vitamin D is converted to a
hormone that regulates calcium absorption and
bone formation.
– Deficiencies are most likely in malnourished
individuals living where there is little sunlight.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
• Vitamin E comprises a group of structurally
similar compounds called tocopherols.
– Like vitamin C, it is an antioxidant: It prevents the
breakdown of vitamin A and polyunsaturated fats
by oxidation.
– Vitamin E apparently is not toxic in overdosage
as are the other fat-soluble vitamins, it is best to
avoid excessively large doses of vitamin E.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
• Vitamin K is a family of structurally related
compounds distinguished from each other by
hydrocarbon side chains of varying length.
– This vitamin is essential to the synthesis of
several blood-clotting factors. It is produced by
intestinal bacteria, so deficiencies are rare.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
Vitamins, Minerals, and Food Labels
– Much is yet to be learned about the functions of vitamins and
minerals in the body, and new information is continuously being
reported.
– The Food and Nutrition Board of the National Academy of
Sciences-National Research Council periodically surveys the
latest nutritional information and publishes Recommended
Dietary Allowances (RDAs).
– The U.S. Food and Drug Administration (FDA), which has
among its many responsibilities setting the rules for food
labeling.
– Since 1994, most packaged food products carry standardized
Nutrition Facts labels.
– In choosing which vitamins and minerals must be listed on the
new labels, the government has focused on those currently of
greatest importance in maintaining good health.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
Antioxidants
– An antioxidant is a substance that prevents oxidation.
– The dietary antioxidants are vitamin C, vitamin E, and
the mineral selenium.
– They defuse free radicals, reactive molecular
fragments with unpaired electrons which gain stability by
picking up electrons from nearby molecules..
– Vitamin E acts by giving up the hydrogen to oxygencontaining free radicals. The hydrogen is then restored
by reaction with vitamin C.
– Selenium is a cofactor in an enzyme that converts
hydrogen peroxide to water before the peroxide can go
on to produce free radicals.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
Minerals
– This group of micronutrients is composed
primarily, but not entirely, of transition group
elements.
– A balanced diet supplies sufficient amounts of
each of these micronutrients.
– Many of the transition elements are necessary
for proper functioning of enzymes, since these
elements are used as cofactors.
– Other minerals are used as building blocks for
the body and some exist as electrolytes.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
• Macrominerals, have required daily amounts
greater than 100 mg per day. These include
calcium, phosphorous, magnesium, potassium,
sodium, chloride, and sulfur.
– Adequate, regular intake of calcium and phosphorous
is necessary for formation and maintenance of bone.
– Magnesium is also necessary for bone metabolism
and is stored in bone tissue; it is also a cofactor in
many different enzymes.
– Sodium, chloride and potassium function as
electrolytes, maintaining osmotic balance in both
intra- and extracellular spaces.
© 2013 Pearson Education, Inc.
19.10 Vitamins and Minerals
• The transition elements, chromium, copper,
magnesium, manganese, molybdenum, selenium
and zinc are classed as micronutrients.
– Our bodies need only minute amounts as cofactors for
enzymes.
– Some are highly toxic if ingested in high amounts.
– Because these are transition element cations with variable
oxidation states, they can serve as transient holders of
electrons during enzymatic reactions.
– Iron is a necessary component of the heme ring present in
both myoglobin and hemoglobin, as well as in the
cytochromes found in the electron transport system.
– Iodine is essential for synthesis of thyroid hormones, which
regulate many functions in the body.
© 2013 Pearson Education, Inc.
Chapter Summary
1. What are enzymes?
• Enzymes are the catalysts for biochemical
reactions.
• They are mostly water-soluble, globular proteins,
and many incorporate cofactors, which are either
metal ions or the nonprotein organic molecules
known as coenzymes.
• One or more substrate molecules (the reactants)
enter an active site lined by those protein side
chains and cofactors necessary for catalyzing the
reaction.
• Six major classes and many subclasses of
reactions are catalyzed by enzymes.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
2. How do enzymes work, and why are they so specific?
• A substrate is drawn into the active site by noncovalent
interactions.
• As the substrate enters the active site, the enzyme shape
adjusts to best accommodate the substrate and catalyze
the reaction (the induced fit).
• Within the enzyme–substrate complex, the substrate is
held in the best orientation for reaction and in a strained
condition that allows the activation energy to be lowered.
• When the reaction is complete, the product is released and
the enzyme returns to its original condition.
• The specificity of each enzyme is determined by the
presence within the active site of catalytically active
groups, hydrophobic pockets, and ionic or polar groups
that exactly fit the chemical makeup of the substrate.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
3. What effects do temperature, pH, enzyme
concentration, and substrate concentration have
on enzyme activity?
• With increasing temperature, reaction rate increases to
a maximum and then decreases as the enzyme protein
denatures.
• Reaction rate is maximal at a pH that reflects the pH of
the enzyme’s site of action in the body.
• In the presence of excess substrate, reaction rate is
directly proportional to enzyme concentration.
• With fixed enzyme concentration, reaction rate first
increases with increasing substrate concentration and
then approaches a fixed maximum at which all active
sites are occupied.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
4.
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How is enzyme activity regulated?
The effectiveness of enzymes is controlled by a variety of activation and
inhibition strategies.
A product of a later reaction can exercise feedback control over an enzyme
for an earlier reaction in a pathway. Feedback control acts through allosteric
control of enzymes that have regulatory sites separate from their active sites.
Binding a regulator induces a change of shape in the active site, increasing or
decreasing the efficiency of the enzyme.
Uncompetitive inhibitors act on the enzyme-substrate complex, blocking a
second substrate from entering the active site; they lower the maximum
reaction rate.
Competitive inhibitors typically resemble the substrate and reversibly block
the active site; they slow the reaction rate but do not change the maximum
rate.
Irreversible inhibitors form covalent bonds to an enzyme that permanently
inactivate it; most are poisons.
Enzyme activity is also regulated by reversible phosphorylation and
dephosphorylation, and by synthesis of inactive zymogens that are later
activated by removal of part of the molecule.
Genetic control is exercised by regulation of the synthesis of enzymes.
© 2013 Pearson Education, Inc.
Chapter Summary, Continued
5.
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What are vitamins and minerals?
Vitamins are organic molecules required in small amounts in the
diet because our bodies cannot synthesize them.
The water-soluble vitamins are coenzymes or parts of coenzymes.
The fat-soluble vitamins have diverse and less well understood
functions.
In general, excesses of water-soluble vitamins are excreted and
excesses of fat-soluble vitamins are stored in body fat, making
excesses of the fat-soluble vitamins potentially more harmful.
Vitamin C, β-carotene (a precursor of vitamin A), vitamin E, and
selenium work together as antioxidants to protect biomolecules
from damage by free radicals.
Minerals are chemical elements needed in small amounts in the
diet. Minerals function as macronutrients (calcium and
phosphorous for bone), electrolytes, and micronutrients used
primarily as enzyme cofactors.
© 2013 Pearson Education, Inc.