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Chapter 16 Amino Acids, Proteins, and Enzymes
16.1 Functions of Proteins
16.2 Amino Acids
16.3 Amino Acids as Acids and Bases
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Functions of Proteins
Proteins perform many different functions in the body.
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Amino Acids
Amino acids • are the building blocks of proteins.• contain a carboxylic acid group and an amino group on
the alpha () carbon.• are ionized in solution.• each contain a different side group (R). R side chain R
│ + │H2N—C —COOH H3N—C —COO−
│ │ H H ionized form
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Examples of Amino Acids
H + │H3N—C—COO−
│ H glycine
CH3
+ │H3N—C—COO−
│ H alanine
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Types of Amino Acids
Amino acids are classified as
• nonpolar (hydrophobic) with hydrocarbon side chains.
• polar (hydrophilic) with polar or ionic side chains.
• acidic (hydrophilic) with acidic side chains.
• basic (hydrophilic) with –NH2 side chains.
Nonpolar Polar
AcidicBasic
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Nonpolar Amino Acids
An amino acid is nonpolar when the R group is H, alkyl, or aromatic.
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Polar Amino Acids
An amino acid is polar when the R group is an alcohol, thiol, or amide.
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Acidic and Basic Amino Acids
An amino acid is • acidic when the R group is a carboxylic acid.• basic when the R group is an amine.
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Learning Check
Identify each as (1) polar or (2) nonpolar. +
A. H3N–CH2–COO− (Glycine)
CH3
| CH–OH + │
B. H3N–CH–COO − (Threonine)
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Solution
Identify each as (1) polar or (2) nonpolar.
+
A. H3N–CH2–COO− (Glycine) (2) nonpolar
CH3
| CH–OH + │
B. H3N–CH–COO − (Threonine) (1) polar
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Fischer Projections of Amino Acids
Amino acids • are chiral except glycine.• have Fischer projections that are stereoisomers.• that are L are the only amino acids used in proteins.
L-Alanine D-Alanine L-Cysteine D-Cysteine
CH2SH
H2N H
COOH
CH2SH
H NH2
COOH
CH3
H NH2
COOH
CH3
H2N H
COOH
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A zwitterion
• has charged −NH3+ and COO– groups.
• forms when both the –NH2 and the –COOH groups in an amino acid ionize in water.
• has equal + and – charges at the isoelectric point (pI).
O O ║ + ║
NH2—CH2—C—OH H3N—CH2—C—O–
glycine zwitterion of glycine
Zwitterions
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In solutions more basic than the pI,
• the —NH3+ in the amino acid donates a proton.
+ OH–
H3N—CH2—COO– H2N—CH2—COO– zwitterion Negative ion at pI pH > pI
Charge: 0 Charge: 1-
Amino Acids as Acids
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In solution more acidic than the pI,• the COO- in the amino acid accepts a proton.
+ H+ +
H3N—CH2—COO– H3N—CH2—COOH zwitterion Positive ionat pI pH< pI
Charge: 0 Charge: 1+
Amino Acids as Bases
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pH and ionization
H+ OH–
+ +
H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–COO–
positive ion zwitterion negative ion
low pH pI high pH
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Separation of Amino Acids
When an electric current is used to separate a mixture of
amino acids• the positively charged amino acids move towards the
negative electrode.• the negatively charged amino acids move toward the
positive electrode. • an amino acid at its pI does not migrate.• the amino acids are identified as separate bands on
the filter paper or thin layer plate.
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Separation of Amino Acids
With an electric current, a mixture of lysine, aspartate,and valine are separated.
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CH3 CH3
+ | |H3N—CH—COOH H2N—CH—COO–
(1) (2)
Which structure represents:
A. Alanine at a pH above its pI?
B. Alanine at a pH below its pI?
Learning Check
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CH3 CH3
+ | |H3N—CH—COOH H2N—CH—COO–
(1) (2)
Which structure represents:
A. Alanine at a pH above its pI? (2)
B. Alanine at a pH below its pI? (1)
Solution
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16.4 Formation of Peptides
Chapter 16 Amino Acids, Proteins, and Enzymes
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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The Peptide Bond
A peptide bond• is an amide bond. • forms between the carboxyl group of one amino acid and
the amino group of the next amino acid.
O CH3 O
+ || + | ||H3N—CH2—C—O– + H3N—CH—C—O–
O H CH3 O + || | | ||
H3N—CH2—C—N—CH—C—O– peptide bond
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Formation of a Dipeptide
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Learning Check
Write the dipeptide Ser-Thr. OH CH3
| | CH2 O HCOH O
+ | ║ + | ║ H3N─CH─C─O– + H3N─CH─C─O–
Ser Thr
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Solution
Write the dipeptide Ser-Thr. OH CH3
| | CH2 O HCOH O + | ║ + | ║
H3N─CH─C─O– + H3N─CH─C─O– Ser Thr
OH CH3
| | CH2 O H HCOH O + | ║ | | ║
NH3─CH─C─N ─CH─C─O– + H2O
Ser-Thr
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Naming Dipeptides
A dipeptide • is named from the free amine (NH3
+) using a -yl ending for the name.
• names the last amino acid with the free carboxyl group (COO-) by its amino acid name.
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Write the three-letter abbreviations and names of the tripeptides that could form from two glycine and one alanine.
Learning Check
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Write the names and three-letter abbreviations of the tripeptides that could form from two glycine and one alanine.
Glycylglycylalanine Gly-Gly-Ala
Glycylalanylglycine Gly-Ala-Gly
Alanylglycylglycine Ala-Gly-Gly
Solution
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Learning Check
What are the possible tripeptides formed from one each of leucine, glycine, and alanine?
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Solution
Tripeptides possible from one each of leucine, glycine, and alanine:
Leu-Gly-Ala
Leu-Ala-Gly
Ala-Leu-Gly
Ala-Gly-Leu
Gly-Ala-Leu
Gly-Leu-Ala
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Learning Check
Write the three-letter abbreviation and name for the
following tetrapeptide.
CH3
│ CH3 S │ │ CH–CH3 SH CH2
│ │ │ CH3 O H CH O H CH2O H CH2 O + │ ║ │ │ ║ │ │ ║ │ │ ║H3N–CH–C–N–CH–C–N–CH–C–N–CH–CO–
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Solution
Ala-Leu-Cys-Met Alanylleucylcysteinylmethionine
CH3
│ CH3 S │ │ CH–CH3 SH CH2
│ │ │ CH3 O H CH O H CH2O H CH2 O
+ │ ║ │ │ ║ │ │ ║ │ │ ║H3N–CH–C–N–CH–C–N–CH–C–N–CH–CO–
Ala Leu Cys Met
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16.5 Levels of Proteins Structure
Chapter 16 Amino Acids, Proteins, and Enzymes
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Primary Structure of Proteins
The primary structure of a protein is• the particular sequence of amino acids.• the backbone of a peptide chain or
protein.
Ala─Leu─Cys─Met
CH3
SH
CH2
CH3
S
CH2
CH2CH O
O-CCH
H
N
O
CCH
H
N
O
CCH
H
N
O
CCHH3N
CH3
CH3CH
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Primary Structures
• The nonapeptides oxytocin and vasopressin have similar primary structures.
• Only the amino acids at positions 3 and 8 differ.
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Primary Structure of Insulin
Insulin• was the first protein to have its
primary structure determined.• has a primary structure of two
polypeptide chains linked by disulfide bonds.
• has a chain A with 21 amino acids and a chain B with 30 amino acids.
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Secondary Structure – Alpha Helix
The secondary structure of analpha helix is• a three-dimensional spatial
arrangement of amino acids in a polypeptide chain.
• held by H bonds between the H of –N-H group and the O of C=O of the fourth amino acid down the chain.
• a corkscrew shape that looks like a coiled “telephone cord”.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Secondary Structure – Beta Pleated Sheet
The secondary structure of a beta pleated sheet• consists of polypeptide chains arranged side by side.• has hydrogen bonds between chains.• has R groups above and below the sheet.• is typical of fibrous proteins such as silk.
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Secondary Structure – Triple Helix
The secondary structure of a triple helix is
• three polypeptide chains woven together.
• typical of collagen, connective tissue, skin, tendons, and cartilage.
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Indicate the type of protein structure.1) primary 2) alpha helix3) beta pleated sheet 4) triple helix
A. polypeptide chains held side by side by H bondsB. sequence of amino acids in a polypeptide chainC. corkscrew shape with H bonds between amino acidsD. three peptide chains woven like a rope
Learning Check
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Indicate the type of protein structure.
1) primary 2) alpha helix
3) beta pleated sheet 4) triple helix
A. 3 polypeptide chains held side by side by H bonds
B. 1 sequence of amino acids in a polypeptide chain
C. 2 corkscrew shape with H bonds between amino
acids
D. 4 three peptide chains woven like a rope
Solution
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• must be obtained from the diet.
• are ten amino acids not synthesized by the body.
• are in meat and diary products.
• are missing (one or more) in grains and vegetables.
Essential Amino Acids
Essential amino acids
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Tertiary Structure
The tertiary structureof a protein • is the overall three-
dimensional shape.• is determined by
attractions and repulsions between the side chains of the amino acids in a peptide chain.
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Crosslinks in Tertiary Structures
Crosslinks in tertiary structures involve attractions and repulsions between the side chains of the amino acids in the polypeptide chain.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Select the type of tertiary interaction.
1) disulfide 2) ionic
3) H bonds 4) hydrophobic
A. leucine and valine
B. two cysteines
C. aspartic acid and lysine
D. serine and threonine
Learning check
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Select the type of tertiary interaction.
1) disulfide 2) ionic
3) H bonds 4) hydrophobic
A. 4 leucine and valine
B. 1 two cysteines
C. 2 aspartic acid and lysine
D. 3 serine and threonine
Solution
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Globular Proteins
Globular proteins • have compact, spherical
shapes.• carry out synthesis,
transport, and metabolism in the cells.
• such as myoglobin store and transport oxygen in muscle.
Myoglobin
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Quaternary Structure
The quaternary structure • is the combination of two
or more protein units.• of hemoglobin consists of
four polypeptide chains as subunits.
• is stabilized by the same interactions found in tertiary structures.
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Fibrous Proteins
Fibrous proteins• consist of long, fiber-like shapes.• such as alpha keratins make up hair, wool, skin, and
nails.• such as feathers contain beta keratins with large
amounts of beta-pleated sheet structures.
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Summary of Protein Structure
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Summary of Protein Structure
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Identify the level of protein structure.1) Primary 2) Secondary3) Tertiary 4) Quaternary
A. beta pleated sheetB. order of amino acids in a proteinC. a protein with two or more peptide chainsD. the shape of a globular proteinE. disulfide bonds between R groups
Learning Check
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Identify the level of protein structure.
1) Primary 2) Secondary
3) Tertiary 4) Quaternary
A. 2 beta pleated sheet
B. 1 order of amino acids in a protein
C. 4 a protein with two or more peptide chains
D. 3 the shape of a globular protein
E. 3 disulfide bonds between R groups
Solution
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Denaturation involves • the disruption of bonds in the secondary, tertiary and
quaternary protein structures.• heat and organic compounds that break apart H
bonds and disrupt hydrophobic interactions.• acids and bases that break H bonds between polar
R groups and disrupt ionic bonds.• heavy metal ions that react with S-S bonds to form
solids.• agitation such as whipping that stretches peptide
chains until bonds break.
Denaturation
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Denaturation of protein occurs when• an egg is cooked. • the skin is wiped with
alcohol.• heat is used to cauterize
blood vessels.• instruments are sterilized
in autoclaves.
Applications of Denaturation
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Tannic acid is used to form a scab on a burn. An egg is hard boiled by placing it in boiling water. What is similar about these two events?
Learning check
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Acid and heat cause the denaturation of protein. They both break bonds in the secondary and tertiary structures of proteins.
Solution
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Chapter 16 Amino Acids, Proteins, and Enzymes
16.6 Enzymes
16.7 Enzyme Action
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Enzymes are Biological Catalysts
Enzymes are proteins that
• Catalyze nearly all the chemical reactions taking place in the cells of the body.
• Increase the rate of reaction by lowering the energy of activation.
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The name of an enzyme
• usually ends in –ase.
• identifies the reacting substance. For example, sucrase catalyzes the reaction of sucrose.
• describes the function of the enzyme. For example, oxidases catalyze oxidation.
• could be a common name, particularly for the digestion enzymes such as pepsin and trypsin.
Names of Enzymes
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Enzymes are classified by the reaction they catalyze.
Class Type of Reactions catalyzedOxidoreductases Oxidation-reductionTransferases Transfer groups of atomsHydrolases HydrolysisLyases Add atoms/remove atoms to or
from a double bondIsomerases Rearrange atomsLigases Use ATP to combine small molecules
Classification of Enzymes
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Match the type of reaction with an enzyme.
1) aminase 2) dehydrogenase
3) isomerase 4) synthetase
A. Converts a cis-fatty acid to a trans-fatty acid.
B. Removes 2 H atoms to form double bond.
C. Combines two molecules to make a new compound.
D. Adds NH3.
Learning Check
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Match the type of reaction with an enzyme
1) aminase 2) dehydrogenase
3) isomerase 4) synthetase
A. 3 Converts a cis-fatty acid to a trans-fatty acid.
B. 2 Removes 2 H atoms to form double bond.
C. 4 Combines two molecules to make a new compound.
D. 1 Adds NH3.
Solution
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The active site • is a region within an
enzyme that fits the shape of the reacting molecule called a substrate.
• contains amino acid R groups that bind the substrate.
• releases products when the reaction is complete.
Active Site
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Enzyme Catalyzed Reaction
In an enzyme-catalyzed reaction• a substrate attaches to the
active site.• an enzyme-substrate (ES)
complex forms.• reaction occurs and products
are released. • an enzyme is used over and
over.
E + S ES E + P
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Lock and Key Model
In the lock-and-key model• the active site has a rigid shape.• an enzyme only binds substrates that exactly fit
the active site.• the enzyme is analogous to a lock.• the substrate is the key that fits that lock.
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Induced-fit Model
In the induced-fit model• enzyme structure is flexible, not rigid.• enzyme and substrate adjust the shape of the active site
to bind substrate.• the range of substrate specificity increases.• shape changes improve catalysis during reaction.
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Example of An Enzyme Catalyzed Reaction
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Learning Check
A. The active site is(1) the enzyme(2) a section of the enzyme(3) the substrate
B. In the induced fit model, the shape of the enzyme when substrate binds
(1) stays the same(2) adapts to the shape of the substrate
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Solution
A. The active site is
(2) a section of the enzyme
B. In the induced fit model, the shape of the enzyme when substrate binds
(2) adapts to the shape of the substrate
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Diagnostic Enzymes
Diagnostic enzymes• determine the
amount of damage in tissues.
• that are elevated may indicate damage or disease in a particular organ.
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Diagnostic Enzymes
Levels of enzymes CK, LDH, and AST
• are elevated following a heart attack.
• are used to determine the severity of the attack.
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Isoenzymes
Isoenzymes• catalyze the same reaction in different tissues in the
body.• can be used to identify the organ or tissue involved
in damage or disease.• such as lactate dehydrogenase (LDH), which
converts lactate to pyruvate, consists of five isoenzymes.
• such as LDH have one form more prevalent in heart muscle and another form in skeletal muscle and liver.
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Isoenzymes
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Chapter 16 Amino Acids, Proteins, and Enzymes
16.8 Factors Affecting Enzyme Activity
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Enzymes• are most active at an
optimum temperature (usually 37°C in humans).
• show little activity at low temperatures.
• lose activity at high temperatures as denaturation occurs.
Temperature and Enzyme Action
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Enzymes• are most active at
optimum pH.• contain R groups of
amino acids with proper charges at optimum pH.
• lose activity in low or high pH as tertiary structure is disrupted.
pH and Enzyme Action
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Optimum pH Values
Enzymes in• the body have an optimum pH of about 7.4.• certain organs, enzymes operate at lower and higher
optimum pH values.
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Enzyme Concentration
As enzyme concentrationincreases
• the rate of reaction increases (at constant substrate concentration).
• more substrate binds
with enzyme.
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Substrate Concentration
As substrate concentration increases • the rate of reaction
increases (at constant enzyme concentration).
• the enzyme eventually becomes saturated giving maximum activity.
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Sucrase has an optimum temperature of 37°C and an optimum pH of 6.2. Determine the effect of the following on its rate of reaction.1) no change 2) increase 3) decrease
A. Increasing the concentration of sucroseB. Changing the pH to 4C. Running the reaction at 70°C
Learning Check
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Sucrase has an optimum temperature of 37°C and an optimum pH of 6.2. Determine the effect of the following on its rate of reaction
1) no change 2) increase 3) decrease
A. 2 Increasing the concentration of sucrase
B. 3 Changing the pH to 4
C. 3 Running the reaction at 70°C
Solution
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Inhibitors• are molecules that cause a loss of catalytic activity.• prevent substrates from fitting into the active sites.
E + S ES E + P
E + I EI no P
Enzyme Inhibition
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Competitive Inhibition
A competitive inhibitor• has a structure that is
similar to that of the substrate.
• competes with the substrate for the active site.
• has its effect reversed by increasing substrate concentration.
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A noncompetitive inhibitor• has a structure that is much
different than the substrate.• distorts the shape of the
enzyme, which alters the shape of the active site.
• prevents the binding of the substrate.
• cannot have its effect reversed by adding more substrate.
Noncompetitive Inhibition
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Malonate and Succinate Dehydrogenase
Malonate• is a competitive
inhibitor of succinate dehydrogenase.
• has a structure that is similar to succinate.
• inhibition is reversed by adding succinate.
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Identify each description as an inhibitor that is
1) Competitive 2) Noncompetitive.
A. Increasing substrate reverses inhibition.B. Binds to enzyme surface, but not to the active site.C. Structure is similar to substrate.D. Inhibition is not reversed by adding more substrate.
Learning Check
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Identify each description as an inhibitor that is:
1) Competitive 2) Noncompetitive
A. 1 Increasing substrate reverses inhibition.B. 2 Binds to enzyme surface, but not to the active site.C. 1 Structure is similar to substrate.D. 2 Inhibition is not reversed by adding more
substrate.
Solution
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16.9 Enzyme Cofactors
Chapter 16 Amino Acids, Proteins, and Enzymes
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Function of Coenzymes
A coenzyme prepares the active site for catalytic activity.
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Water-Soluble Vitamins
Water-soluble vitamins are• soluble in aqueous solutions.• cofactors for many enzymes.• not stored in the body.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Fat-Soluble Vitamins
Fat-soluble vitamins• are A, D, E, and K. • are soluble in lipids, but not in aqueous solutions.• are important in vision, bone formation, antioxidants,
and blood clotting.• are stored in the body.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Learning Check
Identify each compound as a
1) water-soluble vitamin 2) fat-soluble vitamin.
A. Folic acid
B. Retinol (Vitamin A)
C. Vitamin C
D. Vitamin E
E. Niacin
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Solution
Identify each compound as a
1) water-soluble vitamin 2) fat-soluble vitamin.
A. 1 Folic acid
B. 2 Retinol (Vitamin A)
C. 1 Vitamin C
D. 2 Vitamin E
E. 1 Niacin
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Thiamin (Vitamin B1)
Thiamin • was the first B vitamin identified.• is part of the coenzyme thiamin pyrophosphate (TPP), required in the decarboxylation of -keto carboxylic
acids.
• deficiency results in beriberi (fatigue, weight loss, and nerve degeneration).
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Riboflavin (Vitamin B2)
Riboflavin is• made of the sugar alcohol ribitol and flavin.• part of the coenzymes flavin adenine dinucleotide (FAD)
and flavin mononucleotide (FMN).• needed for good vision and healthy skin.
N
N N
NHH3C
H3C
CH2 CH CH CH CH2 OH
OHOHOH
O
O
D-Ribitol
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Niacin (Vitamin B3)
Niacin• is part of the coenzyme
nicotinamide adenine dinucleotide (NAD+) involved in oxidation-reduction reactions.
• deficiency can result in dermatitis, muscle fatigue, and loss of appetite.
• is found in meats, rice, and whole grains.
N
C
O
OH
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Pantothenic Acid (Vitamin B5)
Pantothenic acid• is part of coenzyme A needed for energy production
as well as glucose and cholesterol synthesis.• deficiency can result in fatigue, retarded growth,
cramps, and anemia.• is found in salmon, meat, eggs, whole grains, and
vegetables.
HO CH2 C CH C N CH2 CH2 C OH
O
H
OH OCH3
CH3
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Cobalamin (Vitamin B12)
Cobalamin• consists of four pyrrole
rings with a Co2+.• is a coenzyme for
enzymes that transfer methyl groups and produce red blood cells.
• deficiency can lead to pernicious anemia and nerve damage.
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Ascorbic Acid (Vitamin C)
Vitamin C• is required in collagen
synthesis.• deficiency can lead to
weakened connective tissue, slow-healing wounds, and anemia.
• is found in blueberries, citrus fruits, tomatoes, broccoli, red and green vegetables.
O CHOH
CH2OH
OHHO
O
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Folic Acid (Folate)
Folic acid (folate)• consists of pyrimidine,
p-aminobenzoic acid, and glutamate.
• forms the coenzyme THF used in the transfer of carbon groups and the synthesis of nucleic acids.
• deficiency can lead to abnormal red blood cells, anemia, and poor growth.
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Vitamin A
Vitamin A• is obtained from meats and beta-carotenes in plants.• has beta-carotenes that are converted by liver
enzymes to vitamin A (retinol).
H3C CH3
CH3
CH3 CH3
CH3 CH3H3C CH3
H3C
CH3 CH3
CH2OHH3C CH3
CH3
Beta-carotene
Retinol (vitamin A)
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Vitamin D
Vitamin D (D3)• is synthesized in skin
exposed to sunlight. • regulates the absorption of
phosphorus and calcium during bone growth.
• deficiency can result in weakened bones.
• sources include cod liver oil, egg yolk, and enriched milk.
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Vitamin E
Vitamin E• is an antioxidant in cells.• may prevent the oxidation of unsaturated fatty acids.• is found in vegetable oils, whole grains, and
vegetables.
O
CH3
HO
H3C
CH3
CH3
CH3
CH3 CH3 CH3
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Vitamin K
• Vitamin K1 in plants has a saturated side chain.
• Vitamin K2 in animals has a long unsaturated side chain.
• Vitamin K2 is needed for the synthesis of zymogens for blood clotting.
3n
CH3
CH3
O
O CH3 CH3
Vitamin K2 (menaquinone)
CH3
CH3
O
O CH3 CH3
Vitamin K1 (phylloquinone)
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Learning Check
Identify the vitamin associated with each1) Thiamin (B1) 2) Vitamin A
3) Vitamin K 4) Vitamin D5) Ascorbic Acid
A. Collagen formationB. BeriberiC. Absorption of phosphorus and calcium in bone D. VisionE. Blood clotting
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Solution
Identify the vitamin associated with each
1) Thiamin (B1) 2) Vitamin A
3) Vitamin K 4) Vitamin D
5) Ascorbic Acid
A. 5 Collagen formation
B. 1 Beriberi
C. 4 Absorption of phosphorus and calcium in bone
D. 2 Vision
E. 3 Blood clotting