unit 2 amino and proteins - srm institute of science and … · amino acids and proteins....
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Significance of Proteins
1. Keep the cells and tissues growing, renewing and mending
2. Takepart in some kinds of important physiological
activities
3. Oxidation and supply energy
Structural
Movement
Transport
Storage
Hormone
Protection
Enzymes
Collagen; bones, tendons, cartilageKeratin; hair, skin, wool, nails, feathers
Myosin & Actin; muscle contractions
Hemoglobin; transports O2Lipoproteins; transports lipids
Casein; in milk. Albumin; in eggs
Insulin; regulates blood glucoseGrowth hormone; regulates growthImmunoglobulins; stimulate immunitySnake venom; plant toxins; Sucrase; catalyzes sucrose hydrolysisPepsin; catalyzes protein hydrolysis
Amino acids • Are the building blocks of proteins.• Contain a carboxylic acid group and an amino
group on the alpha (α) carbon.• Are ionized in solution.• Each contain a different side group (R).
RR
│+ │H2N—C —COOH H3N—C —
COO−│ │
HH
ionized form
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
Acidic Basic
Essential amino acids
• Must be obtained from the diet.
• Are the ten amino acids not synthesized by the body.
• Are in meat and diary products.
• Are missing (one or more) in grains and vegetables.
TABLE 19.3
Amino acids • Are chiral except for glycine.• Have Fischer projections that are stereoisomers.• That are L are used in proteins.
L‐alanine D‐alanine L‐cysteine D‐cysteine
CH2SHH2N H
COOH
CH2SH H NH2
COOH
CH3
H NH2
COOH
CH3
H2N H
COOH
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
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 ionat pI pH > pICharge: 0 Charge: 1−
In solutions 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< pICharge: 0 Charge: 1+
H+ OH−
+ +
H3N–CH2–COOH H3N–CH2–COO– H2N–CH2–COO–
positive ion zwitterion negative ion
(at low pH) (at pH) (at high pH)
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– + H2Opeptide bond
The primary structure of a protein is
• The particular sequence of amino acids.
• The backbone of a peptide chain or protein.
CH3
SHCH2
CH3
S
CH2
CH2CH O
O-CCH
H
N
O
CCH
H
N
O
CCH
H
N
O
CCHH3N
CH3
CH3CH
Ala─Leu─Cys─Met
The secondary structures of proteins indicate thethree‐dimensional spatial arrangements of thepolypeptide chains.
An alpha helix has• A coiled shape held in place by hydrogen bonds between the amide groups and the carbonyl groups of the amino acids along the chain.
• Hydrogen bonds between the H of a –N‐H group and the O of C=O of the fourth amino acid down the chain.
A beta‐pleated sheet is a secondary structure that
• 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.
A triple helix• Consists of three alpha
helix chains woven together.
• Contains large amounts glycine, proline, hydroxyproline, and hydroxylysine that contain –OH groups for hydrogen bonding.
• Is found in collagen, connective tissue, skin, tendons, and cartilage.
The tertiary structure of a protein• Gives a specific three dimensional shape to the polypeptide chain.
• Involves interactions and cross links between different parts of the peptide chain.
• Is stabilized byHydrophobic and hydrophilic interactions. Salt bridges.Hydrogen bonds.Disulfide bonds.
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
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.
The quaternary structure• Is the combination of two or more tertiary units.
• Is stabilized by the same interactions found in tertiary structures.
• Of hemoglobin consists of two alpha chains and two beta chains. The hemegroup in each subunit picks up oxygen for transport in the blood to the tissues.
hemoglobin
Protein hydrolysis
• Splits the peptide bonds to give smaller peptides and amino acids.
• Occurs in the digestion of proteins.
• Occurs in cells when amino acids are needed to synthesize new proteins and repair tissues.
• In the lab, the hydrolysis of a peptide requires acid or base, water and heat.
• In the body, enzymes catalyze the hydrolysis of proteins.
+
H3N CH COHOCH3
+
H2O, H+
++
heat,
CH2
OH
H3N CH C
O
N
H
CH C
O
OH
CH3
CH2
OH
CH C
O
OHH3N
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 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.
Transamination of Amino acids
Transamination is the process by which an amino group, usually from glutamate, is transferred to an α‐keto acid, with formation of the corresponding amino acid plus α‐
ketoglutarate.
C
R1
H
NH3
COO + C
R2
O
COO C
R2
H
NH3
COO+C
R1
O
COO
Aminotransferases
amino acid-1 amino acid-2keto acid-2keto acid-1
Deamination of Amino acids
COOHCHNH2
(CH2)2
COOHL-Glu
L-Glu Dehydrogenase
NAD+NADH+H+ COOH
C NH(CH2)2
COOH
COOHC
(CH2)2
COOH
H2O NH3 O
¦Á-ketoglutarate
Sources and Outlet of NH31. Sources:
⑴ Endogenous sources:
① Deamination of AAs‐‐main source
② Catabolism of other nitrogen containing compounds
③ Kidney secretion (Gln)
Alanine Glucose Cycle
protein
amino acid
NH3
pyruvate
¦Á-keto acid
G
muscle
pyruvate
G
NAD+ + H2O
NADH + H+
blood liver
urea
Glu
Ala Ala Ala
Glu
G
¦Á-keto acid
+ NH3
Transportation of NH3 by Gln
CONH2
(CH2)2
CHNH2
COOH
Gln synthetaseCOOH
(CH2)2
CHNH2
COOH
+ NH3
ATP ADP + Pi
Glu GlnGlutaminase
H2O
Formation of Urea1. Site: liver (mitochondria and cytosol)
2. Process --------- ornithine cycle
ornithine NH3 + CO2
H2O
NH3H2O
H2O
urea
arginase
Arg citrulline
Metabolism of amino acids
• Decarboxylation of amino acids
• Metabolism of one carbon unit
• Metabolism of sulfur‐containing AAs
• Metabolism of aromatic AAs
• Metabolism of branched‐chain AAs
Metabolism of Sulphur Containing amino acids
NH2CHCH2
S CH3
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COOH
2
adenosyl transferase
ATPPPi+Pi
NH2CHCH2
S CH3
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COOH
2
RH
RCH3
methyl transferase
NH2CHCH2
SH£¨ £©
COOH
2
S-adenosyl homocysteine
H2O
NH2CHCH2
SH£¨ £©
COOH
2
homocysteine
N5 -CH3FH4
FH4Met synthase
VB12£¨ £©
A
SAM
A
Met
A
Metabolism of Cysteine and Cystine
NH2CHCH2
SH
COOHcysteine
NH2CHCH2
SH
COOHcysteine
+NH2CH
CH2
S
COOHNH2CH
CH2
S
COOH
2H
2Hcystine
Metabolism of Aromatic amino acids
CH2CHNH2COOH
+ O2
CH2CHNH2COOH
+
OH
H2O
tetrahydro- biopterin
dihydro- biopterin
Phe hydroxylase
NADPH+H+NADP+
PheTyr
Metabolism of Tryptophan
N
CH2CHNH2COOH
H
O2
NHCHO
CCHNH2COOH
O
N-formyl kynurenine
H2O
NH2
CCHNH2COOH
OHCOOH
kynurenine
N10 -CHOFH4 synthetase
FH2+ATPADP+Pi
N10 -CHOFH4
Trp
Metabolism of TyrosineCH2CHNH2COOH
OH
CH2CHNH2COOH
OHHO
CO2CH2CH2NH2
OHHO
CH2CH2NH2
OHHO
OH
CH2CH2NHCH3
OHHO
OH
CH2CHNH2COOH
OO
OO
NH
CH2CCOOH
OH
O
OH
OH
CH2COOH
dopa dopamine
dopa quinone norepine-
phrine
indole-5,6- quinone
fumarate +acetoacetate
Tyr
epinephrine
SAM
Tyr transaminase
melanin
hydroxy-phenyl-pyruvate
homogentisate
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