Amino Acids, Peptides, Proteins
Functions of proteins:
Enzymes
Transport and Storage
Motion, muscle contraction
Hormones
Mechanical support
Immune protection (Antibodies)
Generate and transmit nerve impulses
Control growth and differentiation
Lens protein in eye
Feathers
Spider webs
Horns
Milk proteins
Antibiotics
Mushroom poison …..
Luciferin, luciferase
Hemoglobin
Amino Acids
General structure
α carbon, side chain (R group)
Amino Acids
Amino Acids
Amino Acids
Amino Acids
Amino Acids
Amino Acids
Amino Acid
Abbreviations
Nonpolar, aliphatic R group
Glycine
Alanine
Proline
Valine
Leucine
Isoleucine
Methionine
Gly
Ala
Pro
Val
Leu
Ile
Met
G
A
P
V
L
I
M
Aromatic R groups
Phenylalanine
Tyrosine
Tryptophan
Phe
Tyr
Trp
F
Y
W
Polar, uncharged R groups
Serine
Threonine
Cysteine
Asparagine
Glutamine
Ser
Thr
Cys
Asn
Gln
S
T
C
N
Q
Positively charge R groups
Lysine
Histidine
Arginine
Lys
His
Arg
K
H
R
Negatively charged R groups
Aspartate
Glutamate
Asp
Glu
D
E
Amino Acids
Amino Acids
Additional properties of Amino Acids
Numbering of R group carbons
ε
6
CH2
δ
5
CH2
γ
4
CH2
β
3
CH2
+NH
3
α
2
CH
1
COO-
+NH
3
Aromatic side chains absorb UV light
Trp 280 nm
Tyr 280 nm
Phe 260 nm
Peptide bond 210-214 nm
Disulfide bond formation with cysteine
oxidation
reduction
Amino Acids
Nonpolar, aliphatic R group
Gly, Ala, Pro, Val, Leu, Ile, Met
Gly - no steric hindrance
Pro - hinders backbone flexibility
hydrophobic core of soluble proteins
found in transbilayer part of membrane proteins
Aromatic R groups
Phe, Tyr, Trp
hydrophobic, Stacking
Tyr/Trp - H-bonding
Tyr - site of phosphorylation
Polar, uncharged R groups
Ser, Thr, Cys, Asn, Gln
Ser/Thr - H-bonding; phosphorylated, glycosylated; enzyme active sites
Cys - disulfide bonds; enzyme active sites; metal ion binding
Asn/Gln - very polar, H-bonding
Positively charge R groups
Lys, His, Arg
His - pKa close to neutrality (catalysis); ligand for metal ions (Zn2+, Fe2+)
Negatively charged R groups
Asp, Glu
General acids/bases in catalysis (lysozyme)
Chelate divalent metal ions (Mg, Ca, Mn, Zn)
Uncommon Amino Acids
prothrombin
collagen
elastin
myosin
Amino Acids
Optical Activity of Amino Acids
For all AA except glycine the α carbon is bonded to 4 different groups:
Carboxyl, amino, hydrogen, and R group (in Gly, R group is hydrogen)
Chiral center = ___________________
All AA except Gly
Gly
Amino Acids
Optical Activity of Amino Acids
1 Chiral center = ___ stereoisomers
Stereoisomer found in proteins =
Amino acids act as acids and bases
“zwitterion”
amphoteric
Base = proton acceptor, electron pair donor
Acid = proton donor, electron pair acceptor
Amino acid titration Curve
2 buffer
regions
pK1 = a carboxyl group
pK2 = a amino group
isoelectric point (pI)- pH where there is an equal amount of (+)
and (-) charges (overall charge of zero)
isoelectric point (pI) for glycine is at pH = 5.97
pI = (pK1 + pK2)/2
Chemical environment influences pKa
Titration Curve of Histidine
Histidine R group has pKa = 6.0
No other AA side chain has a pKa near neutral pH
So Histidine is really the only AA that can be:
an effective buffer at physiological pH (7.0)
Peptides and Proteins
Peptide - two amino acids joined covalently by a peptide bond
Polypeptide - many AA joined together by peptide bond
(M.W.<10,000)
Protein - macromolecule with one or more polypeptide chains
condensation
Peptides
Ionization
Biologically active Peptides & Polypeptides
Dipeptide
(Nutrasweet)
Other small peptides
Oxytocin (9 aa) - stimulates uterine contractions
Bradykinin (9 aa) - inhibits tissue inflammation
Amanitin - mushroom poison
Polypeptides
Insulin - pancreatic hormone, needed for sugar metabolism,
2 polypeptide chains (30 aa and 21 aa)
Glucagon - pancreatic hormone, opposes action of insulin (29 aa)
Corticotropin - anterior pituitary gland hormone, stimulates
adrenal cortex (39 aa)
Proteins contain other prosthetic groups
Protein Structure