MAINCHAIN PART OF AMINO ACIDS

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•Six hydrophilic (polar) sidechains. All possess =O, O-H
or N-H groups that form hydrogen bonds with water
molecules H 2 O. Often found in active sites of proteins.
•Two acidic (polar) sidechains. These possess carboxyl
groups COO - with a negative charge at physiological pH.
Sidechain pK 3.9 (aspartate) and 4.3 (glutamate). Many
roles (eg: active sites, salt bridges).
•Three basic (polar) sidechains. These possess N-H
groups, of which lysine and arginine are protonated at
physiological pH (pK 10.5, 12.5), while histidine is a
physiological buffer (pK 6.0).
•Nine hydrophobic (nonpolar) sidechains. All
predominantly possess C-H bonds that have little
interaction with water molecules. Tendency to be found
buried in core of water-soluble proteins.
Hydrogen bonds with water.
Usually found at protein surfaces.
OH
CH 2
αCH
αCH
+
NH 3 COO - +
NH 3 COO -
Six hydrophilic amino acid sidechains
CH 3
CH
OH
OH
NH 2
CO
CH 2
CH 2
αCH
αCH
+
NH 3 COO - +
NH 3 COO -
NH 2
CO
CH 2
CH 2
αCH
+
NH 3 COO -
SH
CH 2
αCH
+
NH 3 COO -
Serine Threonine Tyrosine Asparagine Glutamine Cysteine
Ser, S Thr, T Tyr, Y Asn, N Gln, Q Cys, C
Hydrogen bonds with water.
Contain ionisable groups
Two acidic amino acids
NH 3
+
COO -
CH 2
αCH
Aspartate
Asp, D
COO -
NH 3
+
COO -
CH 2
CH 2
αCH
COO -
Glutamate
Glu, E
NH 3
+
αCH
Three basic amino acids
NH 3
+
CH 2
CH 2
CH 2
CH 2
COO -
NH 2 NH 2
+
NH 3
+
C
NH
CH 2
CH 2
CH 2
αCH
COO -
NH
CH
NH 3
+
C
CH 2
αCH
CH
NH +
COO -
Lysine Arginine Histidine
Lys, K Arg, R His, H
Nine hydrophobic amino acid sidechains
No hydrogen bonds
with water
NH 3
+
H
αCH
CH 3
S
CH 2
CH 2
αCH
+
NH 3
COO -
COO -
NH 3
+
CH 3
αCH
CH 2
COO -
αCH
+
NH 3 COO -
CH 3 CH 3
CH
CH
CH
CH 3
αCH
+
NH 3
CH 3
COO -
2
αCH
+
NH 3 COO -
NH
CH 2
αCH
+
NH 3 COO -
CH 3
CH 2 CH 3
CH
αCH
+
NH 3 COO -
Glycine, Gly, G Alanine, Ala, A Valine, Val, V Leucine, Leu, L Isoleucine, Ile, I
Methionine, Met, M Phenylalanine, Phe, F Tryptophan, Trp, W
Buried in proteins.
No hydrogen bonds
with water
CH 2 CH 2
CH 2
αCH
+
NH 2 COO -
Proline, Pro, P
(Note: imino acid)
FUNCTIONAL DIVERSITY OF PROTEINS
INTRODUCTION
TO PROTEINS
FIBROUS PROTEINS:
Structural support Collagen (bones) Tensile strength
Elastin (ligaments) Elastic strength
Proteoglycans (bone) Elastic strength
Keratin (hair, skin) External protection
GLOBULAR PROTEINS:
Enzymes Trypsin (stomach) Cleaves proteins
Lysozyme (tears) Antibactericidal
Transporters Haemoglobin (blood) Oxygen transport
Transferrin (blood) Iron transport
Protection
Immunoglobulins (blood)Immune response
Hormones Insulin (pancreas) Glucose regulator
and DNA-binding, Storage, Toxins, Cell signalling, and many more
HYBRID (MIXED) FIBROUS-GLOBULAR PROTEINS:
Contractile Actin (muscle) Thin filament
Myosin (muscle) Thick filament
Protection Fibrinogen (blood) Clotting