(2) Storage and transport of biochemical molecules (Hb, Mb)
(3) Physical cell support and shape (tubulin, actin, collagen)
(4) Mechanical movement (flagella, mitosis, muscles)
➢ (2) According to polymerization of protein molecules;
➢ Monomeric proteins ➢ Oligomeric proteins (multimeric proteins)
➢ (3) According to conjugation of protein molecules;
106 Da or more。 ➢ Usually insulin (5700 Da) or RNase (126000 Da)
was as the boundary of proteins and polypeptides. ➢ Mr of proteins ≈ Mr of amino acid ×110.
➢Mirror image pairs of amino acids are designated L (levo) and D (dextro)
➢Proteins are assembled from L-amino acids (few D-amino acids occur in nature)
Chapter 3 Amino Acids and the Primary
Structures of Proteins
3.1 Outline of Proteins 3.2 Structures of Amino Acids 3.3 Other Amino Acids and Amino
Acid Derivatives 3.4 Ionization of Amino Acids 3.5 Chemical reactions of AAs: 3.6 Peptide Bonds Link Amino Acids
Ball-and-stick model
Perspective
Fischer projections
Fischer projections - horizontal bonds from a
chiral center extend toward the viewer, vertical bonds extend away from the viewer
in Proteins
3.7 Protein Purification Techniques 3.8 Amino Acid Composition of Proteins 3.9 Determining the Sequence of Amino
Acid residues 3.10 Comparisons of the Primary
4. Configuration of amino acids.
Stereochemistry • Stereoisomers - compounds that have the
same molecular formula but differ in the arrangement of atoms in space • Enantiomers - nonsuperimposable mirror images • Chiral carbons - have four different groups attached
A.Aliphatic R groups: B.Aromatic R groups: C.Sulfur-containing R groups: D.Side chains with alcohol groups: E.Basic R groups: F.Acidic R groups and their amide
conformations of the peptide chain, such as a-helices and b-sheets ➢Tertiary structure - describes the shape of the fully folded polypeptide chain ➢Quaternary structure - arrangement of two or more polypeptide chains into multisubunit molecule
Proline has a nitrogen in the aliphatic ring system
• Proline (Pro, P) - has a three carbon side chain bonded to the a-amino nitrogen
• The heterocyclic pyrrolidine ring restricts the geometry of polypeptides = causes abrupt changes in the direction of the polypeptide chain
Mirror Images of Amino Acid
a
a
Mirror image of Stereoisomers
➢ The 19 chiral amino acids used in the assembly of proteins are all of the L configuration, although a few D-amino acids occur in nature.Why ?
(5) Decoding information in the cell (translation, regulation of gene expression)
(6) Hormones or hormone receptors (growth hormone, insulin receptor)
(7) Other specialized functions (antibodies, toxins etc)
3.2 Structures of Amino Acids
1. General structure of amino acids. More than 200 different AAs are
found in living organisms,including the 20 common (standard) amino acids.
• Proline (Pro, P) 3-carbon chain connects a-C and N = ring structure
Four aliphatic amino acid structures
Important in protein structure and folding since their R groups cluster away from water
COO-
Amino group
+ H3 N
a
Carboxylic group
H
R group
H = Glycine CH3 = Alanine
The R groups are different in 20 AAs.
Zwitterionic form of amino acids
• Under normal cellular conditions amino acids are zwitterions (dipolar ions): Amino group = -NH3+ (Protonated) Carboxyl group = -COO- (Ionized)
derivatives:
A. Aliphatic (hydrophobic) R Groups
• Glycine (Gly, G) - the a-carbon is not chiral since there are two H’s attached (R=H) = smallest and fits easiest into small nitches
5.Classification of the 20 common amino acids
(1).The three-letter and one-letter abbreviations.
(2). Classification of amino acids by chemical construction.
2. Classification of proteins:
(1) According to shape of prlar proteins
Usually water soluble, compact, roughly spherical Hydrophobic interior, hydrophilic surface globular proteins include enzymes, carrier and regulatory proteins
➢ Fibrous proteins
Provide mechanical support – not water soluble Often assembled into large cables or threads α-Keratins: major components of hair and nails Collagen: major component of tendons, skin, bones and teeth
Structures of Proteins Reveal Evolutionary Relationships
3.1 Outline of Proteins