生物化学英文课件Biochemistry-chapter 3

2. Structures of the 20 common (standard) amino acids.
All of the common amino acids found in proteins are α–amino acids.
COO H
H2N
CH
R
3. Ionization of amino acids
➢ Simple proteins ➢ Conjugated proteins
3. Molecular size and Mr of proteins
➢ Proteins are homogeneous. ➢ Mr of proteins vary from about 6000 Da to 1 ×
4. Conformation and structure levels of proteins
➢Primary structure - amino acid linear sequence ➢Secondary structure - regions of regularly repeating
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
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.
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.
Structures of Proteins Reveal Evolutionary Relationships
3.1 Outline of Proteins
1. Proteins is most important biological functional molecules:
(1) Enzymes, the biochemical catalysts (photosynthesis,thermophilic bacteria,HIV-1 revers transcriptase)
COO-
Amino group
+ H3 N
a
Carboxylic group
H
R group
H = Glyciwk.baidu.come 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)
(2) Storage and transport of biochemical molecules (Hb, Mb)
(3) Physical cell support and shape (tubulin, actin, collagen)
(4) Mechanical movement (flagella, mitosis, muscles)
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
Stereochemistry of amino acids
➢19 of the 20 common amino acids have a chiral a-carbon atom (Gly does not)
➢Threonine and isoleucine have 2 chiral carbons each (4 possible stereoisomers each)
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
• 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
➢ 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
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
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
(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)
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
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
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) According to polymerization of protein molecules;
➢ Monomeric proteins ➢ Oligomeric proteins (multimeric proteins)
➢ (3) According to conjugation of protein molecules;
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
2. Classification of proteins:
(1) According to shape of protein molecules;
➢ Globular proteins
Usually water soluble, compact, roughly spherical Hydrophobic interior, hydrophilic surface globular proteins include enzymes, carrier and regulatory proteins
• Four amino acids have saturated side chains: Alanine (Ala, A) Valine (Val, V) Leucine (Leu, L, The occurrence in proteins is the highest.) Isoleucine (Ile, I)
➢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)
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 ?