Polypeptides – a quick review • A protein is a polymer consisting of several amino acids (a polypeptide) • Each protein has a unique 3-D shape or Conformation • This conformation endows each protein with a specific function
Dec 20, 2015
Polypeptides – a quick review
• A protein is a polymer consisting of several amino acids (a polypeptide)
• Each protein has a unique 3-D shape or Conformation
• This conformation endows each protein with a specific function
H
H
N CH
R
C
O
OH
Amino acid structure
Carboxyl groupAmino group
-The physical and chemical properties of the R group (side chains) determine unique characteristics of a particular amino acid.
N terminus-NH2
Carboxyl terminus-COOH
Peptide bond – enzymes join AA via a dehydration reaction:
Thus, during translation, the enzyme peptidyl transferase catalyzes this reaction
Hydrophobic (neutral) R Groups
Hydrophilic (neutral) R groups
Acidic R groups Basic R groups
Proteins have several levels of organization – function is dependent upon structure!
1) Primary structure – sequence of AA in a polypeptide chain
2) Secondary structure – coiling of the peptide chain (alpha helix, beta sheet)
3) Tertiary structure – folding of the secondary structure upon itself
– AA residues that are far apart in the primary structure are brought together, permitting interactions among their side chains (R groups)
4) Quaternary structure – combination of 2 or more like or unlike peptide chain subunits
-Disulfide bridges –
Other bonds contribute:Hydrogen bondsIonic bondsVan der Waals interactions
Tertiary structure:Dependant upon interactions between R groups:
*These help stabilize proteins
Quaternary structure
• Aggregation of 2 or more polypeptide subunits
CODONS on mRNA dictate which charged tRNA’s will come to the ribosome, and therefore which AA’s will be added to the polypeptide.
Mutations – changes in the DNA sequence may alter changes in AA sequence
• Changes in AA sequence MAY alter protein conformation
• Altered conformation results in altered protein function!
UGU to UUUCys to Phe
Disulfide bridge disrupted, tertiary structure altered!