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!