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 C H R C O OH Amino acid structure Carboxyl group Amino group -The physical and chemical properties of the R group (side chains) determine unique characteristics of a particular amino acid. N terminus -NH 2 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 bonds Ionic bonds Van 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 UUU Cys to Phe Disulfide bridge disrupted, tertiary structure altered!