7. Peptides and Proteins Flashcards
Proteins
• Proteins are polymers of amino acids
• Each amino acids in a protein contains an amino group, NH2
, a carboxyl group, COOH, and an R group, all bonded to the central carbon atom.
•The R (rest) group may be a hydrocarbon or they may
contain a functional group. This R gives the AA’s their distinctiveness.
Common amino acids
- Nature uses 20 amino acidsto build all amino acids in living organisms
- Each amino acid has a three letter shorthand code.
- Examples: Ala (alanine), Gly (glycine)
4 groups:
- non-polar side chains
- polar, neutral side chain
- acidic side chain
- basic side chain
Non-polar side chains
-
Polar, neutral side chains
-
Acidic and basic side chains
-
Peptides
• All amino acids present in a proteins are a-amino acids
in which the amino group is bonded to the carbon next to
the carboxyl group
• Two or more amino acids can join together by forming
amide bond, which is known as a peptide bond when they occur in proteins
Peptide formation
• A dipeptide
results when 2 amino acids combine together by
forming a peptide bond using amino group of one amino acid and carboxyl group of another amino acid
• A tripeptide
results when 3 amino acids combine together by
forming two peptide bonds, and so on. Any number of amino acids can link together and form a linear chain like
polymer – polypeptid
Acid - base properties of amino acids
• contain both an acidic group, COOH, and a basic group, NH
A proton is transferred from the COOH group to the NH
2 group producing a dipolar or zwitterion, with simultaneous positive and negative charge.
The pH at which the net positive and negative charges are
balanced is the amino acids isoelectric point-
the overall charge is zero.
Dissociation of peptides
•Acidic media (low pH): zwitterion accepts a proton on the
COO
- group to leave the positively charged NH3+ group
•Basic media (high pH), zwitterion loses a proton from NH3+ group to leave the negatively charged COO
- group
