Amino Acids & Peptides

BIOMEDICAL IMPORTANCE

• the monomer units– L-α-amino

• Cellular functions– Nerve transmission– Biosynthesis of porphyrins– Purines– Pyrimidines – Urea

Biomedical Importance

• Peptides – Neuroendocrine system• as hormones,• hormone-releasing factors, • Neuromodulators,or neurotransmitters

• One- & three-letter • Additional amino acids arise by modification– methylation, formylation, acetylation, prenylation,

and phosphorylation

L- α-Amino acids present in proteins.

• Amino Acids May Have Positive, Negative,• or Zero Net Charge

• Zwitterions – equal number of ionizable groups of opposite

charge

• pKa – Express the Strengths of Weak Acids

Protonic equilibria of aspartic acid.

• The net charge on an amino acid – Depends upon the pKa values of its functional

groups– the pH of the surrounding medium

• At Its Isoelectric pH (pI), an amino acid bears no net charge

For lysine, pI is calculated from:

the pI for aspartic acid

Typical range of pKa values forionizable groups in proteins.

Amino Acid Sequence DeterminesPrimary Structure

Peptide Structures

• Some Peptides Contain Unusual Amino Acids

Glutathione (γ-glutamyl-cysteinylglycine).Note the non-α peptide bond that linksGlu to Cys.

• Peptides Are Polyelectrolytes

• The Peptide Bond Has Partial Double-Bond Character

The four atoms of the peptide bond (colored blue) are coplanar