Chapter 4 - Amino Acids

Amino acids structure

All proteins are composed of 20 amino acids. Contain ionized amino (basic) and carboxyl groups (acidic)

Dipolar ions

AA carry both + and - charge at physiological pH 7.4

Zwitterion

A molecule containing both a positively charged group (NH4+) and a negatively charged group (COO-)

Nonpolar amino acids

Aliphatic (hydrocarbon chain) or aromatic. R groups with no charge. Hydrophobic interior residues

Polar, uncharged amino acids

R groups with no charge but contain a polar functional group. Hydrophilic residues on surface

Polar, charged amino acids

R groups with an extra acidic or basic group. Negatively charged at physiological pH. Hydrophilic residues on structure.

Disulfide bonds

Form when cysteine bonds with another cysteine through oxidation of the two thiol groups

Disulfide bonds importance

protein folding structure and function

Isoelectric point

the pH at which an AA carries no net charge

At a pH above a pI

the amino acid becomes a negative ion

At a pH below a pI

the amino acid becomes a position

peptide bond

C=O-NH linkage

Chiral center

A central molecule that is optically active (they rotate the plane of polarized light). They are asymmetric

All amino acids are optically active except

glycine

enantiomers

Molecules that are non-superimposable mirror images

If the amino group (NH3+) is toward right

It is D form

If the amino group (NH3+) is towards left

It is L form

Ibuprofen

Only one enantiomer is physiologically active

Thalidomide

A mild sedative, but its inactive enantiomer causes severe birth defects

Glutathione (GSH)

An ubiquitous tripeptide and plays a role in cellular metablism.

Glu-Cys-Gly

Glutathione role

Helps inactivate oxygen radicals that potentially damage cellular structures