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Proteins
All peptides and polypeptides are polymers of amino acids. There are 20
amino acids that are relevant to the chemical make-up of mammalian
proteins.
Amino Acids
Consist of a carboxylic acid (-COOH) and an amino (-NH2) functional group
attached to an α-carbon. Distinct R-groups that distinguish one amino acid
from another are also attached. The final bond on the α-carbon is a
hydrogen.
Amino Acid Classifications
Each of the 20 amino acids can be distinguished by the R-groups. Two main
classifications of R-groups are hydrophobic and hydrophilic.
The hydrophobic amino acids tend to repel water and therefore
usually reside in the interior of proteins. The hydrophilic amino acids tend
to interact with water and are usually found on the exterior surfaces of
proteins or in the active sites of enzymes.
Acid-Base Properties of Amino Acids:
R-COOH <——> R-COO– + H+
R-NH3+ <——> R-NH2+ H+
The –COOH and the –NH2 groups and the various R-groups are
capable of ionizing.
Amino acids are the best known examples of Zwitterions – neutral
molecules with a positive and negative electrical charge at different
locations within that molecule.
The Peptide Bond
Peptide bond formation is a condensation reaction (dehydration synthesis)
leading to the polymerization of amino acids into peptides and proteins.
Peptides are small consisting of a few amino acids. A number of hormones
and neurotransmitters are peptides. A dipeptide contains a single peptide
bond formed by the condensation of a carboxyl group of one amino acid
with the amino group of the other amino acid.
Primary Structure of Proteins
Refers to the linear number and order of the amino acids present.
Secondary Structure in Proteins
Within a single protein, different regions of the polypeptide chain assume
different conformations determined by the primary sequence of amino acids.
α-Helix: formation is stabilized by H-bonds between amino
nitrogens and carbonyl carbons of the peptide bonds every 4 amino acids
apart. Helical coiling of the peptide backbone results
- Keratin (hair), Myosin (muscles), Fibrin (blood clots) are 2
or more helices coiled together
Β-Sheets: composed of 2 or more different regions of stretches of at
least 5-10 amino acids. Β-sheets are said to be pleated.
- H-bonding occurs between neighboring polypeptide chains
rather than within one as in the α-helix
- Less common than the α-helix.
- Found extensively only in the protein of silk
Tertiary Structure in Proteins
Quaternary Structure in Proteins
Many proteins contain 2 or more different polypeptide chains that are held
together.
Ex: Hemoglobin, the oxygen carrying protein of the blood, contains 2
α and 2 β subunits.