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High-Yield Terms

  • pH: defined as the negative logarithm of the hydrogen ion (H+) concentration of any given solution

  • pKa: represents a relationship between pH and the equilibrium constant (Ka) for the dissociation of weak acids and bases in solution. Like pH, pKa is the negative logarithm of Ka

  • Isoelectric point: defines the pH at which a molecule or substance carries no net electric charge

  • Hendersen-Hasselbalch equation: defines the relationship between pH and pKa for any dissociation reaction of a weak acid or base such that when the concentration of any conjugate base (A-) and its acid (HA) are equal, the pKa for that dissociation is equivalent to the pH of the solution

  • Buffering: relates to the property that when the pH of a solution is close to the pKa of a weak acid or base, the addition of more acid or base will not result in appreciable change in the pH

Amino Acids: Building Blocks for Protein

Chemical Nature of the Amino Acids

All peptides and polypeptides are polymers of α-amino acids. There are 20 α-amino acids relevant to the makeup of mammalian proteins (see later). Several other amino acids found in the body are in free or combined states (ie, not associated with peptides or proteins). These non–protein-associated amino acids perform specialized functions. Several of the amino acids found in proteins also serve functions distinct from the formation of peptides and proteins, for example, tyrosine in the formation of thyroid hormones or glutamate acting as a neurotransmitter.

The α-amino acids in peptides and proteins (excluding proline) consist of a carboxylic acid (–COOH) and an amino (–NH2) functional group attached to the same tetrahedral carbon atom. This carbon is the α-carbon. Distinct R groups, that distinguish one amino acid from another, are also attached to the α-carbon (except in the case of glycine where the R group is hydrogen). The fourth substitution on the tetrahedral α-carbon of amino acids is hydrogen.

Classification of Amino Acids

Each of the 20 α-amino acids found in proteins can be distinguished by the R group substitution on the α-carbon atom. There are 2 broad classes of amino acids based upon whether the R group is hydrophobic or hydrophilic (Table 1-1).

TABLE 1-1:l-α-Amino Acids Present in Proteins

The hydrophobic amino acids tend to repel the aqueous environment and, therefore, reside predominantly in the interior of proteins. This class of amino ...

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