Amino Acids and Proteins

Overview:

Amino Acids and Proteins describes the amino acid structure and preparations, as well as the isoelectric pH and electrophoresis. Formation of proteins, protein structure, sequencing proteins, detection and denaturation of proteins are also addressed.

The fundamental structure of amino acids is presented and the ways in which those structures differ is stressed. The general R group is noted as being acid, base, cyclic, aromatic, indole, etc. The ten essential amino acids are noted and the importance of ingesting these compounds discussed. The dependence of one amino acid on another within human physiology is stressed using the synthesis of tyrosine from phenylalanine and subsequent results encountered when the body is unable to perform this synthesis.

Properties of amino acids studied include the formation of dipolar ions called zwitterions. These structures are used to describe the reaction of amino acids with acids and bases and the manner in which these zwitterions may behave as a physiological buffer. Also included in the study of properties is the important pH designation of the isoelectric point. The relationship between structure and isoelectric point is noted. From that discussion, the way in which electrophoresis works is explained.

The formation of polypeptides and proteins is addressed via the formation of a peptide link. The importance of blocking the N-terminus or C-terminus is explained. In the reverse of this process, the identification of a polypeptide by sequencing is discussed. The series of steps begins with elimination of disulfide linkages, followed by identification of the N-terminus using the Edman degradation with phenyl isothiiocyanate or the Sanger method with 2,4-dinitrofluorobenzene or some similar method. Complete hydrolysis of a sample to indicate the amino acid population followed, and the process concludes with partial hydrolysis of a sample with subsequent analysis. The student is given several opportunities to determine the sequence of a particular polypeptide.

Proteins are identified and defined. The four ways to describe the structure of proteins are described with examples of each. It is noted that while the first three are expected, the fourth only occurs in limited circumstances.

The detection of proteins always fascinates students because of their interest in forensics. The two important tests for proteins, the Xanthproteic test using nitric acid is one the student knows from experience, although perhaps not by name. There are very few students who reach this level of chemistry without experiencing the yellow coloration that arises from a nitric acid spill on the fingers. The second test, Ninhydrin, involves formation of Ruhemann’s purple, a classic way in which fingerprints may be detected and lifted.

The fragile structure of proteins is nowhere better demonstrated than through discussion of the denaturation process. Students are often surprised by the methods by which the biological activity of proteins may be destroyed.