Amino Groups

Amino groups are widely distributed in biological substances such as proteins, polynucleotides, polysaccharides, and lipids. They play important roles in electrostatic interactions because of their nucleophilicity and positive charge. The amino groups in the nucleotides of nucleic acids are involved in pairing bases. Modification of these groups causes serious errors in nucleic acid replication, translation, and gene expression. Amino groups in proteins are important for maintaining their structure and solubility and, sometimes, for manifestating their biological function. In particular, an amino group plays a pivotal role in the enzymes that utilize pyridoxal phosphate derivatives as coenzymes.

 There are two kinds of amino groups in proteins. One is the N-terminal a-amino group, that has a pKa of 6 to 8, and the other is the Ɛ -amino group of lysine residues that have pKa values generally between 8 and 10.5. The N-terminal amino group is very important for elucidating the primary structure of a protein because a free N-terminal amino group is indispensable for Edman Degradation. N-terminal amino groups in proteins are often protected by acylation as a posttranslational modification. Attaching ubiquitin molecules to the amino groups of proteins induces their degradation . Amino groups are reactive nucleophiles and are widely utilized to immobilize proteins on solid supports for affinity chromatography.

1. Chemical Modification of Amino Groups in Proteins

Many modification methods for amino groups have been invented based on their excellent nucleophilicities (1). An amino group is a strong nucleophile only in its nonprotonated form, so it is most reactive at high pH. Because of the differences in pKa values between a- and  -amino groups, the former may be selectively modified by controlling the pH of the reaction medium. Because many amino groups are exposed and not involved in the protein function, modification of amino groups is suitable for introducing reporter groups, such as chromophores, into proteins and for radiolabeling them. The number of amino groups present and subsequently the extent of their modification with some modifying reagents, is determined by trinitrophenylation with 2,4,6-trinitrobenzene sulfonic acid. The integral number of amino groups present in a protein can be counted. Representative modification methods for amino groups are shown in Table 1.

Table 1. Chemical Modification of Amino Groups of Proteins

References

1.T. Imoto and H. Yamada (1989) In Protein Structure: A Practical Approach (T. E. Creighton, ed.), IRL Press, Oxford, UK, pp. 247–277.