Tyrosine side chains, iodination

Careful acetylation of pepsin with ketene has yielded preparations with no free amino groups, but with full activity. More extensive acetylation inactivates the enzyme. Tyrosine side chains appear to be somewhat more important to the enzyme, but some activity remains after partial iodination, and treatment with nitrous acid leaves 50 per cent of the activity. 

The predominant mechanism of radiolabelling of antibodies with radioiodine involves electrophilic substitution of the iodine into the activated phenolic ring of tyrosine side-chains as shown in Scheme 1. Provided that low substitution ratios are used it is unlikely that radiolabelling will take place at any other site in the large immunoglobulin molecule but in proteins lacking tyrosine residues... 

The addition of a radioactive iodine atom to a protein molecule typically has little effect on the resultant protein activity, unless the active center is modified in the process. The size of an iodine atom is relatively small and does not result in many steric problems with large molecules. The sites of potential protein modification are tyrosine and histidine side chains. Tyrosine may be modified with a total of two iodine atoms per phenolate group, whereas histidine can incorporate one iodine. Sulfhydryl modification at cysteine residues is typically unstable. 

Figure 12.5 IODO-GEN is a water-insoluble oxidizing agent that can react with 1251 - to form a highly reactive mixed halogen species, 125IC1. This intermediate can add radioactive iodine atoms to tyrosine or histidine side chain rings.

The reaction of IODO-GEN with iodide ion in solution results in oxidation with subsequent formation of a reactive, mixed halogen species, IC1 (Fig. 266). Either 125I or 13 1 can be used in this reaction. The IC1 then rapidly reacts with any sites within target molecules that can undergo electrophilic substitution reactions. Within proteins, any tyrosine and histidine side-chain groups can be modified with iodine within... 

The synthesis of the spirocyclic core [70-77] is obviously the most difficult task, the biomimetic approach being the most frequent way of preparing it. The strategy is based on the hipervalent iodine-mediated oxidative hydroxylation of a tyrosinal derivative followed by a cis-bisepoxidation. The shortest way [75] involved the introduction of the side chain as an amide of tyrosine ethyl ester. Aranorosin was obtained after DIBAL reduction to the aldehyde, oxidation to the dienone with phenyliodosyl bis(trifluoroacetate) (PIFA) and final epoxidation (Scheme 24). 

Many other amino acids, in addition to the ones listed here, are known to exist. They occur in some, but by no means all, proteins. Figure 3.4 shows some examples of the many possibilities. They are derived from the common amino acids and are produced by modification of the parent amino acid after the protein is synthesized by the organism in a process called posttranslational modification. Hydroxyproline and hydroxylysine differ Ifom the parent amino acids in that they have hydroxyl groups on their side chains they are found only in a few connective-tissue proteins, such as collagen. Thyroxine differs from tyrosine in that it has an extra iodine-containing aromatic group on the side... 

It is thought that L-thyroxine is formed by the electrophilic iodination of L-tyrosine followed by dimerization and loss of the side chain with the mediation of PLP (pyridoxal phosphoric acid). 

Theoretically, the condensation of mono- and diiodotyrosine could yield four different compounds 3,3 -diiodothyronine, 3,3, 5 -triiodothyronine, 3,5,3 -triio-dothyronine, and thyroxine (see Fig. 8-4). The exact mechanism of the formation of these various iodinated derivatives is not clear, but it has been suggested that they are formed by the condensation of two molecules of iodotyrosines with loss of a side chain. In this reaction, the iodinated tyrosine is converted by the loss of two atoms of hydrogen to the quinoid form, which acts as a free radical and condenses with other molecules of iodinated tyrosine to yield a diphenyl ring. During such a reaction, an oxygen bridge is formed, and one of the side chains of tyrosine is split to yield an a-amino acrylic acid. The a-amino acrylic acid is further metabolized to yield ammonia and pyruvic acid. 

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