Carbonyl groups, amino acid residues into

Once the pore is open, K+ ions appear to move outward in single file. Analysis of crystals shows four K+ ions interacting with the backbone carbonyl groups of the amino acid residues that form the ion selectivity filter (Fig. 6-8B). It is thought that two K+ ions occupy sites 1 and 3 (Fig. 6-8B, orange) and then switch to sites 2 and 4 (gray). The K+ ion in site 4 would dissociate into the extracellular space, the K+ ion in site 2 would move to site 3 and another... 

FIGURE 6.3 Introduction of carbonyl groups into amino acid residues. 

Methods for determining the kinds of amino acid residues modified depend somewhat upon the type of reagent used the introduction of a carbonyl group into a protein provides a handle which can facilitate the identification of the labeled residue, as will be seen later in this chapter. 

Table 6.1 lists the water-soluble vitamins with their structures and coenzyme forms. Certain portions of the coenzymes are especially important in their biological activities, and they are indicated by arrows. For example, in case of coenzyme A, a thiol ester is formed between its -SH residue and the acyl group being transferred. And in the case of pyridoxal phosphate, its carbonyl residue forms a Schiff base with the amino group of the amino acid that is being decarboxylated. Fat-soluble vitamins (Table 6.2) are also transformed into biologically active substances. However, with the possible exception of vitamin K, these do not operate as prosthetic groups or cosubstrates in specific enzyme reactions. 

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