Amino acids essential residues

The A domains of all mammalian and some other GH13 a-amylases have a chloridebinding site that is essential for activity together with a short, inserted downstream amino acid loop (residues 304 through 311 in salivary amylase). This loop is flexible and interacts with substrate after binding so that the catalytic residues are correctly positioned for hydrolysis. It is absent from enzymes that do not require chloride ions for activity. The C domain has no known function, but it may facilitate solubility, in part, because of glycan attached to asparagine-413 in this domain. 

Fig. 18. Amino acids essential for bacterial sialidase action. The positions of the amino acid residues interacting with different parts of the Neu5Ac molecule or forming a hydrophobic pocket (by Leu, Trp and Met indicated by a dotted line at the left side of the sialic acid molecule) are shown. Vc, Vibrio cholerae sialidase [791] St, Salmonella typhimurium sialidase [790] and Cp, Clostridium perfringens sialidase [R.G. Kleineidam, personal communication].

Iodine is an essential element to humans and presents in the human body in minute amounts (15-20mg in adults), of which more than 80% exists in the thyroid gland. Iodine in the human body mainly comes from food intake and inhalation of atmospheric iodine. In the thyroid, iodine is added to the essential amino acid thyroxine residue on thyroglobulin. 

At present hexacyanoferrate(III) is the most widely applicable oxidant (see the table). Oxidants are generally used at a concentration of 0.5 mM, thus avoiding excessive oxidation of sulfhydryl groups and other side chains of the enzyme while still allowing the paracatalytic modification to proceed at an acceptably rapid rate. In some instances, the inactivation of the enzyme may be faster in the absence of substrate than in the presence of oxidant plus substrate. In such instances, direct oxidation of essential amino acids, cysteinyl residues in particular, apparently precedes paracatalytic modification. 

Carboxypeptidases are zinc-containing enzymes that catalyze the hydrolysis of polypeptides at the C-terminal peptide bond. The bovine enzyme form A is a monomeric protein comprising 307 amino acid residues. The structure was determined in the laboratory of William Lipscomb, Harvard University, in 1970 and later refined to 1.5 A resolution. Biochemical and x-ray studies have shown that the zinc atom is essential for catalysis by binding to the carbonyl oxygen of the substrate. This binding weakens the C =0 bond by... 

This led to the conclusion that these amino acids were essential for the resolution capability and only 6 new libraries of 18 compounds had to be synthesized with these amino acid residues to define the position 3. Surprisingly, the separation abilities of all six libraries were very similar. Therefore, tyrosine was chosen for continuing deconvolution, since it is convenient as its aromatic ring can easily be detected by UV spectrometry. The last step, defining position 5, required the synthesis and testing of 6 individual hexapeptides. 

AVP and OT are cyclic nonapeptides with a disulphide bridge between the cysteine residues 1 and 6, resulting in a six-amino acid ring and a COOH-terminal a-amidated three-residue tail. OT differs only in two amino acids from AVP lie in position 3, which is essential for OT recqrtor (OTR) stimulation and Leu in position 8. AVP has a Phe in position 3 and an Arg in position 8. Arg 8 is essential for acting upon vasopressin receptors (Fig. 1). Lysipressin, found in pigs and some marsupials, has a Lys in position 8 [1]. 

The voltage sensor is the part of a channel protein responsible for detection of the membrane potential. A voltage sensor of the voltage-dependent Na+ channel was predicted by Hodgkin and Huxley in 1952. Positively charged amino acid residues in S4 of each repeat play an essential role as the voltage sensor. 

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