Amino acid nitrogen atoms

Note that the second nitrogen atom of urea originates from the amino group of aspartate. Cytosolic amino acid nitrogen is the source of aspartate nitrogen, as shown in Equation (20.18) through (20.20) ... 

The effect of pH is rarely of use for pK measurement it is more often of use in identifying the site of protonation/deprotonation when several basic or acidic sites are present. Knowing the incremental substitutent effects Z of amino and ammonium groups on benzene ring shifts in aniline and in the anilinium ion (40), one can decide which of the nitrogen atoms is protonated in procaine hydrochloride (problem 24). 

The most widely used method for the laboratory synthesis of a-amino acids is a modification of the malonic ester synthesis (Section 21.7). The key reagent is diethyl acetamidomalonate, a derivative of malonic ester that aheady has the critical nitrogen substituent in place at the a-caibon atom. The side chain is introduced by alkylating diethyl acetamidomalonate in the same way as diethyl malonate itself is alkylated. 

Histidine is one of the 20 naturally occurring amino acids commonly found in proteins (see Chapter 4). It possesses as part of its structure an imidazole group, a five-membered heterocyclic ring possessing two nitrogen atoms. The pAl for dissociation of the imidazole hydrogen of histidine is 6.04. 

On the basis of the dissociation constant values, it seems sensible to conclude that, in these moderately basic carbinolamines, the hydrogen atom of the hydroxyl group is suflQciently acid to be eliminated under the influence of an alkali and by its transfer to the nitrogen atom of the mesomeric anion, the formation of the amino-aldehyde form may result. Instead of the amino-aldehyde, however, the corresponding bimolecular ether (15a-c) can be obtained. " It can be concluded that the formation of the bimolecular ether (S l or 8 2 mechanism) and the formation of the amino-aldehyde (B-SeI or B-Se2 mechanism) are competitive reactions. It seems probable that where the first reaction can occur the latter one is pushed into the background. The triple tautomeric system postulated by Gadamer... 

Just as proteins are biopolymers made of amino acids, nucleic acids are biopolv-mers made of nucleotides joined together to form a long chain. Each nucleotide is composed of a nucleoside bonded to a phosphate group, and each nucleoside is composed of an aldopentose sugar linked through its anomeric carbon to the nitrogen atom of a heterocyclic purine or pyrimidine base. 

The catabolism of proteins is much more complex than that of fats and carbohydrates because each of the 20 amino acids is degraded through its own unique pathway. The general idea, however, is that the amino nitrogen atom is removed and the substance that remains is converted into a compound that enters the citric acid cycle. 

Most amino acids lose their nitrogen atom by a transamination reaction in which the -NH2 group of the amino acid changes places with the keto group of ct-ketoglutarate. The products are a new a-keto acid plus glutamate. The overall process occurs in two parts, is catalyzed by aminotransferase enzymes, and involves participation of the coenzyme pyridoxal phosphate (PLP), a derivative of pyridoxine (vitamin UJ. Different aminotransferases differ in their specificity for amino acids, but the mechanism remains the same. 

Zero order reaction A reaction whose rate is independent of reactant concentration, 289,295-298, 317q Zinc, 86-87,550 Zn-Cn2+ voltaic cell, 481-485 Zwitterion Form of an amino acid in which there is a separation of charge between the nitrogen atom of the NH2 group (+) and one of the oxygen atoms of the COOH group (—), 623-624... 

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