Amino acids isomeric forms

Cyclotene is the precursor of dihydrocyclopentapyrazines after having reacted with ammonia respectively amino acids to form cyclotene imine. Figure 2 demonstrates not only the isomerization and transformation of this compound but also the formula of bis-dihydro-cyclopentapyrazines as special reaction products. They are formed by condensation of cyclotene-imine and represent a mixture of 4 di-astereomeric compounds. These compounds produce identical mass spectra (Shibamoto et a 1,16). We have carried out corresponding experiments by heating Furaneol with phenylalanine or serine in an autoclave at 180 0 C. As Figure 3 demonstrates some monocyclic pyrazines are formed by reaction of the degradation products from Furaneol. 

The amino acid residues forming the three-dimensional structure of the active site are chiral. As a result the active site is chiral. It can bind only one isomeric form of a hex-ose sugar, in this case the D-isomer. 

As the number of amino acids per protein molecule increases (with some varieties repeated), the number of possible isomeric sequences increases rapidly. Using only 12 different amino acids to form a protein of molecular weight about 34,000 containing 288 amide bonds, the number of possible isomers which could exist has been calculated to be 10 °°. If each molecule weighs on average about lO g, a collection of one of each would weigh 10 ° g which is about 10 times the total mass of the planet. The protein alphabet is thus extremely versatile and there are many more... 

The next five transition metals iron, cobalt, nickel, copper and zinc are of undisputed importance in the living world, as we know it. The multiple roles that iron can play will be presented in more detail later in Chapter 13, but we can already point out that, with very few exceptions, iron is essential for almost all living organisms, most probably because of its role in forming the amino acid radicals required for the conversion of ribonucleotides to deoxyribonucleotides in the Fe-dependent ribonucleotide reductases. In those organisms, such as Lactobacilli6, which do not have access to iron, their ribonucleotide reductases use a cobalt-based cofactor, related to vitamin B12. Cobalt is also used in a number of other enzymes, some of which catalyse complex isomerization reactions. Like cobalt, nickel appears to be much more extensively utilized by anaerobic bacteria, in reactions involving chemicals such as CH4, CO and H2, the metabolism of which was important... 

All amino acids except glycine exist in these two different isomeric forms but only the L isomers of the a-amino acids are found in proteins, although many D amino acids do occur naturally, for example in certain bacterial cell walls and polypeptide antibiotics. It is difficult to differentiate between the D and the L isomers by chemical methods and when it is necessary to resolve a racemic mixture, an isomer-specific enzyme provides a convenient way to degrade the unwanted isomer, leaving the other isomer intact. Similarly in a particular sample, one isomer may be determined in the presence of the other using an enzyme with a specificity for the isomer under investigation. The other isomer present will not act as a substrate for the enzyme and no enzymic activity will be demonstrated. The enzyme L-amino acid oxidase (EC 1.4.3.2), for example, is an enzyme that shows activity only with L amino acids and will not react with the D amino acids. 

Many of the organic compounds found in nature are chiral. More importantly, most natural compounds in living organisms are not only chiral, but are present in only one of their optical isomeric forms. Such compounds include amino acids, proteins, enzymes and sugars. 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

In the second part of the reaction (see A, lb), these steps take place in the opposite direction pyridoxamine phosphate and the second 2-oxoacid form a ketimine, which is isomerized into aldimine. Finally, the second amino acid is cleaved and the coenzyme is regenerated. 

In the absence of substrates, the aldehyde group of pyridoxal phosphate is covalently bound to a lysine residue of the transaminase (1). This type of compound is known as an aldimine or Schiffs base. During the reaction, amino acid 1 (A, la) displaces the lysine residue, and a new aldimine is formed (2). The double bond is then shifted by isomerization. 

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