Isoleucine Isoleudne

Valine, methionine, isoleucine, and threonine are all metabolized through the propionic acid pathway (also used for odd-carbon fatty acids). Defidency of either enzyme results in neonatal ketoacidosis from failure to metabolize ketoacids produced from these four amino adds. The defidendes may be distinguished based on whether meth)dmalonic adduria is present. A diet low in protein or a semisynthetic diet with low amounts of valine, methionine, isoleudne, and threonine is used to treat both deficiencies. 

Figure 7.10 shows the results depicted by Givand et al. [7]. It can be seen that the leucine content in isoleudne crystals is greater than that in the equilibrium mother liquor and, on the other hand, the isoleucine content in leucine crystals is significantly... 

Isoleudne The largest amounts of isoleucine are found in milk and egg proteins (6-7%), while meat and grains contain 4-5% of leucine (average content is 4.6%). 

Isoleucine, threonine, 4-hydroxyproline and 3-hydroxyproline also have an asymmetric carbon atom in the side chain (two chiral centres in total). They can therefore occur in four stereoisomeric forms (this number is equal to 2 = 2 = 4, where = number of asymmetric carbon atoms) known as l-, d-, L-allo- and d-allo- according to the configuration of the carbon carrying the amino group. The unique RIS system is not widely used in biochemistry. The stereoisomers of isoleudne are thus called L-isoleucine or (2S,3S)-isoleucine (2-4), D-isoleucine or (2R,3R)-isoleucine (2-77), L-a//o-isoleudne or (2S,3R)-isoleudne (2-78) and D-fl//o-isoleucine or (2fi,3S)-isoleucine (2-79). Proteins contain only L-isoleudne. The product of enzymatic or thermal isomerisation of L-isoleucine is D-oZZo-isoleudne. Likewise, proteins contain L-threonine or (2S,3R)-threonine (2-4) and L-4-hydroxyproline or (2S,4S)-4-hydroxyprohne (2-6). Cystine (as well as lanthionine, see Sedion 2.5.1.3.4) contains two identical asymmetric carbons in the molecule and, therefore, only exists in three different isomers. Proteins contain just L-cystine (2-4), its isomer is D-cystine (2-80) and the mesoform meso-cystine (2-81) is a symmetrical molecule. 

Aspartate, derived from oxaloacetate via transamination (Givan, 1980 Ireland and Joy, 1985), serves as a common precursor of lysine, methionine, threonine, and isoleucine. The synthesis of isoleudne is, in turn, closely related to the synthesis of the other branched chain amino acids. The series of reactions outlined in Fig. 1 is based on the results of multiple approaches used to elucidate the synthesis of these amino acids in higher plants (Bryan, 1980). Among the notable advances in this area are delineation of a variety of potential regulatory mechanisms, identification of several sets of isozymes, and the subcellular localization of a number of pathway enzymes. 

---