D-Ornithine 4,5-Aminomutase

Besides a lysine 5,6-aminomutase, Clostridium sticklandii also has a D-ornithine 4,5-aminomutase (EC 5.4.3.5) [78, 79], D-Ornithine is generated from L-ornithine by ornithine racemase [80], The two genes encoding D-ornithine 4,5-aminomutase have been cloned, sequenced, and expressed in E. coli [81]. The enzyme is an a2/ 2-heterotetramer, consisting of 12 800 Da and 82 900 Da subunits. The protein requires Bn and pyridoxal phosphate as cofactors. Similar to the lysine 5,6-aminomutase, a conserved base-ofi)/histidine-on cobalamin binding motif is present in the 82 900 Da protein. 

Considering that few biosynthetic pathways to / -amino acids have yet been investigated, it seems nevertheless possible to conclude that most / -amino acids that are structurally related to proteinogenic a-amino acids are generated from a-amino acids by aminomutases (Table 1.6.1). 

So far, two types of aminomutase have been investigated in detail. Lysine 2,3-aminomutase from Clostridium subterminale SB4 is the example par excellence for the SAM-dependent type of aminomutase. Several other enzymes belonging to the same family are known. Examples are biotin synthase [82], pyruvate formate lyase [83, 84], and anaerobic ribonucleotide reductase [85]. 

The second class of aminomutase is represented by D-ornithine 4,5-aminomutase and /Mysine 5,6-aminomutase. Although cobalamin is used as cofactor instead of SAM, the mechanism of the aminomutase reaction is similar. 

Interestingly, all known cobalamin-dependent aminomutases seem to catalyze a shift of the co amino group whereas the SAM dependent lysine 2,3-aminomutase catalyzes the shift of the a amino group. The latter is true for phenylalanine 2,3-aminomutase in Taxus brevifolia and tyrosine 2,3-aminomutase in Cortinarius vio- 

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ORNITHINE TRANSCARBAMOYLASE d-ORNITHINE 4,5-AMINOMUTASE ORNITHINE AMINOTRANSFERASE ORNITHINE DECARBOXYLASE Ornithine carbamoyltransferase,... 

D-ornithine 4,5-aminomutase catalyzes the reversible interconversion of d-ornithine 11 to (2R,4S)-diaminopentanoic acid 46 [78] (Scheme 1.6.17). 

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