Biochemical characterization of aspartate aminotransferase allozymes from common wheat

摘要

Six allozymes of aspartate aminotransferase (AAT, EC 2.6.1.1): three plastidial (AAT-2 zone) and three cytosolic (AAT-3 zone) were isolated from common wheat (Triticum aestivum) seedlings and highly purified by a five-step purification procedure. The identity of the studied proteins was confirmed by mass spectrometry. The molecular weight of AAT allozymes determined by gel filtration was 72.4 +/- 3.6 kDa. The molecular weights of plastidial and cytosolic allozymes estimated by SDS-PAGE were 45.3 and 43.7 kDa, respectively. The apparent Michaelis constant (K (m)) values determined for four substrates appeared to be very similar for each allozyme. The values of the turnover number (k (cat)) and the k (cat)/K (m) ratio calculated for allozymes with L-aspartate as a leading substrate were in the range of 88.5-103.8 s(-1)/10,412-10,795 s(-1) M-1 for AAT-2 zone and 4.6-7.0 s(-1)/527-700 s(-1) M-1 for AAT-3 zone. These results clearly demonstrated much higher catalytic efficiency of AAT-2 allozymes. Therefore, partial sequences of cDNA encoding AATs from different zones were obtained using the RT-PCR technique. Comparison of the AAT-2 and AAT-3 amino acid sequences from active site regions revealed five non-conservative substitutions, which impact on the observed differences in the isozymes catalytic efficiency is discussed.