Cooperative effects in the binding of pyridoxal 5'-phosphate to mitochondrial apo-aspartate aminotransferase |
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Authors: | A M Garzillo G Marino B Pispisa |
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Institution: | 1. Istituto di Chimica Organica e Biologica and Istituto Chimico, Università di Napoli, Via Mezzocannone 16, 80134 Napoli, Italy |
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Abstract: | Titrations of mitochondrial apo-aspartate aminotransferase with pyridoxal 5'-phosphate in the presence of AMP, contrary to what has been observed in the case of the cytosolic isoenzyme (1983) FEBS Lett. 153, 98-102], show sigmoidal isotherms, with Hill coefficients ranging from nH = 1.4, in the absence of AMP, to nH = 1.8, in the presence of 5.9 mM AMP. The experimental data were successfully fitted by the Monod-Wyman- Changeaux model. The best fit, in the absence of AMP, was obtained with L = 30, KR = 4.72 X 10(-7) M and KT = 1.18 X 10(-5) M. Binding curves in the presence of AMP fit the model by keeping KR as a constant. This implies that AMP could bind to the apoenzyme only in the T state. In contrast, binding curves in the presence of phosphate ion (Pi) showed a less pronounced cooperativity, the Hill coefficient dropping to nH = 1.0 in the presence of 0.1 mM Pi. The above results suggest a regulatory role of AMP and Pi in the reconstitution of aspartate aminotransferase. |
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Keywords: | Pyridoxal 5′-phosphate AMP |
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