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  • Title: Co-operative interactions between the catalytic sites in Escherichia coli aspartate transcarbamylase. Role of the C-terminal region of the regulatory chains.
    Author: Xi XG, Van Vliet F, Ladjimi MM, De Wannemaeker B, De Staercke C, Piérard A, Glansdorff N, Hervé G, Cunin R.
    Journal: J Mol Biol; 1990 Nov 20; 216(2):375-84. PubMed ID: 2254935.
    Abstract:
    In aspartate transcarbamylase (ATCase) each regulatory chain interacts with two catalytic chains each one belonging to a different trimeric catalytic subunit (R1-C1 and R1-C4 types of interactions as defined in Fig. 1). In order to investigate the interchain contacts that are involved in the co-operative interactions between the catalytic sites, a series of modified forms of the enzyme was prepared by site-directed mutagenesis. The amino acid replacements were devised on the basis of the previously described properties of an altered form of ATCase (pAR5-ATCase) which lacks the homotropic co-operative interactions between the catalytic sites. The results obtained (enzyme kinetics, bisubstrate analog influence and pH studies) show that the R1-C4 interaction is essential for the establishment of the enzyme conformation that has a low affinity for aspartate (T state), and consequently for the existence of co-operativity between the catalytic sites. This interaction involves the 236-250 region of the aspartate binding domain of the catalytic chain (240s loop) and the 143-149 region of the regulatory chain which comprises helix H3'.
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