The teichoic acids.

The rifamycin synthetase is primed with a 3-amino-5-hydroxybenzoate starter unit by a loading module that contains domains homologous to the adenylation and thiolation domains of nonribosomal peptide synthetases. Adenylation and thiolation activities of the loading module were reconstituted in vitro and shown to be independent of coenzyme A, countering literature proposals that the loading module is a coenzyme A ligase. Kinetic parameters for covalent arylation of the loading module were measured directly for the unnatural substrates benzoate and 3-hydroxybenzoate. This analysis was extended through competition experiments to determine the relative rates of incorporation of a series of substituted benzoates. Our results show that the loading module can accept a variety of substituted benzoates, although it exhibits a preference for the 3-, 5-, and 3,5-disubstituted benzoates that most closely resemble its biological substrate. The considerable substrate tolerance of the loading module of rifamycin synthetase suggests that the module has potential as a tool for generating substituted derivatives of natural products.

Actinomycetales↗

PubMed2001

Role of hydrogen bonding in the interaction between a xylan binding module and xylan.

NMR studies of the internal family 2b carbohydrate binding module (CBM2b-1) of Cellulomonas fimi xylanase 11A have identified six polar residues and two aromatic residues that interact with its target ligand, xylan. To investigate the importance of the various interactions, free energy and enthalpy changes have been measured for the binding of xylan to native and mutant forms of CBM2b-1. The data show that the two aromatic residues, Trp 259 and Trp 291, play a critical role in the binding, and similarly that mutants N264A and T316A have no affinity for the xylose polymer. Interestingly, mutations E257A, Q288A, N292A, E257A/Q288A, E257A/N292A, and E257A/N292A/Q288A do not significantly diminish the affinity of CBM2b-1 for the xylose polymers, but do influence the thermodynamics driving the protein-carbohydrate interactions. These thermodynamic parameters have been interpreted in light of a fresh understanding of enthalpy-entropy compensation and show the following. (1) For proteins whose ligands are bound on an exposed surface, hydrogen bonding confers little specificity or affinity. It also displays little cooperativity. Most specificity and affinity derive from binding between the face of sugar rings and aromatic rings. (2) Loss of hydrogen bonding interactions leads to a redistribution of the remaining bonding interactions such that the entropic mobility of the ligand is maximized, at the expense (if necessary) of enthalpically favorable bonds. (3) Changes in entropy and enthalpy in the binding between polysaccharide and a range of mutants can be interpreted by considering changes in binding and flexibility, without any need to consider solvent reorganization.

Actinomycetales↗