Enzymic properties of members of the ribonuclease A superfamily, like the activity on RNA, the preference for either cytosine or uracil in the primary binding site B1, the preference for the other side of the cleaved phosphodiester bond, the B2 site, and features of the two noncatalytic phosphate-binding sites P0 and P2 are discussed in several articles in this multi-author review, and are summarized in this closing article. A special feature of members of the ribonucleases 1 family is their destabilizing action on double-stranded nucleic acid structures. A feature of the ribonuclease A superfamily is the frequent occurrence of gene duplications, both in ancestral vertebrate lineages and in recently evolved taxa. Three different bovine ribonucleases 1 have been identified in pancreas, semen and brain, respectively, which are the result of two gene duplications in an ancestral ruminant. Similar gene duplications have been identified in other ribonuclease families in several mammalian and other vertebrate taxa. The ribonuclease family, of which the human members have been assigned numbers 2, 3 and 6, underwent a still mysterious pattern of gene duplications and functional expression as proteins with ribonuclease activity and other physiological properties.

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The success of knowledge-based homology modelling is critically dependent on the predictive potency of the program structure-based calculations, which attempt to translate homologous sequences into three-dimensional structures, and on the actual relevance of the crystal structure for the protein topology. As quality control, experimental data for selected parameters of the protein's conformation are required. Using the crystal structure of the sialidase of Salmonella typhimurium as framework for model building of the homologous enzyme from Clostridium perfringens, a set of energy-minimised conformers is derived. These proteins present e.g. Tyr, Trp and His residues with an assessable area on the surface, since the side chains of these amino acid residues are responsive to chemically induced dynamic nuclear polarization (CIDNP), monitored by NMR. Hence, as first lesson, a comparative analysis for model-derived and experimentally determined values can be performed. The second lesson of this study concerns the notable impact of single amino acid substitutions (Tyr/Phe, Cys/Ser) on the surface accessibility of the CIDNP-reactive amino acid side chains in mutant forms of the sialidase. Corroborating the predictions from the theoretical calculations, the spectra of the engineered mutants reveal marked and non-uniform alterations. Thus, the effect of apparently rather conservative amino acid substitutions on a distinct conformational aspect of this protein, even at distant sites, should not be underestimated.