![]() Suppose the percentages of α-helices and β-sheets in a protein are symbolized by α and β, respectively. This can be further elucidated through the following illustration. As demonstrated in a recent study on predicting protein subcellular locations ( Chou and Elrod, 1999), the lower the number of discriminating objects, the higher the rate of correct prediction would be in identifying them. to foretell one among an infinite number of possible values. In the latter case, however, the prediction has to deal with a target of continuous quantity, i.e. In the former case, the prediction is actually to foretell one class among only a few possible clusters. `More difficult' refers to the fact that, compared with the structural class prediction, the odds are much lower for the secondary structure content prediction. Nevertheless, the classification thus obtained is still closely correlated with the percentages of the secondary structure contents ( Chou et al., 1998). ![]() `More basic' refers to the fact that the protein structural classes, as classified by many investigators, are solely based on the percentages of the secondary structure content (see, for example, Klein and Delish, 1986 Nakashima et al., 1986 Chou, 1989 Kneller et al., 1990 Chou, 1995 Liu and Chou, 1997), though it must be mentioned here that a more reasonable scheme of classifying structural classes is actually based upon the evolutionary relationships of proteins and on the principles that govern their 3D structure (Murzin et al., 1995). Compared with structural class prediction, secondary structure content prediction is both more basic and more difficult. To help reach such a goal, various approaches targeting different levels and aspects of protein structure were initiated, such as secondary structure prediction (see, for example, Chou and Fasman, 1978 Fasman, 1989), structural class prediction (see, for example, Nakashima et al., 1986 Chou, 1989 Chou, 1995 Chou and Zhang, 1995 Bahar et al., 1997 Liu and Chou, 1997), domain class prediction ( Chou et al., 1998) and secondary structure content prediction ( Krigbaum and Knutton, 1973 Muskal and Kim, 1992 Zhang et al., 1996, 1998). One of the biggest challenges in molecular biology is how to predict the three-dimensional (3D) structure of a protein given only its amino acid sequence. ![]()
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