function prediction and the n-gram biological language model from natural language processing has been used to filter the missing proteins. Finally, the conclusion and future perspectives are proposed.
References
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Chapter 2
Linguistic Feature Analysis of Protein Sequences
2.1Motivation and Basic Idea
Proteins play an important role in the function of complex biological systems. But the relationship between primary sequences, three-dimensional structures and functions of proteins is one of the most important unanswered questions in biology. With the completion of the Human Genome Project and all kinds of work in assessing biological sequences accurately, a large number of genomic and proteomic sequences are available for different organisms at present. The exponential increase of these data provides an opportunity for us to attack the sequence–structure–function mapping problem with sophisticated data-driven methods. Such methods have been successfully used in the domain of natural language processing. There are analogies between biological sequences and natural language. In linguistics, some words and phrases can form a meaningful sentence, while in biology, some tactic nucleotides denote genes and some fixed protein sequences can determine the structure and function of the protein.1 But is there a “language” in biological sequences?
Mantegna2 analyzed the linguistic features of noncoding DNA and emphasized that there exists a “language” in noncoding DNA. Although there are some insufficiencies in the work,3–5 many methods used in natural language processing have been used in biological sequences. N-grams of DNA6 and protein7 have been extracted. A bio-dictionary has been built and used to annotate proteins.8 Latent semantic analysis has been used to characterize the secondary structure of proteins.9 Probabilistic models from speech recognition have been used to enhance the protein domain discovery.10
The n-gram analysis method is one of the most frequently used techniques in computational linguistics. It takes the assumption that only the previous n − 1 words in a sentence have an effect on the probabilities for the next word.11 It has been successfully used in automatic speech recognition, document classification, information extraction, statistical machine translation and other challenging tasks in natural language. In this chapter, the n-grams of whole genome protein sequences have been extracted, their Zipf’s law has been analyzed and some statistical features have been extracted from the n-grams.
2.2Comparative n-gram Analysis
Amino acids are treated as words, since each amino acid carries a chemical “meaning”. In order to extract the n-gram from whole genome protein sequences, all the proteins of the same organism were arranged in series and split by blank, e.g. protein1 protein2 protein3 etc. Due to the large size of the genomic data, the suffix array12,13 was used to reduce the computational cost. To extract the n-gram statistical data, we developed a toolkit that can carry out the following functions:
1.Count protein number and length.
2.Count n-grams and most frequent n-grams.
3.Count n-grams of specified length.
4.Determine relative frequencies of specific n-grams across organisms.
5.Assess the distribution of n-gram frequencies in a specific organism.
The method was applied to protein sequences derived from whole genome sequences of 20 organisms. The protein sequence data was downloaded from the Swiss-prot database.14 The number of proteins varies from 484 (Mycoplasma genitalium) to 25612 (Human).
We developed a modification of Zipf-like analysis that could reveal differences between word usage in different organisms. First, the amino acid n-grams of a given length were sorted in descending order by frequency for the organism of choice. The comparative n-gram plots comparing the n-grams of one organism to those of other organisms