R/desc-13-PAAC.R

#' Pseudo Amino Acid Composition (PseAAC) Descriptor
#'
#' This function calculates the Pseudo Amino Acid Composition (PseAAC)
#' descriptor (dim: \code{20 + lambda}, default is 50).
#'
#' @param x A character vector, as the input protein sequence.
#'
#' @param props A character vector, specifying the properties used.
#' 3 properties are used by default, as listed below:
#' \describe{
#' \item{\code{'Hydrophobicity'}}{Hydrophobicity value of the 20 amino acids}
#' \item{\code{'Hydrophilicity'}}{Hydrophilicity value of the 20 amino acids}
#' \item{\code{'SideChainMass'}}{Side-chain mass of the 20 amino acids}}
#'
#' @param lambda The lambda parameter for the PseAAC descriptors, default is 30.
#'
#' @param w The weighting factor, default is 0.05.
#'
#' @param customprops A \code{n x 21} named data frame contains \code{n}
#' customized property. Each row contains one property.
#' The column order for different amino acid types is
#' \code{'AccNo'}, \code{'A'}, \code{'R'}, \code{'N'},
#' \code{'D'}, \code{'C'}, \code{'E'}, \code{'Q'},
#' \code{'G'}, \code{'H'}, \code{'I'}, \code{'L'},
#' \code{'K'}, \code{'M'}, \code{'F'}, \code{'P'},
#' \code{'S'}, \code{'T'}, \code{'W'}, \code{'Y'},
#' \code{'V'}, and the columns should also be \emph{exactly} named like this.
#' The \code{AccNo} column contains the properties' names.
#' Then users should explicitly specify these properties
#' with these names in the argument \code{props}.
#' See the examples below for a demonstration.
#' The default value for \code{customprops} is \code{NULL}.
#'
#' @return A length \code{20 + lambda} named vector
#'
#' @note Note the default \code{20 * 3} \code{prop} values have already been
#' independently given in the function. Users can also specify
#' other (up to 544) properties with the Accession Number in
#' the \code{\link{AAindex}} data, with or without the default
#' three properties, which means users should explicitly specify
#' the properties to use. For this descriptor type, users need to
#' intelligently evaluate the underlying details of the descriptors
#' provided, instead of using this function with their data blindly.
#' It would be wise to use some negative and positive control comparisons
#' where relevant to help guide interpretation of the results.
#'
#' @author Nan Xiao <\url{https://nanx.me}>
#'
#' @seealso See \code{\link{extractAPAAC}} for amphiphilic pseudo
#' amino acid composition descriptor.
#'
#' @export extractPAAC
#'
#' @references
#' Kuo-Chen Chou. Prediction of Protein Cellular Attributes
#' Using Pseudo-Amino Acid Composition.
#' \emph{PROTEINS: Structure, Function, and Genetics}, 2001, 43: 246-255.
#'
#' Kuo-Chen Chou. Using Amphiphilic Pseudo Amino Acid Composition
#' to Predict Enzyme Subfamily Classes. \emph{Bioinformatics}, 2005, 21, 10-19.
#'
#' JACS, 1962, 84: 4240-4246. (C. Tanford). (The hydrophobicity data)
#'
#' PNAS, 1981, 78:3824-3828 (T.P.Hopp & K.R.Woods). (The hydrophilicity data)
#'
#' CRC Handbook of Chemistry and Physics, 66th ed.,
#' CRC Press, Boca Raton, Florida (1985). (The side-chain mass data)
#'
#' R.M.C. Dawson, D.C. Elliott, W.H. Elliott, K.M. Jones,
#' Data for Biochemical Research 3rd ed., Clarendon Press Oxford (1986).
#' (The side-chain mass data)
#'
#' @examples
#' x <- readFASTA(system.file("protseq/P00750.fasta", package = "protr"))[[1]]
#' extractPAAC(x)
#'
#' myprops <- data.frame(
#'   AccNo = c("MyProp1", "MyProp2", "MyProp3"),
#'   A = c(0.62, -0.5, 15), R = c(-2.53, 3, 101),
#'   N = c(-0.78, 0.2, 58), D = c(-0.9, 3, 59),
#'   C = c(0.29, -1, 47), E = c(-0.74, 3, 73),
#'   Q = c(-0.85, 0.2, 72), G = c(0.48, 0, 1),
#'   H = c(-0.4, -0.5, 82), I = c(1.38, -1.8, 57),
#'   L = c(1.06, -1.8, 57), K = c(-1.5, 3, 73),
#'   M = c(0.64, -1.3, 75), F = c(1.19, -2.5, 91),
#'   P = c(0.12, 0, 42), S = c(-0.18, 0.3, 31),
#'   T = c(-0.05, -0.4, 45), W = c(0.81, -3.4, 130),
#'   Y = c(0.26, -2.3, 107), V = c(1.08, -1.5, 43)
#' )
#'
#' # use 3 default properties, 4 properties from the
#' # AAindex database, and 3 cutomized properties
#' extractPAAC(
#'   x,
#'   customprops = myprops,
#'   props = c(
#'     "Hydrophobicity", "Hydrophilicity", "SideChainMass",
#'     "CIDH920105", "BHAR880101",
#'     "CHAM820101", "CHAM820102",
#'     "MyProp1", "MyProp2", "MyProp3"
#'   )
#' )
extractPAAC <- function(
  x, props = c("Hydrophobicity", "Hydrophilicity", "SideChainMass"),
  lambda = 30, w = 0.05, customprops = NULL) {
  if (protcheck(x) == FALSE) {
    stop("x has unrecognized amino acid type")
  }

  if (nchar(x) <= lambda) {
    stop('Length of the protein sequence must be greater than "lambda"')
  }

  AAidx <- read.csv(system.file("sysdata/AAidx.csv", package = "protr"), header = TRUE)

  tmp <- data.frame(
    AccNo = c("Hydrophobicity", "Hydrophilicity", "SideChainMass"),
    A = c(0.62, -0.5, 15), R = c(-2.53, 3, 101),
    N = c(-0.78, 0.2, 58), D = c(-0.9, 3, 59),
    C = c(0.29, -1, 47), E = c(-0.74, 3, 73),
    Q = c(-0.85, 0.2, 72), G = c(0.48, 0, 1),
    H = c(-0.4, -0.5, 82), I = c(1.38, -1.8, 57),
    L = c(1.06, -1.8, 57), K = c(-1.5, 3, 73),
    M = c(0.64, -1.3, 75), F = c(1.19, -2.5, 91),
    P = c(0.12, 0, 42), S = c(-0.18, 0.3, 31),
    T = c(-0.05, -0.4, 45), W = c(0.81, -3.4, 130),
    Y = c(0.26, -2.3, 107), V = c(1.08, -1.5, 43)
  )

  AAidx <- rbind(AAidx, tmp)

  if (!is.null(customprops)) AAidx <- rbind(AAidx, customprops)

  aaidx <- AAidx[, -1]
  row.names(aaidx) <- AAidx[, 1]

  n <- length(props)

  # Standardize H0 to H

  H0 <- as.matrix(aaidx[props, ])

  H <- matrix(ncol = 20, nrow = n)
  for (i in 1:n) {
    H[i, ] <-
      (H0[i, ] - mean(H0[i, ])) / (sqrt(sum((H0[i, ] - mean(H0[i, ]))^2) / 20))
  }
  AADict <- c(
    "A", "R", "N", "D", "C", "E", "Q", "G", "H", "I",
    "L", "K", "M", "F", "P", "S", "T", "W", "Y", "V"
  )
  dimnames(H) <- list(props, AADict)

  # Compute (big) Theta

  Theta <- vector("list", lambda)

  xSplitted <- strsplit(x, split = "")[[1]]

  N <- length(xSplitted)

  for (i in 1:lambda) {
    for (j in 1:(N - i)) {
      Theta[[i]][j] <- mean((H[, xSplitted[j]] - H[, xSplitted[j + i]])^2)
    }
  }

  # Compute (small) theta

  theta <- sapply(Theta, mean)

  # Compute first 20 features

  fc <- summary(factor(xSplitted, levels = AADict), maxsum = 21)
  Xc1 <- fc / (1 + (w * sum(theta)))
  names(Xc1) <- paste("Xc1.", names(Xc1), sep = "")

  # Compute last lambda features

  Xc2 <- (w * theta) / (1 + (w * sum(theta)))
  names(Xc2) <- paste("Xc2.lambda.", 1:lambda, sep = "")

  # Combine (20 + lambda) features

  Xc <- c(Xc1, Xc2)

  Xc
}

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protr documentation built on May 18, 2019, 9:02 a.m.