nma.pdb: Normal Mode Analysis

In bio3d: Biological Structure Analysis

Normal Mode Analysis

Description

Perform elastic network model (ENM) C-alpha normal modes calculation of a protein structure.

Usage

## S3 method for class 'pdb'
nma(pdb, inds = NULL, ff = 'calpha', pfc.fun = NULL,
                  mass = TRUE, temp = 300.0, keep = NULL, hessian = NULL,
                  outmodes = NULL, ... )

build.hessian(xyz, pfc.fun, fc.weights = NULL, pdb = NULL, ...)

## S3 method for class 'nma'
print(x, nmodes=6, ...)

Arguments

pdb	an object of class pdb as obtained from function read.pdb.
inds	atom and xyz coordinate indices obtained from atom.select that selects the elements of pdb upon which the calculation should be based. If not provided the function will attempt to select the calpha atoms automatically (based on function atom.select).
ff	character string specifying the force field to use: ‘calpha’, ‘anm’, ‘pfanm’, ‘reach’, or ‘sdenm’.
pfc.fun	customized pair force constant (‘pfc’) function. The provided function should take a vector of distances as an argument to return a vector of force constants. If provided, 'pfc.fun' will override argument ff. See examples below.
mass	logical, if TRUE the Hessian will be mass-weighted.
temp	numerical, temperature for which the amplitudes for scaling the atomic displacement vectors are calculated. Set ‘temp=NULL’ to avoid scaling.
keep	numerical, final number of modes to be stored. Note that all subsequent analyses are limited to this subset of modes. This option is useful for very large structures and cases where memory may be limiting.
hessian	hessian matrix as obtained from build.hessian. For internal purposes and generally not intended for public use.
outmodes	atom indices as obtained from atom.select) specifying the atoms to include in the resulting mode object.
xyz	a numeric vector of Cartesian coordinates.
fc.weights	a numeric matrix of size NxN (where N is the number of calpha atoms) containg scaling factors for the pariwise force constants. See examples below.
x	an nma object obtained from nma.pdb.
nmodes	numeric, number of modes to be printed.
...	additional arguments to build.hessian, aa2mass, pfc.fun, and print. One useful option here for dealing with unconventional residues is ‘mass.custom’, see the aa2mass function for details.

Details

This function calculates the normal modes of a C-alpha model of a protein structure. A number of force fields are implemented all of whhich employ the elastic network model (ENM).

The ‘calpha’ force field - originally developed by Konrad Hinsen - is the recommended one for most applications. It employs a spring force constant differentiating between nearest-neighbour pairs along the backbone and all other pairs. The force constant function was parameterized by fitting to a local minimum of a crambin model using the AMBER94 force field.

See load.enmff for details of the different force fields.

By default nma.pdb will diagonalize the mass-weighted Hessian matrix. The resulting mode vectors are moreover scaled by the thermal fluctuation amplitudes.

The implementation under default arguments reproduces the calculation of normal modes (VibrationalModes) in the Molecular Modeling Toolkit (MMTK) package. To reproduce ANM modes set ff='anm', mass=FALSE, and temp=NULL.

Value

Returns an object of class ‘nma’ with the following components:

modes	numeric matrix with columns containing the normal mode vectors. Mode vectors are converted to unweighted Cartesian coordinates when mass=TRUE. Note that the 6 first trivial eigenvectos appear in columns one to six.
frequencies	numeric vector containing the vibrational frequencies corresponding to each mode (for mass=TRUE).
force.constants	numeric vector containing the force constants corresponding to each mode (for mass=FALSE)).
fluctuations	numeric vector of atomic fluctuations.
U	numeric matrix with columns containing the raw eigenvectors. Equals to the modes component when mass=FALSE and temp=NULL.
L	numeric vector containing the raw eigenvalues.
xyz	numeric matrix of class xyz containing the Cartesian coordinates in which the calculation was performed.
mass	numeric vector containing the residue masses used for the mass-weighting.
temp	numerical, temperature for which the amplitudes for scaling the atomic displacement vectors are calculated.
triv.modes	number of trivial modes.
natoms	number of C-alpha atoms.
call	the matched call.

Note

The current version provides an efficent implementation of NMA with execution time comparable to similar software (when the entire Hessian is diagonalized).

The main (speed related) bottleneck is currently the diagonalization of the Hessian matrix which is performed with the core R function eigen. For computing a few (5-20) approximate modes the user can consult package ‘irlba’.

NMA is memory extensive and users should be cautions when running larger proteins (>3000 residues). Use ‘keep’ to reduce the amount of memory needed to store the final ‘nma’ object (the full 3Nx3N Hessian matrix still needs to be allocated).

We thank Edvin Fuglebakk for valuable discussions on the implementation as well as for contributing with testing.

Author(s)

Lars Skjaerven

References

Skjaerven, L. et al. (2014) BMC Bioinformatics 15, 399. Grant, B.J. et al. (2006) Bioinformatics 22, 2695–2696. Hinsen, K. et al. (2000) Chemical Physics 261, 25–37.

See Also

fluct.nma, mktrj.nma, dccm.nma, overlap, rmsip, load.enmff.

Examples

## Fetch stucture
pdb <- read.pdb( system.file("examples/1hel.pdb", package="bio3d") )

## Calculate normal modes
modes <- nma(pdb)

## Print modes
print(modes)

## Plot modes
plot(modes)

## Visualize modes
#m7 <- mktrj.nma(modes, mode=7, file="mode_7.pdb")

## Not run: 
## Use Anisotropic Network Model
modes <- nma(pdb, ff="anm", mass=FALSE, temp=NULL, cutoff=15)

## Use SSE information and SS-bonds
sse      <- dssp(pdb, resno=FALSE, full=TRUE)
ss.bonds <- matrix(c(76,94, 64,80, 30,115, 6,127),
                   ncol=2, byrow=TRUE)


## User defined energy function
## Note: Must take a vector of distances
"my.ff" <- function(r) {
   ifelse( r>15, 0, 1 )
}

## Modes with a user defined energy function
modes <- nma(pdb, pfc.fun=my.ff)


## A more manual approach
sele <- atom.select(pdb, chain='A', elety='CA')
xyz <- pdb$xyz[sele$xyz]

hessian <- build.hessian(xyz, my.ff)
modes <- eigen(hessian)

## Dealing with unconventional residues
pdb <- read.pdb("1xj0")

## nma(pdb)
#modes <- nma(pdb, mass.custom=list(CSX=121.166))

## End(Not run)

bio3d documentation built on Oct. 30, 2024, 1:08 a.m.