Transcript Molecular Dynamics, Monte Carlo and Docking Lecture 21 Introduction to Bioinformatics

Molecular Dynamics, Monte
Carlo and Docking
Lecture 21
Introduction to Bioinformatics
MNW2
If you throw up a stone, it is
Physics.
If you throw up a stone, it is
Physics. If it lands on your head,
it is Biophysics.
If you throw up a stone, it is
Physics. If it lands on your head,
it is Biophysics.
If you write a computer program,
it is Informatics.
If you throw up a stone, it is
Physics. If it lands on your head,
it is Biophysics.
If you write a computer program,
it is Informatics. If there is a bug
in it, it is Bioinformatics
Allowed phi-psi angles
Red areas are preferred, yellow areas are
allowed, and white is avoided
2.3a Hamiltonian equations of motion
Hamiltonian equations (one degree of freedom):
H – Hamiltonian function, Hamiltonian,
q, p –Canonical variables: generalized coordinate (q) and
momentum [impulses] (p).
q = coordinates
p = momentum
v(t) = (r(t + t) - r(t - t))/2t
Molecular systems strive to be in the lowest energy state
Molecular Dynamics
Knowledge of the atomic forces and masses can be used to solve the position of
each atom along a series of extremely small time steps (on the order of
femtoseconds = 10-15 seconds). The resulting series of snapshots of structural
changes over time is called a trajectory. The use of this method to compute
trajectories can be more easily seen when Newton's equation is expressed in the
following form:
The "leapfrog" method is a common numerical approach to calculating
trajectories based on Newton's equation. The steps can be summarized as
follows:
Force field
The potential energy of a system can be expressed as a sum of valence (or bond),
crossterm, and nonbond interactions:
The energy of valence interactions comprises bond stretching (Ebond), valence angle
bending (Eangle), dihedral angle torsion (Etorsion), and inversion (also called out-ofplane interactions) (Einversion or Eoop) terms, which are part of nearly all force fields
for covalent systems.
A Urey-Bradley term (EUB) may be used to account for interactions between atom pairs involved in 1-3 configurations (i.e., atoms bound to
a common atom):
Evalence = Ebond + Eangle + Etorsion + Eoop + EUB
Modern (second-generation) forcefields include cross terms to account for such
factors as bond or angle distortions caused by nearby atoms. Crossterms can
include the following terms: stretch-stretch, stretch-bend-stretch, bend-bend,
torsion-stretch, torsion-bend-bend, bend-torsion-bend, stretch-torsion-stretch.
The energy of interactions between nonbonded atoms is accounted for by van der
Waals (EvdW), electrostatic (ECoulomb), and (in some older forcefields) hydrogen
bond (Ehbond) terms:
Enonbond = EvdW + ECoulomb + Ehbond
Force field
energy
f=
distance
a/r12 - b/r6
Van der Waals forces
The Lennard-Jones potential is mildly attractive as two uncharged molecules or atoms approach one
another from a distance, but strongly repulsive when they approach too close. The resulting potential is
shown. At equilibrium, the pair of atoms or molecules tend to go toward a separation corresponding to the
minimum of the Lennard--Jones potential (a separation of 0.38 nanometers for the case shown in the
Figure)
F = kqiqj/r2
Figure: Snapshots of ubiquitin pulling with constant velocity at
three different time steps.
ERGODIC
(l) of or relating to a process in which a sequence or sizable sample is equally representative of the whole (as in regard to a statistical
parameter);
(2) involving or relating to the probability that any state will recur, especially having zero probability that any state will never recur.
(WEBSTER'S DICTIONARY)
A collection of systems forms an ergodic ensemble if the modes of behavior found in any one system from time to time resemble its
behavior at other temporal periods and if the behavior of any other system when chosen at random also is like the one system. We do not
require identical performance, only quite similar time averages and number averages. (If you cannot tell one youth from another or one adult
from another, they belong to an ergodic ensemble.) In an ergodic population, any single individual is representative of the entire population.
The salient characteristics of this individual are essentially identical with any other member of the group. (Iberall)
#! /usr/bin/perl
#===============================================================================
#
# $Id: mcdemo.pl,v 1.1.1.1 2003/03/12 16:13:28 jkleinj Exp $
#
# mcdemo: Demo program for MC simulation of the number pi
#
# (C) 2003 Jens Kleinjung
#
# Dr Jens Kleinjung, Room P440
| [email protected]
# Bioinformatics Unit, Faculty of Sciences
| Tel +31-20-444-7783
# Free University Amsterdam
| Fax +31-20-444-7653
# De Boelelaan 1081A, 1081 HV Amsterdam | http://www.cs.vu.nl/~jkleinj
#
#===============================================================================
# preset parameters
$hits = 1;
$miss = 1;
for ($i=0; $i<100000; $i++)
{
# assign random x,y coordinates
$x = rand;
$y = rand;
# calculate radius
$r = sqrt(($x*$x)+($y*$y));
# sum up hits and misses
if ($r <= 1)
{ $hits++; }
else
{ $miss++; }
# calculate pi
$pi = (4*$hits)/($hits +$miss);
# print pi if ($i%100 == 0) { print("$i $pi\n"); }
}
#===============================================================================
In many conformational search methods based on Monte Carlo (MC),
after a MC move, the system is energy minimised, i.e. put in the
lowest local energy conformation, for example by gradient descent
(steepest descent).
antibody HyHEL-63 (cyan) complexed with Hen Egg White Lysozyme
The X-ray structure of the antibody HyHEL-63 (cyan) uncomplexed and complexed with Hen Egg White Lysozyme (yellow) has shown that there are small but significant, local
conformational changes in the antibody paratope on binding. The structure also reveals that most of the charged epitope residues face the antibody. Details are in
Li YL, Li HM, Smith-Gill SJ and Mariuzza RA (2000) The conformations of the X-ray structure Three-dimensional structures of the free and antigen-bound Fab from monoclonal
antilysozyme antibody HyHEL-63. Biochemistry 39: 6296-6309.
Salt links and electrostatic interactions provide much of the free energy of binding. Most of the charged residues face in interface in the X-ray structure. The importance of the salt link
between Lys97 of HEL and Asp27 of the antibody heavy chain is revealed by molecular dynamics simulations. After 1NSec of MD simulation at 100°C the overall conformation of
the complex has changed, but the salt link persists. Details are described in Sinha N and Smith-Gill SJ (2002) Electrostatics in protein binding and function. Current Protein & Peptide
Science 3: 601-614.