COLLAPSE TRANSITIONS IN PROTEIN-LIKE LATTICE POLYMERS - THE EFFECT OFSEQUENCE PATTERNS

The collapse transition of lattice protein-like heteropolymers has bee n studied by means of the Monte Carlo method. The protein model has be en reduced to the alpha-carbon trace restricted to a high coordination lattice. The sequences of model heteropolymers contain two types of m ers: hydrophobic/nonpolar (H) and hydrophilic/polar (P). Interactions of HH and PP pairs were assumed to be negative (weaker attractions of PP pairs) while the contact energy for HP pairs was equal to zero. All sequence-specific short-range interactions have been neglected in the present studies. It has been found that homopolymeric chains undergo a smooth collapse transition to a dense globular state. The globule la cks any signatures of local ordering that could be interpreted as a mo del of protein secondary structure. Heteropolymers with the sequences of hydrophilic and hydrophobic residues characteristic for alpha- and beta-type proteins undergo a somewhat sharper (though continuous) coll apse transition to a dense globular state with elements of local order ing controlled by the sequence. The helical pattern induces more secon dary structure than the beta-type pattern. For all examined sequences the level of local ordering was lower than the average secondary struc ture content of globular proteins. The results are compared with other theoretical work and with known experimental facts. The implications for the reduced modeling of protein systems are briefly discussed. (C) 1997 John Wiley & Sons, Inc.