The influence of a homologous protein impurity on lysozyme crystal growth

The effect of a structurally similar protein impurity, turkey (Melengris gallopavo) egg-white lysozyme (TEWL) on crystallization of the host protein, hen-egg-white lysozyme (HEWL) from chicken (Gallus gallus) was studied under varying impurity and host solution concentrations. A change in morphology is observed when crystals of HEWL are grown in the presence of TEWL. As the relative amount of TEWL increases, TEWL crystals become more elongated in the [0 0 1] direction. Elongation is more pronounced in samples with lower initial concentrations of HEWL than in samples with higher initial concentrations. This behavior is consistent with that of impurities in small molecule crystal growth and with predictions based on the Kubota-Mullin model. The observed effect on the growth process can be attributed to the apparent inhibition in the [1 1 0] crystal growth direction of HEWL by TEWL since slowly growing faces become dominant faces in crystal growth. Incorporation of TEWL into HEWL crystals grown in a sitting drop batch method was measured using cation exchange chromatography. The results indicate that impurity incorporation is associated with increasing supersaturation. This conclusion is consistent with a kinetically controlled process of impurity incorporation. The observed impurity effects are most probably associated with the interchange of glutamine in position 41 of HEWL by histidine in TEWL. (C) 1999 Elsevier Science B.V. AU rights reserved.