The influence of a homologous protein impurity on lysozyme crystal growth

The effect of a structurally similar protein impurity, turkey (Melengris ga llopavo) egg-white lysozyme (TEWL) on crystallization of the host protein, hen-egg-white lysozyme (HEWL) from chicken (Gallus gallus) was studied unde r varying impurity and host solution concentrations. A change in morphology is observed when crystals of HEWL are grown in the presence of TEWL. As th e 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 concent rations. This behavior is consistent with that of impurities in small molec ule crystal growth and with predictions based on the Kubota-Mullin model. T he 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 sl owly growing faces become dominant faces in crystal growth. Incorporation o f TEWL into HEWL crystals grown in a sitting drop batch method was measured using cation exchange chromatography. The results indicate that impurity i ncorporation is associated with increasing supersaturation. This conclusion is consistent with a kinetically controlled process of impurity incorporat ion. The observed impurity effects are most probably associated with the in terchange of glutamine in position 41 of HEWL by histidine in TEWL. (C) 199 9 Elsevier Science B.V. AU rights reserved.