LACTOFERRIN - THE ROLE OF CONFORMATIONAL-CHANGES IN ITS IRON-BINDING AND RELEASE

The relative rates of ferric ion removal from human milk lactoferrin and human serum transferrin by the synthetic tricatecholate sequestering agent N,N',N''-tris(5-sulfo-2,3-dihydroxybenzoyl)-1,5,10-triazadecane (3,4-LICAMS) have been compared at pH 7.4 and 37-degrees-C. Hyperbolic plots of the observed pseudo-first-order rate constants with increasing concentrations of 3,4-LICAMS were obtained for both proteins and are consistent with a similar mechanism of iron removal from both proteins by this ligand. However, the average limiting rate of iron removal from milk lactoferrin (k(max) = 6.5(4) x 10(-4) min-1) is about 100-fold slower than that found from serum transferrin (k(max) = 6.3(4) x 10(-2) min-1) under the same conditions. This accounts for essentially all of the greater thermodynamic stability of lactoferrin. These observations are explained in light of the two conformations, ''closed'' and ''open'', seen in protein crystal structures. Lactoferrin's ''closed'' form is more stable relative to transferrin, leading to a slower rate of iron release. This slower rate of iron removal from lactoferrin is consistent with a passive bacteriostatic function of lactoferrin via iron sequestration. Moreover it demonstrates that the bacteriostasis likely has a kinetic as well as a thermodynamic component.