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

The relative rates of ferric ion removal from human milk lactoferrin a nd human serum transferrin by the synthetic tricatecholate sequesterin g agent (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 t he observed pseudo-first-order rate constants with increasing concentr ations of 3,4-LICAMS were obtained for both proteins and are consisten t with a similar mechanism of iron removal from both proteins by this ligand. However, the average limiting rate of iron removal from milk l actoferrin (k(max) = 6.5(4) x 10(-4) min-1) is about 100-fold slower t han 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 g reater 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 m ore stable relative to transferrin, leading to a slower rate of iron r elease. This slower rate of iron removal from lactoferrin is consisten t with a passive bacteriostatic function of lactoferrin via iron seque stration. Moreover it demonstrates that the bacteriostasis likely has a kinetic as well as a thermodynamic component.