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Composition of pH-Sensitive Triad in C-Lobe of Human Serum Transferrin. Comparison to Sequences of Ovotransferrin and Lactoferrin Provides Insight into Functional Differences in Iron Release

Halbrooks, Peter J. and Giannetti, Anthony M. and Klein, Joshua S. and Bjorkman, Pamela J. and Larouche, Julia R. and Smith, Valerie C. and MacGillivray, Ross T. A. and Everse, Stephen J. and Mason, Anne B. (2005) Composition of pH-Sensitive Triad in C-Lobe of Human Serum Transferrin. Comparison to Sequences of Ovotransferrin and Lactoferrin Provides Insight into Functional Differences in Iron Release. Biochemistry, 44 (47). pp. 15451-15460. ISSN 0006-2960. https://resolver.caltech.edu/CaltechAUTHORS:20170131-140942114

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Abstract

The transferrins (TF) are a family of bilobal glycoproteins that tightly bind ferric iron. Each of the homologous N- and C-lobes contains a single iron-binding site situated in a deep cleft. Human serum transferrin (hTF) serves as the iron transport protein in the blood; circulating transferrin binds to receptors on the cell surface, and the complex is internalized by endocytosis. Within the cell, a reduction in pH leads to iron release from hTF in a receptor-dependent process resulting in a large conformational change in each lobe. In the hTF N-lobe, two critical lysines facilitate this pH-dependent conformational change allowing entry of a chelator to capture the iron. In the C-lobe, the lysine pair is replaced by a triad of residues:  Lys534, Arg632, and Asp634. Previous studies show that mutation of any of these triad residues to alanine results in significant retardation of iron release at both pH 7.4 and pH 5.6. In the present work, the role of the three residues is probed further by conversion to the residues observed at the equivalent positions in ovotransferrin (Q-K-L) and human lactoferrin (K-N-N) as well as a triad with an interchanged lysine and arginine (K534R/R632K). As expected, all of the constructs bind iron and associate with the receptor with nearly the same KD as the wild-type monoferric hTF control. However, interesting differences in the effect of the substitutions on the iron release rate in the presence and absence of the receptor at pH 5.6 are observed. Additionally, titration with KCl indicates that position 632 must have a positively charged residue to elicit a robust rate acceleration as a function of increasing salt. On the basis of these observations, a model for iron release from the hTF C-lobe is proposed. These studies provide insight into the importance of charge and geometry of the amino acids at these positions as a partial explanation for differences in behavior of individual TF family members, human serum transferrin, ovotransferrin, and lactoferrin. The studies collectively highlight important features common to both the N- and C-lobes of TF and the critical role of the receptor in iron release.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/bi0518693DOIArticle
http://pubs.acs.org/doi/abs/10.1021/bi0518693PublisherArticle
ORCID:
AuthorORCID
Bjorkman, Pamela J.0000-0002-2277-3990
Additional Information:© 2005 American Chemical Society. Received 14 September 2005. Published online 2 November 2005. Published in print 1 November 2005. This work was supported by USPHS Grant R01 DK 21739 (A.B.M.) and R01-DK60770 (P.J.B.) and the Howard Hughes Medical Institute (P.J.B.). J.S.K. was supported by biology funds from the Lawrence Ferguson Endowment.
Funders:
Funding AgencyGrant Number
NIHR01 DK 21739
NIHR01-DK60770
Howard Hughes Medical Institute (HHMI)UNSPECIFIED
Lawrence Ferguson EndowmentUNSPECIFIED
Issue or Number:47
Record Number:CaltechAUTHORS:20170131-140942114
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20170131-140942114
Official Citation:Composition of pH-Sensitive Triad in C-Lobe of Human Serum Transferrin. Comparison to Sequences of Ovotransferrin and Lactoferrin Provides Insight into Functional Differences in Iron Release Peter J. Halbrooks, Anthony M. Giannetti, Joshua S. Klein, Pamela J. Björkman, Julia R. Larouche, Valerie C. Smith, Ross T. A. MacGillivray, Stephen J. Everse, and Anne B. Mason Biochemistry 2005 44 (47), 15451-15460 DOI: 10.1021/bi0518693
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:73894
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:01 Feb 2017 00:17
Last Modified:03 Oct 2019 16:32

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