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Structure of human haptoglobin
Structural highlights
DiseaseHPT_HUMAN The disease is caused by mutations affecting the gene represented in this entry. FunctionHPT_HUMAN As a result of hemolysis, hemoglobin is found to accumulate in the kidney and is secreted in the urine. Haptoglobin captures, and combines with free plasma hemoglobin to allow hepatic recycling of heme iron and to prevent kidney damage. Haptoglobin also acts as an Antimicrobial; Antioxidant, has antibacterial activity and plays a role in modulating many aspects of the acute phase response. Hemoglobin/haptoglobin complexes are rapidely cleared by the macrophage CD163 scavenger receptor expressed on the surface of liver Kupfer cells through an endocytic lysosomal degradation pathway.[1] Uncleaved haptoglogin, also known as zonulin, plays a role in intestinal permeability, allowing intercellular tight junction disassembly, and controlling the equilibrium between tolerance and immunity to non-self antigens.[2] Publication Abstract from PubMedCD163, a macrophage-specific receptor, plays a critical role in scavenging hemoglobin released during hemolysis, protecting against oxidative effects of heme iron. In the bloodstream, hemoglobin is bound by haptoglobin, leading to its immediate endocytosis by CD163. While haptoglobin's structure and function are well understood, CD163's structure and its interaction with the haptoglobin-hemoglobin complex have remained elusive. Here, we present the cryo-electron microscopy structure of the entire extracellular domain of human CD163 in complex with haptoglobin-hemoglobin. The structure reveals that CD163 assembles into trimers (and to some extent dimers), binding haptoglobin-hemoglobin in their center. Key acidic residues in CD163 interact with lysine residues from both haptoglobin and hemoglobin. Calcium-binding sites located near the haptoglobin-hemoglobin interface in CD163 provide explanation for the calcium dependence of the interaction. Furthermore, we show that the interaction facilitating CD163 oligomerization mimics ligand binding and is also calcium dependent. This structural insight into CD163 advances our understanding of its role in hemoglobin scavenging as well as its broader relevance to structurally related scavenger receptors. The Cryo-EM structure of human CD163 bound to haptoglobin-hemoglobin reveals molecular mechanisms of hemoglobin scavenging.,Etzerodt A, Mikkelsen JH, Torvund-Jensen M, Hennig D, Boesen T, Graversen JH, Moestrup SK, Kollman JM, Andersen CBF Nat Commun. 2024 Dec 30;15(1):10871. doi: 10.1038/s41467-024-55171-4. PMID:39738064[3] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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