HX

Hemopexin-like repeats.
HX
SMART accession number:SM00120
Description: Hemopexin is a heme-binding protein that transports heme to the liver. Hemopexin-like repeats occur in vitronectin and some matrix metalloproteinases family (matrixins). The HX repeats of some matrixins bind tissue inhibitor of metalloproteinases (TIMPs).
Interpro abstract (IPR018487):

Hemopexin (EC 3.2.1.35) is a serum glycoprotein that binds haem and transports it to the liver for breakdown and iron recovery, after which the free hemopexin returns to the circulation [(PUBMED:12042069)]. Hemopexin prevents haem-mediated oxidative stress. Structurally hemopexin consists of two similar halves of approximately two hundred amino acid residues connected by a histidine-rich hinge region. Each half is itself formed by the repetition of a basic unit of some 35 to 45 residues. Hemopexin-like domains have been found in two other types of proteins, vitronectin [(PUBMED:9572850)], a cell adhesion and spreading factor found in plasma and tissues, and matrixins MMP-1, MMP-2, MMP-3, MMP-9, MMP-10, MMP-11, MMP-12, MMP-14, MMP-15 and MMP-16, members of the matrix metalloproteinase family that cleave extracellular matrix constituents [(PUBMED:14619953)]. These zinc endopeptidases, which belong to MEROPS peptidase subfamily M10A, have a single hemopexin-like domain in their C-terminal section. It is suggested that the hemopexin domain facilitates binding to a variety of molecules and proteins, for example the HX repeats of some matrixins bind tissue inhibitor of metallopeptidases (TIMPs).

The hemopexin domain exhibits the shape of an oblate ellipsoidal disk. The polypeptide chain is organised in four beta-sheet (blades) I to IV, which are almost symmetrically arranged around a central axis in consecutive order, giving rise to the formation of a four-bladed propeller. Each propeller blade or repeat is made up of four antiparallel beta-strands connected in a W-like strand topology, and is strongly twisted [(PUBMED:8969305), (PUBMED:20147493)].

This entry represents the repeats found in hemopexin and related domains.

Family alignment:
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There are 10863 HX domains in 2912 proteins in SMART's nrdb database.

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