Glycosyltransferase Family 47

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• Author: Daniel Tehrani and Charlie Corulli
• Responsible Curator: Breeanna Urbanowicz

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Glycosyltransferase Family GT47
Fold	?
Mechanism	?
Active site residues	Known/unknown
CAZy DB link
http://www.cazy.org/GT47.html

Substrate specificities

Glycosyltransferases in GT47 catalyze the transfer of a wide variety of monosaccharides from activated donor sugar nucleotides onto a diversity of acceptor substrates found in plants, animals, insects, and bacteria1. Donor sugar nucleotides for discrete clades of GT47 enzymes include UDP-Arabinofuranose (UDP-Araf, i.e. ARAD1), UDP-Arabinopyranose (UDP-Arap), UDP-Xylose (UDP-Xyl, i.e. IRX-10), UDP-Galactose (UDP-Gal , i.e. MUR3), UDP-Galacturonic acid (UDP-GalA, i.e. XUT1), and UDP-Glucuronic acid (UDP-GlcA, i.e. EXT1)1-7. Acceptor substrates for the GT47 enzymes include a diverse collection of plant cell wall polysaccharides in various linkages and heparan sulfate backbone synthesis in mammals1,7. Plants The GT47 family is highly diversified in plants, having an association with the biosynthesis of almost every class of plant cell wall polysaccharide. Although the enzymatic functions for the vast majority of plant GT47s are currently unknown, identified members have been reported to use UDP-GalA, UDP-Gal, UDP-Arap, UDP-Araf, and UDP-Xyl as activated sugar donors. Known acceptor polysaccharide substrates include xyloglucan, xylan, galacto-glucomannan, xylo-galacturonan, and rhamnogalacturonan I. Xyloglucan: Xyloglucan is a hemicellulose and a major component of the plant primary cell wall8. Xyloglucan forms polymer-polymer interactions with cellulose, creating a network through cross-linking cellulose microfibrils. These interactions are influenced by the diversity of sidechains found on xyloglucan which can vary depending on plant species, tissue, and stage of growth9. Various members of the GT47 family have been reported to contribute to the synthesis of the numerous sidechains found on xyloglucan, with the two most notable xyloglucan-modifying GT47s being MUR3 and XLT2. These two enzymes catalyze the regiospecific addition of β-D-Gal forming the Gal-β1,2-Xyl-α- (‘L) sidechains of xyloglucan. The GT47s XST1 and XDT are reported to transfer UDP-Araf and UDP-Arap respectively to the 3rd xylose of xyloglucan, forming the Araf-α1,2-Xyl-α- (‘S) and Arap-α1,2- Xyl-α- (‘D) sidechain motifs. XUT1 is reported to transfer UDP-GalA, forming the GalA-β1,2-Xyl-α- (‘Y) sidechain. More recently, the GT47 xyloglucan beta-xylopyranosyltransferase (XBT) has been identified to transfer UDP-Xyl to form the Xyl-β1,2-Xyl-α-(‘U) sidechain10. The quantity of GT47s reported to act on xyloglucan is indicative of the important role this family has in contributing to the diversity of this polymer. Xylan: Unlike the previously mentioned sidechain modifications of xyloglucan, GT47s can additionally contribute to the synthesis of polymer backbones as observed with xylan. Xylan is a hemicellulosic polysaccharide and a major component of the plant secondary cell wall. This polysaccharide is composed of a β1,4-Xyl backbone, synthesized through the actions of multiple GTs. The GT47 IRX10 is one such GT which functions in a complex with two other GTs, IRX9 (GT43) and IRX14 (GT43), using UDP-Xyl as a donor substrate11. This xylan synthase complex (XSC) contributes to the synthesis of the xylan backbone, although IRX10 is the only enzyme in the complex which displays an enzymatic function in extending xylan in vivo. Mutations in either IRX9 or IRX14 have been observed to contribute to xylan deficiencies in plants indicating that both proteins have an essential yet currently unknown role in the complex12. Loss of function mutations have additionally identified IRX7 as another potential xylan modifying GT47 participating in the synthesis of the xylan reducing end tetrasaccharide β-D-Xyl-1,4-[β-D-Xyl-1,3-α-l-Rha-1,2-α-D-GalA-1,4-D-Xyl] although more evidence is required to elucidate this function13. Mannan: Mannan is a hemicellulosic polysaccharide prominently found in the plant primary cell wall. Galactoglucomannan is a class of mannan with a backbone interspersed with β1,4-Glc which can be further substituted with α1,6-Gal residues. Recently, it was shown that the α1,6-Gal residues of this polymer can additionally be substituted with β1,2-Gal. Loss of function mutations in Arabidopsis have identified the GT47 MBGT1 as a likely candidate in synthesizing the Galβ-1,2-Galα-1,6- sidechains by adding the terminal galactose to the structure14. Pectin: Pectin encompasses a diverse group of polymers which include homogalacturonan, rhamnogalacturonan I, rhamnogalacturonan II, and xylogalacturonan. Pectic polysaccharides play many crucial roles in plants such as intercellular adhesion, stress response, seed germination, morphogenesis, and cell communication1,15. Members of the GT47 family have been reported to synthesize sidechain additions on xylogalacturonan and rhamnogalacturonan I. Loss of function mutations in Arabidopsis have identified the GT47 XGD1 as a xylosyltransferase, synthesizing the addition of β1,4-Xyl residues on the GalA backbone of xylogalacturonan3. The GT47 ARAD1 has been identified to contribute to the synthesis of arabinose sidechains on rhamnogalacturonan I, identified via the analysis of isolated RG-I from arad1 Arabidopsis mutants16. Extensin: Unlike the previously mentioned polysaccharides, extensins are rod-like glycoproteins which form crosslinked networks in the plant cell wall. These networks are reported to play a crucial role in regulating cell wall growth and development17. A unique member of the GT47 family, ExAD, is reported to synthesize the addition of the fourth arabinofuranose (Araf) on Araf substituted C4-hydroxyprolines (Hyps) creating Hyp-Araf4, a unique feature found on extensins15. Animals The abundance of GT47 family enzymes in mammals is more restricted and includes only members of the Exostosin (EXT) and Exostoslin-Like (EXTL) family of enzymes involved in heparan sulfate biosynthesis. Heparan sulfate is comprised of a repeat disaccharide polymer of ( GlcAβ1,4GlcNAcα1,4-)n that is further elaborated with extensive sulfation along the polymer chain. The disaccharide backbone repeat is elongated by the co-polymerase activity of the heterodimeric EXT1-EXT2 complex7. EXT1 and EXT2 are homologous two domain enzymes, and each protein chain contains a GT47 β1,4-GlcA transferase-like and a GT64 α1,4GlcNAc transferase-like domain. Surprisingly, only the GT47 domain of EXT1 and GT64 domain of EXT2 exhibit catalytic activity, while the other domains in each subunit are nonfunctional7. Additional EXT homologs include the EXTL proteins, EXTL1-3. EXTL1 and EXTL3 are two domain proteins, each harboring a GT47 and GT64 domain like EXT1 and EXT2. However, only the GT64 domains exhibit α1,4GlcNAc transferase activity, while their corresponding GT47 domains are inactive. In contrast, EXTL2 is a single GT64 domain enzyme with a α1,4GlcNAc transferase activity, while the corresponding GT47 domain present in other EXTs is missing. Thus, among the five mammalian EXT or EXTL homologs, only EXT1 contains a functional GT47 domain exhibiting β1,4-GlcA transferase activity.

Kinetics and Mechanism

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Catalytic Residues

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Three-dimensional structures

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Family Firsts

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References

1. Cantarel BL, Coutinho PM, Rancurel C, Bernard T, Lombard V, and Henrissat B. (2009). The Carbohydrate-Active EnZymes database (CAZy): an expert resource for Glycogenomics. Nucleic Acids Res. 2009;37(Database issue):D233-8. DOI:10.1093/nar/gkn663 |

2. Davies, G.J. and Sinnott, M.L. (2008) Sorting the diverse: the sequence-based classifications of carbohydrate-active enzymes. The Biochemist, vol. 30, no. 4., pp. 26-32. DOI:10.1042/BIO03004026.