Glycosyltransferase Family 108

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Glycosyltransferase Family GT108
Clan	GH-S
Mechanism	inverting
Reactions	GDP-Man β-1,2-mannosyltransferase 2.4.1-, donor is GDP-α-Man; 1,2-β-oligomannan phosphorylase 2.4.1.340, product is α-mannose-1-phosphate
Active site residues	known
CAZy DB link
http://www.cazy.org/GT108.html

Substrate specificities

The glycosyltransferases in family GT108 were originally identified by bioinformatics analysis using GH130 sequences as a query. This identified a tandem repeat of seven genes on chromosome 10 of Leishmania mexicana and varying numbers of orthologs in other trypanosomatids [1]. A null mutant lacking the entire array of seven genes lost the ability to synthesize 1,2-β-oligomannan (termed mannogen). These enzymes were termed mannosyltransferase/phosphorylases (MTPs) owing to ability to both synthesize mannogen from GDPMan and/or Man-1-P, as well as an ability to catalyze phosphorolysis of mannogen to form Man-1-P. Specifically, the L. mexicana enzymes MPT3, MPT4, MPT6 and MPT7 catalyze the phosphorolysis of mannogen to give Man-1-P, as well as the reverse reaction to synthesize mannogen by mannosyltransfer from Man-1-P. The ‘’L. mexicana’’ enzymes MPT1 and MPT2 act as GDP-Man dependent β-1,2-mannosyltransferases, elongating mannogen, and lack detectable phosphorolytic activity.

Kinetics and Mechanism

In the glycoside cleavage reaction Asp83 of Leishmania mexicana MPT4 acts as a general acid, protonating the glycosidic leaving group via a proton relay through the -1 subsite mannose 3-OH, allowing phosphate to displace the anomeric glycoside leaving group. In the reverse reaction involving mannosyl transfer from Man-1-P (or GDP-Man for MPT1 and 2), this residue acts as a general base, deprotonating the 2-OH of the sugar nucleophile to promote glycosidic bond formation. The proposed mechanism is similar to that for family GH130 β-1,2-mannoside phosphorylases [2]

Catalytic Residues

Asp83 is the catalytic general base in the Leishmania mexicana MPT4 [1]. Activity as a mannosyltransferase or phosphorylase is achieved by a His/Arg switch: the GDP-Man transferases MTP1 and MTP2 contain a His residue (His168 in MTP1 and His161 in MTP2) in the active site; at the equivalent position the phosphorolytic MTPs such as MTP4 contain an Arg residue (Arg150) [1].

Three-dimensional structures

The three-dimensional structures of several GT108 proteins have been reported. The Structural Genomics of Pathogenic Protozoa Consortium (SGPP) deposited the first structure of a member of this family as a 'hypothetical protein' from L. major [3]; the function of this protein remains unknown. The structures of several MPTs from L. mexicana have been reported [1]. All proteins have a five-bladed β-propeller fold. Similar folds are predicted for other family members [1]. A complex of the D94N variant of L. mexicana MPT2 with mannobiose highlighted this conserved residue as the likely general acid for the reverse phosphorolytic reaction. Superposition of the structures of the MPTs with GH130 β-mannoside phosphorylases highlighted conserved active site residues in the -1 subsite, and conserved interactions with the sugar residue. The Arg/His residues that correlates with GDP-Man transferase/mannoside phosphorylase activity were located in the active site cleft, in a position that was proposed to result in specific interactions with GDP or phosphate in the respective catalytic reactions [1].

Family Firsts

First catalytic residue identification: Asp83 in Leishmania mexicana MPT4 and other MPT enzymes [1]. The His/Arg switch distinguishing GDP-Man dependent transferase activity, and phosphorolytic activity was also identified in this study.
First 3-D structure: The Leishmania major strain Friedlin protein LMJF_10_1260 was determined by X-ray crystallography [3].

References

All Medline abstracts: PubMed