Glycoside Hydrolase Family 77

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• Author: ^^^Stefan Janecek^^^
• Responsible Curator: ^^^Stefan Janecek^^^

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Glycoside Hydrolase Family GH77
Clan	GH-H
Mechanism	retaining
Active site residues	known
CAZy DB link
https://www.cazy.org/GH77.html

Substrate specificities

Glycoside hydrolase family GH77 is the member of the α-amylase clan GH-H [1], together with GH13 and GH70 [2]. The family contains only one enzyme specificity - the amylomaltase (EC 2.4.1.25), that is known as disproportionating enzyme (D-enzyme) in plants [3] or 4-α-glucanotransferase in bacteria [4] and archaeons [5]. As of June 2014, it has more than 2,000 members [6] with ~1,950 from Bacteria, and about 30 from each Archaea and Eukarya (plants and green algae).

Amylomaltase catalyses the glucan-chain transfer from one α-1,4-glucan to another α-1,4-glucan (or to 4-hydroxyl group of glucose) or within a single linear glucan molecule to produce a cyclic α-1,4-glucan with degree of polymerization starting from 17 [3, 4, 5]. Cyclodextrin glucanotransferase, a member of the α-amylase family GH13, also produces cyclic α-1,4-glucans, but with a small degree of polymerization (6-8), called cyclodextrins [7].

Five tertiary structures have already been determined and only several tens enzymes have been biochemically characterized [6]. The main structural feature that discriminates the family GH77 amylomaltases from typical α-amylase family GH13 members is the lack of domain C [8] that succeeds the catalytic (β/α)8-barrel in the family GH13. The eight-fold symmetry of the catalytic barrel is disrupted by several insertions between the barrel β-strands that form the so-called subdomains B1, B2 and B3 [8]. Subdomain B1 consists of a highly twisted four-stranded antiparallel β-sheet with two α-helices and it is also present in other enzymes from the α-amylase clan GH-H (known as domain B). Subdomain B2 has predominantly an α-helical structure and it is unique to amylomaltases. Subdomain B3 could have a role of domain C from the α-amylase family [8].

Interestingly, primary structures of amylomaltases from borreliae contain unique sequence features [9], i.e. natural mutations in functionally important positions, such as extremely well-conserved and functional arginine in position i-2 with respect to the catalytic nucleophile is replaced naturally by a lysine. It is worth mentioning that this arginine positioned two residues before the catalytic nucleophile in the conserved sequence region II was considered to belong to the four residues conserved invariantly throughout the α-amylase family [10]. Its substitution is therefore of a special interest because the GH77 protein from Borrelia burgdorferi does exhibit the real amylomaltase activity [11]. Since, however, the lysine could eventually play the role of the original arginine, it is not possible to say unambiguously that the catalytic triad alone is enough for a GH-H protein to be a real functional member of the α-amylase family.

Kinetics and Mechanism

GH77 members employ the retaining reaction mechanism as used in the family GH13, they fold into a (β/α)8-barrel with the catalytic machinery consisting of a strand β4-aspartic acid (catalytic nucleophile), β5-glutamic acid (proton donor) and β7-aspartic acid (transition-state stabilizer), and they share their own 4-7 characteristic conserved sequence regions.

Catalytic Residues

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Three-dimensional structures

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Family Firsts

First stereochemistry determination

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First catalytic nucleophile identification

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First general acid/base residue identification

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First 3-D structure

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References

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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 |

9. Machovic M, and Janecek S. The invariant residues in the α-amylase family: just the catalytic triad. Biologia 2003; 58(6) 1127-32. (PDF)

10. Janecek S. How many conserved sequence regions are there in the α-amylase family? Biologia 2002; 57(Suppl. 11) 29-41. (PDF)