Glycoside Hydrolase Family 1

Glycoside Hydrolase Family GH1
Clan	GH-A
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/fam/GH1.html

Substrate specificities

The most common known enzymatic activities in this family, at the current time, are those of b-glucosidases and b-galactosidases: indeed typically both activities are found within the same active site, often with similar kcat values, but with substantially higher Km values for the galactosides. However, other commonly found activities are 6-phospho-b-glucosidase and 6-phospho-b-galactosidase, b-mannosidase, b-D-fucosidase and b-glucuronidase. Family GH1 enzymes are found across a broad spectrum of life forms. Enzymes of medical interest include the human lactase/phlorizin hydrolase whose deficiency leads to lactose intolerance. In plants Family 1 enzymes are often involved in the processing of glycosylated aromatics such as saponins and some plant hormones stored in inactive glycosylated forms. Indeed some have been identified as plant oncogenes due to aberrant control of auxin levels. One of the work horses of glycosidase enzymology, the almond emulsin b-glucosidase, even though not fully sequences, is deduced to belong to Family 1 by limited sequence analysis ¹.

Kinetics and Mechanism

Family 1 b-glycosidases are retaining enzymes, as first shown by NMR² and follow a classical Koshland double-displacement mechanism. Enzymes that have been well-studied kinetically include the almond emulsin enzyme, for which a particularly nice and important set of studies on rate-limiting steps and inhibition was reported in the mid 1980’s ^3,4 and the Agrobacterium sp. b-glucosidase which has been the subject of a series of kinetic evaluations, including detailed steady state ^5,6 and pre-steady state kinetic analyses in which the roles of each substrate hydroxyl in catalysis have also been carefully probed. ⁷

Catalytic Residues

The catalytic nucleophile was first identified in the Agrobacterium sp. b-glucosidase as Glu358 in the sequence YITENG through trapping of the 2-deoxy-2-fluoroglucosyl-enzyme intermediate and subsequent peptide mapping.{Withers, 1990 #52} The acid/base catalyst was first identified as Glu 170 in this same enzyme through detailed mechanistic analysis of mutants at that position, which included azide rescue experiments. ⁹ Family GH1 enzymes, as is typical of Clan GHA, have an asparagine residue preceding the acid/base catalyst in a typical NEP sequence. The asparagine engages in important hydrogen bonding interactions with the substrate 2-hydroxyl. Interestingly, the plant myrosinases of GH1 cleave thioglycosides bearing an anionic aglycone, and have evolved an active site in which the acid/base glutamate is replaced by glutamine. Substrates are sufficiently reactive not to require the acid catalyst, while the role of base catalyst is played by exogenous ascorbate, which binds to the glycosyl enzyme REF.

Three-dimensional structures

Three-dimensional structures are available for a large number of Family 1 enzymes, the first solved being that of the white clover enzyme (Tolley). As members of Clan GHA they have a classical (a/b)₈ TIM barrel fold with the two key active site glutamic acids being approximately 200 residues apart in sequence and located at the C-terminal ends of b-strands 4 (acid/base) and 7 (nucleophile).¹⁰ Known oligomeric forms of these enzymes include dimers and tetramers (others?).

PICTURES? OVERALL STRUCTURE; ACTIVE SITE; CARTOON OF ACTIVE SITE SHOWING INTERACTIONS?