Lupine Publishers | Scholarly Journal of Food and Nutrition
Abstract
α-Galactosidases as natural
biocatalysts have important application value in various industries. In this
paper, we reviewed their physiological functions, biological sources,
classification, and protein structure and substrate specificity relationships
of Glycoside hydrolase (GH) family 27, aiming at providing references for
related experiments and studies.
Keywords: GH27α-Galactosidase, Loop region, Substrate specificity,
Thermo stability, Stability at low pH 29
Abbreviations: GH: Glycoside Hydrolase
Introduction
Galacto-oligosaccharides (i.e.
stachyose and raffinose) commonly exist in food and feed and are indigestible
by human and animals, thereby causing flatulence, gastrointestinal disturbance
and low feed efficiency. [1] α-Galactosidases (EC. 3.2.1.22) has hydrolysis
ability to degrade these anti-nutritional factors, decrease the viscosity of
the diet, reduce the occurrence of diarrhoea, destroy the structure of the cell
wall of the plant, promote the nutrition release, improve the utilization
efficiency of the nutrients in the feed, increases lean meat rate, enhances
immune function and disease resistance of animals. [2,3] α-Galactosidases has
great application value in industrial processes of feed, food, and beet sugar
production. α-Galactosidases are widely distributed in fungi, bacteria, plants
and human (www.cazy.org). Fungal α-galactosidases have maximal activity at pH
3-5, but bacterial α-galactosidases are the optimal pH of 6-7.5.1, [4-6] Some
thermostable α-galactosidases have been identified from thermophilic fungi,
such as thermomyces lanuginosus, Talaromecys emersonii, and Rhizomucor miehei.
[7- 9] Due to high processing temperatures and acidic environment of the
gastrointestinal tract, the highly efficient, thermostable and acidicphilic
α-galactosidases with broad substrate specificity is of great interest. [10]
Base on the sequence similarities, α-galactosidases are divided into Glycoside
Hydrolase (GH) families 4, 27, 36, 57, 97, and 110. [11] Most fungal
α-galactosidases belong to GH27 and have conserved YLKYDNC catalytic motif and
DD(G/C) W binding motif. [12,13] The resolved crystal structures of two GH27
α-galactosidases from Trichoderma reesei (1t0oA) and Saccharomyces cerevisiae
(ScAGal, 3LRK) share a (β/α)8 barrel fold and a retaining reaction mechanism.
[13,14] Sequence analysis indicated Loops 1, 2, and 4 of α-galactosidase from
T. reesei and corresponding loops 1-3 and 6 from ScAGal create new binding
sites for formation and breakdown of a covalent glycosyl enzyme
intermediate.13,14 In our study, two α-galactosidases Gal27A and Gal27B of
family GH27 from the thermophilic Neosartorya fischeri had similar tertiary
structures but varied in loop regions and substrate specificity. [15,16] Gal27A
had far separate loops and showed higher activity towards raffinose, which was
4.9 - and 3.8- fold for that of melibiose and stachyose.
[15] Whereas stachyose was a
preferred substrate for Gal27B with closely proximate loops and its activity to
stachyose was 9.6- and 4.4-fold of melibiose and raffinose, respectively.16
Introduction of loop 4 of Gal27A into Gal27B elevated the activity to raffinose
and broaded the substrate specificity (data not shown). Kinetic analysis of
prolyl oligopeptidase indicated the loop splitting decreased the affinity of
the enzyme to the substrate. [17] Sitedirected mutagenesis revealed loops
facing the active site of prolyl oligopeptidase can regulate the substrate
gating and specificity. [18] Thus the flexibility and motility of loops are
presumed to be involved in enzyme-substrate interactions. The transformation of
the zymoproteins is an important source to obtain excellent zymoproteins for
various industries. With the development and accumulation of structural
biological information of protein structure and function, protein rational
design will inevitably become an important means to improve the properties of
enzyme proteins.
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