Glucanases are

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

that break down large polysaccharides via hydrolysis. The product of the hydrolysis reaction is called a

glucan A glucan is a polysaccharide derived from D-glucose, linked by glycosidic bonds. Glucans are noted in two forms: alpha glucans and beta glucans. Many beta-glucans are medically important. They represent a drug target for antifungal medications of ...

, a linear polysaccharide made of up to 1200 glucose monomers, held together with glycosidic bonds. Glucans are abundant in the

endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the embryo and ...

cell walls of cereals such as barley,

rye Rye (''Secale cereale'') is a grass grown extensively as a grain, a cover crop and a forage crop. It is a member of the wheat tribe (Triticeae) and is closely related to both wheat (''Triticum'') and barley (genus ''Hordeum''). Rye grain is u ...

sorghum ''Sorghum'' () is a genus of about 25 species of flowering plants in the grass family (Poaceae). Some of these species are grown as cereals for human consumption and some in pastures for animals. One species is grown for grain, while many othe ...

, rice, and wheat. Glucanases are also referred to as lichenases, hydrolases, glycosidases, glycosyl hydrolases, and/or laminarinases. Many types of glucanases share similar amino acid sequences but vastly different substrates. Of the known endo-glucanases, 1,3-1,4-β-glucanase is considered the most active.

Structure

β-glucanases

The secondary and tertiary structures of β-glucanases involves the stacking of multiple β-sheets, each of which are made of several anti-parallel strands that bend and form a cleft crossing the active site of the enzyme. This type of structure has been called the "

jelly roll fold The jelly roll or Swiss roll fold is a protein fold or supersecondary structure composed of eight beta strands arranged in two four-stranded sheets. The name of the structure was introduced by Jane S. Richardson in 1981, reflecting its resemblance ...

Some common β-glucanases

* 1,3-β-glucanases (laminarinases, EC 3.2.1.39) * Endo-1,3(4)-β-glucanase * β-1,3-glucanase, an enzyme in plants that breaks down β-1,3-glucans such as callose or curdlan * β-1,6 glucanase, an enzyme that breaks down β-1,6-glucans * Cellulase, an enzyme that perform the hydrolysis of 1,4-beta-D-glycosidic linkages in cellulose, lichenin and cereal β-D-glucans. *

Xyloglucan-specific endo-beta-1,4-glucanase In enzymology, a xyloglucan-specific endo-beta-1,4-glucanase () is an enzyme that catalyzes the chemical reaction :xyloglucan + H2O \rightleftharpoons xyloglucan oligosaccharides Thus, the two substrates of this enzyme are xyloglucan and H2O ...

Xyloglucan-specific exo-beta-1,4-glucanase In enzymology, a xyloglucan-specific exo-beta-1,4-glucanase () is an enzyme that catalyzes the chemical reaction :xyloglucan + H2O \rightleftharpoons xyloglucan oligosaccharides (exohydrolysis of 1,4-beta-D-glucosidic linkages in xyloglucan) T ...

α-glucanases

* α-1,4-glucanase, an enzyme that breaks down α-1,4-glucans * α-1,6-glucanase, an enzyme that breaks down α-1,6-glucans * Pullulanase, a specific kind of glucanase that degrade pullulan The functional formation of the enzyme-substrate complex is dictated by the induced-fit mechanism.

Mechanism of Enzyme Action

The main function of glucanase is to catalyze the hydrolysis of glycosidic bonds in polysaccharides. This function is not highly specific, and the enzymes distinguish among substrates mostly by the types of bonds present and α- or β- configuration. In 1953, Dr. D. E. Koshland proposed a double-displacement mechanism for this enzyme action. The first step of his proposed mechanism is rate-limiting step independent of the concentration of the substrate and involves an amino acid

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

and an acid/base catalyst. In this step, the nucleophile, with help from the acid residue, displaces the aglycone and forms a covalent glycosyl-enzyme intermediate. The second step involves a water molecule, assisted by the conjugate base of the acid catalyst, rendering the free sugar while retaining an anomeric configuration of the molecule. Glucanases can also catalyze transglycosylation, resulting in new β-glycosidic bonds between donor and acceptor saccharides. This reaction, which has the same region- and stereo-specificity as the hydrolysis reaction, involves either the direct reversal of hydrolysis (known as

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...

) or kinetic control of a glycosyl donor substrate. Hydrolysis_Condensation_reaction

Microbial Occurrence and Agricultural Significance

Microbial Production

Bacteria such as '' Escherichia coli'', and '' Bacillus'' spp. produce 1,3-1,4-β-glucanases in order to degrade and use polysaccharides from their environment as an energy source. These bacterial glucanases are an example of convergent evolution as they share similarity or relation with plant glucanase

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secondary Secondary may refer to: Science and nature * Secondary emission, of particles ** Secondary electrons, electrons generated as ionization products * The secondary winding, or the electrical or electronic circuit connected to the secondary winding i ...

, or tertiary structure. Glucanases have also been found to be secreted by fungi such as ''

Trichoderma harzianum ''Trichoderma harzianum'' is a fungus that is also used as a fungicide. It is used for foliar application, seed treatment and soil treatment for suppression of fungal pathogens causing various fungal plant diseases. Commercial biotechnological p ...

'', '' Saccharomyces cerevisiae'' and the anaerobic fungi ''

Orpinomyces ''Orpinomyces'' is a genus of fungi in the family Neocallimastigaceae Neocallimastigomycota is a phylum containing anaerobic fungi, which are symbionts found in the digestive tracts of larger herbivores. Anaerobic fungi were originally place ...

'' and Neocallimastigomycota, found in the digestive tracts of herbivores. ''T. harzianum'' is also used as a

fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...

, which is linked to the ability of its β-gluanases to hydrolyze

phytopathogenic Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...

fungi via a mycoparasitic attack.

Beer and Wine

Barley 1,3-1,4-β-glucanases are heat inactivated during malting, which can cause the build-up of high molecular-weight glucans which in turn result in reduced extract yield, lower filtration rates, and even gelatinous precipitates in the finished product. As a remedy, heat-resistant bacterial 1,3-1,4-β-glucanases are added. Used in enological practices during the aging process of wine, particularly when aged on lees with microxygenation. The enzyme aids in autolysis of yeast cells to release polysaccharides and mannoproteins, which is believed to aid in the color and texture of the wine.

Livestock Feed

In the production of feedstuff for broiler chickens and piglets, it has been found that β-glucanases improve digestibility of barley-based diets.