glucansucrase
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Glucansucrase (also known as
glucosyltransferase Glucosyltransferases are a type of glycosyltransferase that enable the transfer of glucose. Examples include: * glycogen synthase * glycogen phosphorylase Glycogen phosphorylase is one of the phosphorylase enzymes (). Glycogen phosphorylase c ...
) is an
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
in the
glycoside hydrolase Glycoside hydrolases (also called glycosidases or glycosyl hydrolases) catalyze the hydrolysis of glycosidic bonds in complex sugars. They are extremely common enzymes with roles in nature including degradation of biomass such as cellulose (ce ...
family GH70 used by
lactic acid Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as nat ...
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
to split sucrose and use resulting
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
molecules to build long, sticky
biofilm A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...
chains. These
extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
homopolysaccharides are called α- glucan
polymers A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...
. Glucansucrase enzymes can synthesize a variety of glucans with differing solubilities,
rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with Plasticity (physics), plastic flow rather than deforming Elasticity (phy ...
, and other properties by altering the type of glycosidic linkage, degree of branching, length, mass, and conformation of the polymers. Glucansucrases are classified according to the glycosidic linkage they catalyze. They can be mutansucrases, dextransucrases, alternansucrases, or reuteransucrases. This versatility has made glucansucrase useful for industrial applications. Glucansucrase's role in cariogenesis is a major point of interest. Glucan polymers stick to teeth in the human mouth and cause
tooth decay Tooth decay, also known as cavities or caries, is the breakdown of teeth due to acids produced by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicatio ...
. *


Structure

Glucansucrases are large, extracellular proteins with average molecular masses around 160,000 Daltons. Therefore crystallography studies have only been carried out for fragments of the enzymes, not complete structures. However, glucansucrase is very similar to
α-amylase α-Amylase is an enzyme (EC 3.2.1.1; systematic name 4-α-D-glucan glucanohydrolase) that hydrolyses α bonds of large, α-linked polysaccharides, such as starch and glycogen, yielding shorter chains thereof, dextrins, and maltose: :Endohyd ...
, another sugar-cutting enzyme. Glucansucrase thus has many of the same structural features. For example, both enzymes have three domains in their catalytic core and a (β/α)8 barrel. Glucansucrase has five major domains: A, B, C, IV, and V. The domains in glucansucrase, however, have a different arrangement than those in α-amylase. The folding characteristics of α-amylase and glucansucrase are still very similar, but their domains are permuted. Domains A, B, IV, and V are built from two discontiguous parts of the polypeptide chain, causing the chain to follow a U-shape. From the N- to C-terminus, the polypeptide chain goes in the following order: V, IV, B, A, C, A, B, IV, V (see figure at top right). The C domain is the only one made up of a continuous polypeptide sequence. Domain A contains the (β/α)8 barrel and the catalytic site. In the catalytic site, three residues in particular play important roles for enzymatic activity: a
nucleophilic 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 ...
aspartate, an acid/base glutamate, and an additional aspartate to stabilize the
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked ...
. Domain B makes up a twisted antiparallel β sheet. Some of the loops in domain B help shape the groove near the catalytic site. Additionally, some amino acids between domains A and B form a
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
binding site near the nucleophilic aspartate. The Ca2+ ion is necessary for enzyme activity.


Reaction and Mechanism

Glucansucrase has two parts to its reaction. First it cleaves a
glycosidic bond A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. A glycosidic bond is formed between the hemiacetal or hemiketal group ...
to split sucrose. Products of the reaction are the constituent
monosaccharides Monosaccharides (from Greek '' monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units ( monomers) from which all carbohydrates are built. They are usually colorless, water ...
glucose and fructose. This glucose is added to a growing glucan chain. Glucansucrase uses the energy released from bond cleavage to drive glucan synthesis. Both sucrose breakdown and glucan synthesis occur in the same active site. The first step is carried out through a transglycosylation mechanism involving a glycosyl-enzyme intermediate in subsite-1. Glutamate is likely the catalytic acid/base, aspartate the nucleophile, and another aspartate the transition state stabilizer. These three residues are all highly conserved and mutating them leads to a significant decrease in enzymatic activity. The glucansucrase mechanism has historically been controversial in the scientific literature. The mechanism involves two displacements. The first originates from a glycosidic cleavage of the sucrose substrate between subsites -1 and +1. This releases fructose and forms a sugar-enzyme intermediate when the glucose unit attaches to the nucleophile. The second displacement is transfer of a glucosyl moiety to an acceptor, such as a growing glucan chain. The debate in the past was over whether the glucosyl group attached to the non-reducing or reducing end of an incoming acceptor. Additional investigations pointed to a non-reducing mechanism with a single active site.


Evolution

Glucansucrase proteins likely evolved from an amylase enzyme precursor. The two enzymes have similar folding patterns and protein domains. In fact, past attempts to produce drugs targeting glucansucrase have not been successful because the drugs also disrupted amylase, which is necessary to break down starches. This occurred because the active sites of the two enzymes are nearly the same. Glucansucrase likely maintained a highly-conserved active site as it underwent a different evolutionary path.


Health

Glucansucrase allows the oral bacteria '' Streptococcus mutans'' to metabolize sucrose into lactic acid. This lactic acid lowers the pH around teeth and dissolves
calcium phosphate The term calcium phosphate refers to a family of materials and minerals containing calcium ions (Ca2+) together with inorganic phosphate anions. Some so-called calcium phosphates contain oxide and hydroxide as well. Calcium phosphates are whi ...
in
tooth enamel Tooth enamel is one of the four major tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the normally visible part of the tooth, covering the crown. The other major tissues are dentin, ...
, leading to tooth decay. Additionally, the synthesis of glucan aids ''S. mutans'' in adhering to the surface of teeth. As the polymers accumulate, they help more acid-producing bacteria stay on teeth. Consequently, glucansucrase is such an attractive drug target to prevent tooth decay. If ''S. mutans'' can no longer break down sucrose and synthesize glucan, calcium phosphate is not degraded and bacteria cannot adhere as easily to teeth.


Industry

Bacteria with glucansucrase enzymes are used extensively in industry for a variety of applications. The polymer
dextran Dextran is a complex branched glucan ( polysaccharide derived from the condensation of glucose), originally derived from wine. IUPAC defines dextrans as "Branched poly-α-d-glucosides of microbial origin having glycosidic bonds predominantly C-1 ...
is one prominent example of a very useful polymer. It is produced at commercial scale for uses in
veterinary medicine Veterinary medicine is the branch of medicine that deals with the prevention, management, diagnosis, and treatment of disease, disorder, and injury in animals. Along with this, it deals with animal rearing, husbandry, breeding, research on nutri ...
, separation technology,
biotechnology Biotechnology is the integration of natural sciences and engineering sciences in order to achieve the application of organisms, cells, parts thereof and molecular analogues for products and services. The term ''biotechnology'' was first used ...
, the food industry for gelling, viscosifying, and emulsifying, in human medicine as a prebiotic,
cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
-lowering agent or
blood plasma Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the intr ...
expander, and more.


See also

* Alternansucrase


References


External links


Glucansucrase: Molecule of the Month
by David Goodsell, RCSB Protein Data Bank * {{PDBe-KB2, Q5SBN3, Glucansucrase Enzymes Dentistry