Sucrose phosphorylase () is an important enzyme in the metabolism of

sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...

and regulation of other metabolic intermediates. Sucrose phosphorylase is in the class of hexosyltransferases. More specifically it has been placed in the retaining

glycoside hydrolases 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 (cel ...

family although it catalyzes a transglycosidation rather than hydrolysis. Sucrose phosphorylase catalyzes the conversion of sucrose to D-fructose and α-D-glucose-1-phosphate. It has been shown in multiple experiments that the enzyme catalyzes this conversion by a

double displacement A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...

mechanism.

Reaction

The method by which sucrose phosphorylase converts sucrose to D-fructose and alpha-D-glucose-1-phosphate has been studied in great detail. In the reaction, sucrose binds to the enzyme, at which point

fructose Fructose, or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and galacto ...

is released by the enzyme-substrate complex. A covalent glucose-enzyme complex results, with beta-linkage between an oxygen atom in the

carboxyl group In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

of an aspartyl residue and C-1 of glucose. The covalent complex was experimentally isolated by chemical modification of the protein using NaIO4 after addition of the substrate, supporting the hypothesis that reaction catalyzed by sucrose phosphorylase proceeds through the ping-pong mechanism. In the final enzymatic step, the

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 ...

is cleaved through reaction with a

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

group, yielding α-D-glucose-1-phosphate. In a separate reaction, α-D-glucose-1-phosphate is converted to

glucose-6-phosphate Glucose 6-phosphate (G6P, sometimes called the Robison ester) is a glucose sugar phosphorylated at the hydroxy group on carbon 6. This dianion is very common in cells as the majority of glucose entering a cell will become phosphorylated in this wa ...

by the action of

phosphoglucomutase Phosphoglucomutase () is an enzyme that transfers a phosphate group on an α-D-glucose monomer from the 1 to the 6 position in the forward direction or the 6 to the 1 position in the reverse direction. More precisely, it facilitates the interconve ...

. Glucose-6-phosphate is an extremely important intermediate for several pathways in the human body, including

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvate (). The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH ...

gluconeogenesis Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrat ...

, and the

pentose phosphate pathway The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt and the HMP Shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (5-carbon sugars) as well as ribose 5-pho ...

. The function of sucrose phosphorylase is especially significant due to the role α-D-glucose-1-phosphate in energy metabolism.

Structure

The structure of sucrose phosphorylase has been identified in numerous experiments. The enzyme consists of four major domains, namely A, B, B’, and C. Domains A, B’ and C exist as dimers around the active site. The size of the enzyme, as determined by sedimentation

centrifugation Centrifugation is a mechanical process which involves the use of the centrifugal force to separate particles from a solution according to their size, shape, density, medium viscosity and rotor speed. The denser components of the mixture migrate ...

, was found to be 55 KDa, consisting of 488 amino acids. The active has been shown to contain two binding sites, one designated a water site where hydroxylic molecules such as 1,2-cyclohexanediol and

ethylene glycol Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...

may bind, and another designated as the acceptor site where the sugar molecule binds. Though the function of the water site has not been completely elucidated, the enzyme's stability in aqueous solutions indicates that the water site may be involved in hydrolysis of the glycosidic bond. The acceptor site is surrounded by three active residues that have been found to be essential in enzymatic activity. Using specific mutagenic assays, Asp-192 was found to be the catalytic

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 ...

of the enzyme, “attacking C-1 of the glucosyl moiety of sucrose”. In fact,

in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...

manipulation has shown that

D-xylose Xylose ( grc, ξύλον, , "wood") is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional gro ...

L-sorbose Sorbose is a ketose belonging to the group of sugars known as monosaccharides. It has a sweetness that is equivalent to sucrose (table sugar). The commercial production of vitamin C (ascorbic acid Vitamin C (also known as ascorbic acid and ...

, and

L-arabinose Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group. For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, or structural ...

can replace fructose as the glucosyl acceptor. The only requirement of the acceptor molecule is that the hydroxyl group on the C-3 be cis-disposed to the oxygen atom of the glycosidic bond. Glu-232 acts as the Bronsted acid-base catalyst, donating a proton to the displaced hydroxyl group on C-1 of the glucoside. The most significant residue in the enzymatic activity, however, is Asp-295. Upon cleavage of the fructofuranosyl

moiety Moiety may refer to: Chemistry * Moiety (chemistry), a part or functional group of a molecule ** Moiety conservation, conservation of a subgroup in a chemical species Anthropology * Moiety (kinship), either of two groups into which a society is ...

from sucrose, the resultant glucose forms a covalent intermediate with the enzyme. The carboxylate side chain of Asp-295 hydrogen bonds with the hydroxyl groups at C-2 and C-3 of the glucosyl residue. This interaction is maximized during 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 wi ...

of this covalent complex, lending support to the ping-pong mechanism. Finally, phosphorylation of the glucosyl residue at C-1 forms a transient positive charge on the glucosyl carbon, promoting breakage of the

ester bond In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ar ...

between Asp-192 and the sugar residue. Cleavage yields the product, α-D-glucose-1-phosphate.

Regulation

Since the discovery and characterization of sucrose phosphorylase, few documented experiments discuss mechanisms of regulation for the enzyme. The known methods of regulation are

transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...

al, affecting the amount of enzyme present at any given time. Global regulation of DNA molecules containing the gene for sucrose phosphorylase is performed by

catabolite repression Carbon catabolite repression, or simply catabolite repression, is an important part of global control system of various bacteria and other microorganisms. Catabolite repression allows microorganisms to adapt quickly to a preferred (rapidly metaboli ...

. First discovered in

Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

bacteria, both

Cyclic AMP Cyclic adenosine monophosphate (cAMP, cyclic AMP, or 3',5'-cyclic adenosine monophosphate) is a second messenger important in many biological processes. cAMP is a derivative of adenosine triphosphate (ATP) and used for intracellular signal transd ...

(cAMP) and

cAMP Receptor Protein cAMP receptor protein (CRP; also known as catabolite activator protein, CAP) is a regulatory protein in bacteria. CRP protein binds cAMP, which causes a conformational change that allows CRP to bind tightly to a specific DNA site in the promote ...

(CRP) function in sucrose phosphorylase regulation. The cAMP-CRP complex formed when both molecules combine acts as a positive regulator for transcription of the sucrose phosphorylase gene. The complex binds to the

promoter region In genetics, a promoter is a sequence of DNA to which proteins bind to initiate transcription of a single RNA transcript from the DNA downstream of the promoter. The RNA transcript may encode a protein (mRNA), or can have a function in and of i ...

to activate transcription, enhancing the creation of sucrose phosphorylase. Genetic regulation of sucrose phosphorylase is also performed by

metabolites In biochemistry, a metabolite is an intermediate or end product of metabolism. The term is usually used for small molecules. Metabolites have various functions, including fuel, structure, signaling, stimulatory and inhibitory effects on enzymes, c ...

. Through experimentation it is known that genes encoding for the sucrose phosphorylase enzyme can be induced by sucrose and

raffinose Raffinose is a trisaccharide composed of galactose, glucose, and fructose. It can be found in beans, cabbage, brussels sprouts, broccoli, asparagus, other vegetables, and whole grains. Raffinose can be hydrolyzed to D-galactose and sucrose by ...

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, using ...

, on the other hand, represses the transcription of the sucrose phosphorylase gene. These metabolites undoubtedly function in this way because of their implications in cellular metabolism. There has been little research on methods of the

allosteric regulation In biochemistry, allosteric regulation (or allosteric control) is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme's active site. The site to which the effector binds is termed the ''allosteric site ...

of sucrose phosphorylase, so at this point the function of allosteric molecules can only be hypothesized. Due to the nature of its function in metabolic pathways, it is likely that sucrose phosphorylase is additionally regulated by other common metabolites. For example, the presence of ATP would probably inhibit sucrose phosphorylase since ATP is a product of the catabolic pathway. Conversely, ADP would likely stimulate sucrose phosphorylase to increase levels of ATP. Further research on the subject would be required to support or refute these ideas.

Function

As mentioned above, sucrose phosphorylase is a very important enzyme in metabolism. The reaction catalyzed by sucrose phosphorylase produces the valuable byproducts α-D-glucose-1-phosphate and fructose. α-D-glucose-1-phosphate can be reversibly converted by

to glucose-6-phosphate, which is an important intermediate used in glycolysis. In addition, fructose can be reversibly converted into

fructose 6-phosphate Fructose 6-phosphate (sometimes called the Neuberg ester) is a derivative of fructose, which has been phosphorylated at the 6-hydroxy group. It is one of several possible fructosephosphates. The β-D-form of this compound is very common in cells ...

, also found in the glycolytic pathway. In fact, fructose-6-phosphate and glucose-6-phosphate can be interconverted in the glycolytic pathway by phosphohexose isomerase. The final product of glycolysis,

pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic aci ...

, has multiple implications in metabolism. During

anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...

conditions, pyruvate con be converted into either lactate or

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...

, depending on the organism, providing a quick source of energy. In

aerobic Aerobic means "requiring air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Aerobic respiration, the aerobic process of cellu ...

conditions, pyruvate can be converted into

Acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for ...

, which has many possible fates including

catabolism Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipids, ...

in the

Citric Acid Cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins ...

for energy use and

anabolism Anabolism () is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy, known also as an endergonic process. Anabolism is the building-up aspect of metabolism, whereas catabolism is the breaking-do ...

in the formation of

fatty acids In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, f ...

for energy storage. Through these reactions, sucrose phosphorylase becomes important in the regulation of metabolic functions. The regulation of sucrose phosphorylase can also be used to explain its function in terms of energy consumption and preservation. The cAMP-CRP complex that enhances transcription of the sucrose phosphorylase gene (Reid and Abratt 2003) is only present when glucose levels are low. The purpose of sucrose phosphorylase, therefore, can be linked to the need for higher glucose levels, created through its reaction. The fact that glucose acts as a feedback inhibitor to prevent the formation of sucrose phosphorylase further supports its catalytic role in the creation of glucose for energy use or storage. The glucose-6-phosphate molecule created from the original α-D-glucose-1-phosphate product is also involved in the

. Through a series of reactions, glucose-6-phosphate can be converted into

ribose-5-phosphate Ribose 5-phosphate (R5P) is both a product and an intermediate of the pentose phosphate pathway. The last step of the oxidative reactions in the pentose phosphate pathway is the production of ribulose 5-phosphate. Depending on the body's state, r ...

, which is used for a variety of molecules such as

nucleotides Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules w ...

coenzymes A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that ass ...

, DNA, and

RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

. These connections reveal that sucrose phosphorylase is also important for the regulation of other cellular molecules.

References

External links

The Mechanism of Action of Sucrose Phosphorylase

BRENDA Sucrose Phosphorylase Reaction Mechanism
{{Portal bar, Biology, border=no Transferases EC 2.4.1 Peripheral membrane proteins