polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

galactose Galactose (, '' galacto-'' + '' -ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molec ...

pulmonate Pulmonata or pulmonates, is an informal group (previously an order, and before that a subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group includ ...

Caenogastropoda Caenogastropoda is a taxonomic clade, a large diverse group which are mostly sea snails and other marine gastropod mollusks, but also includes some freshwater snails and some land snails. The clade is the most diverse and ecologically successfu ...

perivitelline fluid The perivitelline fluid is an extracellular fluid found in the eggs of most gastropods and constitutes the main source of nutrition and defense for their embryos. It replaces the egg yolk of other animals, which in snail eggs is reduced to non-nutri ...

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one o ...

Helix pomatia ''Helix pomatia'', common names the Roman snail, Burgundy snail, or escargot, is a species of large, edible, air-breathing land snail, a pulmonate gastropod terrestrial mollusc in the family Helicidae.MolluscaBase eds. (2021). MolluscaBase. ...

Achatina fulica ''Lissachatina fulica'' is a species of large land snail that belongs in the subfamily Achatininae of the family Achatinidae. It is also known as the Giant African land snail.Aplexa nitens'' and '' Otala lactea'', '' Bulimnaea megasoma'', '' Ariolimax columbianis'', ''

Ariophanta ''Ariophanta'' is a genus of air-breathing land snails, terrestrial pulmonate gastropod mollusks in the subfamily Ariophantinae of the family Ariophantidae. MolluscaBase eds. (2021). MolluscaBase. Ariophanta Des Moulins, 1829. Accessed throug ...

'', ''

Biomphalaria glabrata ''Biomphalaria glabrata'' is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails. ''Biomphalaria glabrata'' is an intermediate snail host for the trematode ''Sch ...

'', and '' Strophochelius oblongus''. This polysaccharide was also identified in the Caenogastropoda '' Pila virens'' and ''

Viviparus ''Viviparus'', common name the river snails, is a genus of large, freshwater snails with an operculum, aquatic gastropod mollusks.Bouchet, P. (2014). Viviparus Montfort, 1810. Accessed through: World Register of Marine Species at http://www ...

'', ''

Pomacea canaliculata ''Pomacea canaliculata'', commonly known as the golden apple snail or the channeled apple snail, is a species of large freshwater snail with gills and an operculum, an aquatic gastropod mollusc in the family Ampullariidae, the apple snails ...

,'' and '' Pomacea maculata''. In adult

gastropods The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. Ther ...

, galactogen is confined to the albumen gland, showing a large variation in content during the year and reaching a higher peak in the reproductive season. During the reproductive season, this polysaccharide is rapidly restored in the albumen gland after being transferred to the eggs, decreasing its total amount only after repeated ovipositions. In ''

Pomacea canaliculata ''Pomacea canaliculata'', commonly known as the golden apple snail or the channeled apple snail, is a species of large freshwater snail with gills and an operculum, an aquatic gastropod mollusc in the family Ampullariidae, the apple snails ...

'' snails, galactogen would act, together with

perivitellins Perivitellins are egg proteins found in the perivitelline fluid of many gastropods. They are multifunctional complexes providing the developing embryo with nutrition, protection from the environment, and defense against predators. Despite the cen ...

, as a main limiting factor of reproduction. This polysaccharide has been identified in the Golgi zone of the secretory cells from the albumen gland in the form of discrete granules 200 Å in diameter. The appearance of galactogen granules within the secretory globules suggests that this is the site of biosynthesis of the polysaccharide. Apart from the albumen gland, galactogen is also found as a major component of the

perivitelline fluid The perivitelline fluid is an extracellular fluid found in the eggs of most gastropods and constitutes the main source of nutrition and defense for their embryos. It replaces the egg yolk of other animals, which in snail eggs is reduced to non-nutri ...

from the snail eggs, comprising the main energy source for the developing embryo.

Structure

Galactogen is a

polymer 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 a ...

of

galactose Galactose (, '' galacto-'' + '' -ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molec ...

with species-specific structural variations. In this

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

, the D-galactose are predominantly β (1→3) and β (1→6) linked; however some species also have β (1→2) and β (1→4). The galactogen of the aquatic

Basommatophora Basommatophora was a term that was previously used as a taxonomic informal group, a group of snails within the informal group Pulmonata, the air-breathing slugs and snails. According to the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005), w ...

(e.g. ''

Lymnaea ''Lymnaea'' is a genus of small to large-sized air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the subfamily Lymnaeinae ( of the family Lymnaeidae, the pond snails.Bouchet, P.; Rosenberg, G. (2013). Lymnaea Lamarck, 179 ...

'', ''

Biomphalaria ''Biomphalaria'' is a genus of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the family Planorbidae, the ram's horn snails and their allies.MolluscaBase (2018). Biomphalaria Preston, 1910. Accessed through: World Regi ...

'') is highly branched with only 5-8 % of the sugar residues in linear sections, and β(1→3) and β(1→6) bonds alternate more-or-Iess regularly. In the terrestrial

Stylommatophora Stylommatophora is an orderPhilippe Bouchet, Jean-Pierre Rocroi, Bernhard Hausdorf, Andrzej Kaim, Yasunori Kano, Alexander Nützel, Pavel Parkhaev, Michael Schrödl and Ellen E. Strong. 2017. Revised Classification, Nomenclator and Typification ...

(e.g. ''

Helix A helix () is a shape like a corkscrew or spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helices, ...

'', ''

Arianta ''Arianta'' is a medium-sized genus of European land snails, terrestrial pulmonate gastropod mollusks in the family Helicidae. Species of snails within this genus make and use calcareous love darts. Species Species within the genus ''Arianta ...

'', ''

Cepaea ''Cepaea'' is a genus of medium-sized air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Helicidae. The shells of species within this genus are often brightly colored and patterned with stripes. The two species ...

'', ''

Achatina ''Achatina'' is a genus of medium-sized to very large, air-breathing, tropical land snails, terrestrial pulmonate gastropod mollusks in the family Achatinidae.Bouchet, P. (2014). Achatina Lamarck, 1799. Accessed through: World Register of Marine ...

'') up to 20% of the sugar residues are linear β(1→3) bound. The galactogen of ''Ampullarius sp'' species has an unusually large proportion of linearly arranged sugars, with 5% β(1→3), 26% β(1→6), and 10% β(1→2). Other analyses in ''

Helix pomatia ''Helix pomatia'', common names the Roman snail, Burgundy snail, or escargot, is a species of large, edible, air-breathing land snail, a pulmonate gastropod terrestrial mollusc in the family Helicidae.MolluscaBase eds. (2021). MolluscaBase. ...

'' suggested a dichotomous structure, where each galactopyranose unit bears a branch or side chain. Molecular weight determinations in galactogen extracted from the eggs of ''

Helix pomatia ''Helix pomatia'', common names the Roman snail, Burgundy snail, or escargot, is a species of large, edible, air-breathing land snail, a pulmonate gastropod terrestrial mollusc in the family Helicidae.MolluscaBase eds. (2021). MolluscaBase. ...

'' and '' Limnaea stagnalis'' were estimated in 4x10⁶ and 2.2x10⁶, respectively. In these snails galactogen contains only D-galactose. Depending upon the origin of the galactogen, apart from D-galactose, L-galactose, L-fucose, D-glucose, L-glucose and phosphate residues may also be present; for instance, the galactogen from ''Ampullarius'' sp. contains 98% of D-galacotose and 2% of L- fucose, and the one isolated from '' Pomacea maculata'' eggs consist in 68% of D-galactose and 32% of D-glucose. Phosphate-substituted galactose residues are found in the galactogen of individual species from various snail genera such as ''

Biomphalaria ''Biomphalaria'' is a genus of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the family Planorbidae, the ram's horn snails and their allies.MolluscaBase (2018). Biomphalaria Preston, 1910. Accessed through: World Regi ...

'', ''

Helix A helix () is a shape like a corkscrew or spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helices, ...

'' and ''

Cepaea ''Cepaea'' is a genus of medium-sized air-breathing land snails, terrestrial pulmonate gastropod mollusks in the family Helicidae. The shells of species within this genus are often brightly colored and patterned with stripes. The two species ...

''. Therefore, current knowledge indicates it could be considered either a homopolysaccharide of or a heteropolysaccharide dominated by galactose.

Metabolism

Galactogen is synthesized by secretory cells in the albumen gland of adult female snails and later transferred to the egg. This process is under neurohormonal control, notably by the brain galactogenin. The biochemical pathways for

glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one o ...

and galactogen synthesis are closely related. Both use

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 ...

as a common precursor and its conversion to activated galactose is catalyzed by UDP-glucose 4-epimerase and galactose-1-P uridyl-transferase. This enables glucose to be the common precursor for both

glycogenesis Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to chains of glycogen for storage. This process is activated during rest periods following the Cori cycle, in the liver, and also activated by insulin in re ...

and galactogenesis. In fact, both

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

s are found in the same secretory cells of the albumen gland and are subject to independent seasonal variations.

Glycogen Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria. The polysaccharide structure represents the main storage form of glucose in the body. Glycogen functions as one o ...

accumulates in autumn as a general energy storage for hibernation, whereas galactogen is synthesized during spring in preparation of egg-laying. It is commonly accepted that galactogen production is restricted to embryo nutrition and therefore is mainly transferred to eggs. Little is known about the galactogen-synthesizing

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. A ...

s. A D-

galactosyltransferase Galactosyltransferase is a type of glycosyltransferase which catalyzes the transfer of galactose. An example is B-N-acetylglucosaminyl-glycopeptide b-1,4-galactosyltransferase. The biosynthesis of disaccharides, oligosaccharides and polysacchar ...

was described in the albumen gland of ''

Helix pomatia ''Helix pomatia'', common names the Roman snail, Burgundy snail, or escargot, is a species of large, edible, air-breathing land snail, a pulmonate gastropod terrestrial mollusc in the family Helicidae.MolluscaBase eds. (2021). MolluscaBase. ...

''. This enzyme catalyzes the transfer of D-galactose to a (1→6) linkage and is dependent upon the presence of acceptor galactogen. Similarly, a β-(1→3)-galactosyltransferase activity has been detected in albumen gland extracts from ''Limnaea stagnalis''. In embryos and fasting newly hatched snails, galactogen is most likely an important donor (via galactose) of metabolic intermediates. In feeding snails, the primary diet is glucose-containing

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...

and

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...

. These

polymer 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 a ...

s are digested and contribute glucose to the pathways of intermediary metabolism. Galactogen consumption begins at the gastrula stage and continues throughout development. Up to 46-78 % of egg galactogen disappears during embryo development. The remainder is used up within the first days after hatching. Only snail embryos and hatchlings are able to degrade galactogen, whereas other animals and even adult snails do not. β- galactosidase may be important in the release of galactose from galactogen; however, most of the catabolic pathway of this polysaccharide is still unknown.

Other functions

Besides being a source of energy, few other functions have been described for galactogen in the snail eggs, and all of them are related to embryo defense and protection. Given that carbohydrates retain water, the high amount of this polysaccharide would protect the eggs from desiccation from those snails that have aerial oviposition. Besides, the high viscosity that the polysaccharide may confer to the

perivitelline fluid The perivitelline fluid is an extracellular fluid found in the eggs of most gastropods and constitutes the main source of nutrition and defense for their embryos. It replaces the egg yolk of other animals, which in snail eggs is reduced to non-nutri ...

has been suggested as a potential antimicrobial defense. Since galactogen is a β-linked

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

, such as

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...

or

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymer, heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls.Scheller HV, Ulvskov H ...

s, specific biochemical adaptations are needed to exploit it as a nutrient, such as specific glycosidases. However, apart from snail embryos and hatchlings, no animal seems to be able to

catabolize 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, lipid ...

galactogen, including adult snails. This fact led to consider galactogen as part of an antipredation defense system exclusive of gastropods, deterring predators by lowering the nutritional value of eggs.

References

{{Reflist Polysaccharides Carbohydrates

Occurrence and distribution

Structure

Metabolism

Other functions

References