ALDOSAS: CHARACTERISTICS, TYPES, NUMBER OF CARBONS - BIOLOGY - 2025

The aldoses are monosaccharides having a terminal aldehyde group. They are simple sugars since they cannot be hydrolyzed to produce other simpler sugars. They have between three and seven carbon atoms. Like ketoses, aldoses are polyhydric sugars.

In nature, the most abundant aldoses are arabinose, galactose, glucose, mannose, ribose, and xylose. In photosynthetic organisms, the biosynthesis of these sugars takes place from fructose-6-phosphate, a sugar from the Calvin cycle. Heterotrophic organisms get glucose and galactose from their food.

Source: NEUROtiker

characteristics

In aldohexoses all carbons are chiral, with the exception of carbon 1, which is the carbonyl carbon of the aldehyde group (C-1), as well as carbon 6, which is a primary alcohol (C-6). All chiral carbons are secondary alcohols.

In all aldoses, the absolute configuration of the chiral center furthest from the carbonyl carbon of the aldehyde group can be that of D-glyceraldehyde or L-glyceraldehyde. This determines whether the aldose is a D or L enantiomer.

In general, aldoses with n-carbons that have 2 ^n-2 stereoisomers. In nature, aldosas with the D configuration are more abundant than aldosas with the L configuration.

The aldehyde function of the aldoses reacts with a secondary hydroxyl group in an intramolecular reaction to form a cyclic hemiacetal. The cyclization of the aldose converts the carbonyl carbon into a new chiral center, called anomeric carbon. The position of the -OH substituent on the anomeric carbon determines the D or L configuration.

The aldoses whose anomeric carbon atom has not formed glycosidic bonds are called reducing sugars. This is because aldoses can donate electrons, reducing to an oxidizing agent or electron acceptor. All aldoses are reducing sugars.

Types of aldosas and their functions

Glucose as the main source of energy for living beings

Glycolysis is the universal central pathway for glucose catabolism. Its function is to produce energy in the form of ATP. Pyruvate, formed in glycolysis, can follow the lactic fermentation pathway (in skeletal muscle) or the alcoholic fermentation pathway (in yeast).

Pyruvate can also be fully oxidized to carbon dioxide through a process known as respiration. It encompasses the pyruvate dehydrogenase complex, the Krebs cycle, and the electron transport chain. Compared to fermentation, respiration produces much more ATP per mole of glucose.

Glucose and galactose as components in disaccharides

Glucose is present in disaccharides such as cellobiose, isomalt, lactose, maltose, and sucrose.

The hydrolysis of lactose, a sugar present in milk, produces D-glucose and D-galactose. Both sugars are covalently linked by a bond of carbon 1 of galactose (β configuration, with carbon 4 of glucose). Lactose is a reducing sugar because the anomeric carbon of glucose is available, Gal (β 1 -> 4) Glc.

Sucrose is one of the products of photosynthesis and is the most abundant sugar in many plants. Hydrolysis produces D-glucose and D-fructose. Sucrose is not a reducing sugar.

Glucose as a component in polysaccharides

Glucose is present in polysaccharides that serve as energy reserve substances, such as starch and glycogen in plants and mammals, respectively. It is also present in carbohydrates that serve as structural support, such as cellulose and chitin from plants and invertebrates, respectively.

Starch is the reserve polysaccharide of plants. It is found as insoluble granules composed of two types of glucose polymers: amylose and amylopectin.

Amylose is an unbranched chain of linked D-glucose residues (α 1 -> 4). Amylopectin is a branched chain of glucose residues (α 1 -> 6).

Glycogen is the reserve polysaccharide of animals. Glycogen resembles amylopectin in that it has a chain of glucose residues (α 1 -> 4) attached, but with many more branches (α 1 -> 6).

Cellulose is part of the cell wall of plants, especially in the stems and wood-forming components of the plant body. Similar to amylose, cellulose is a chain of unbranched glucose residues. It has between 10,000 and 15,000 D-glucose units, linked by β 1 -> 4 bonds.

Chitin is made up of modified glucose units, such as N-acetyl-D-glucosamine. They are linked by β 1 -> 4 bonds.

Mannose as a component of glycoproteins

Glycoproteins possess one or more oligosaccharides. Glycoproteins are generally found on the surface of the plasma membrane. Oligosaccharides can be attached to proteins through serine and threonine residues (O-linked) or to asparagine or glutamine residues (N-linked).

For example, in plants, animals, and single-celled eukaryotes, the N-linked oligosaccharide precursor is added in the endoplasmic reticulum. It has the following sugars: three glucoses, nine mannoses and two N-acetylglucosamines, which is written Glc ₃ Man ₉ (GlcNac) ₂.

Ribose in metabolism

In vascular plants and animals, glucose can be oxidized via pentose phosphate to produce ribose 5-phosphate, a pentose that will form part of nucleic acids. Specifically, ribose becomes part of RNA, while deoxyribose becomes part of DNA.

Ribose is also part of other molecules, such as adenosine triphosphate (ATP), nicotinamide adenine dinucleotide (NADH), flavin adenine dinucleotide (FADH ₂), and phosphorylated adenine nicotinamide dinucleotide (NADPH).

ATP is a molecule whose function is to provide energy in different processes in cells. NADH and FADH ₂ participate in glucose catabolism, specifically in redox reactions. NADPH is one of the products of glucose oxidation in the pentose phosphate pathway. It is a source of reducing power in the biosynthesis pathways of cells.

Arabinose and xylose as structural components

The cell wall of plants is composed of cellulose and hemicellulose. The latter consists of heteropolysaccharides that have short branches composed of hexoses, D-glucose, D-galactose, and D-mannose, and pentoses such as D-xylose and D-arabinose.

In plants, the Calvin cycle is a source of phosphorylated sugars, such as D-fructose-6-phosphate, which can be transformed into D-glucose-6-phosphate. This metabolite is converted by several steps, enzymatically catalyzed, into UDP-xylose and UDP-arabinose, which serve for the biosynthesis of hemicellulose.

Carbon number of aldoses

Alldotriose

It is a three-carbon aldose, with a D- or L-glyceraldehyde configuration. There is only one sugar: glyceraldehyde.

Aldotetrosa

It is a four-carbon aldose, with a D- or L-glyceraldehyde configuration. Example: D-erythrose, D-treose.

Aldopentose

It is a five-carbon aldose, with a D- or L-glyceraldehyde configuration. Examples: D-ribose, D-arabinose, D-xylose.

Aldohexose

It is a six-carbon aldose, with a D- or L-glyceraldehyde configuration. Examples: D-glucose, D-mannose, D-galactose.

References

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