Nucleotides, Monosaccharides, and Polysaccharides

Posted on Mar 30, 2025 in Biology

Nucleotides

The nucleotides are formed by the union of one molecule of phosphoric acid and a nucleoside via the hydroxyl group of the fifth carbon (carbon 5′) of the pentose. It is, therefore, a phosphoric ester of nucleoside. The nucleotides are strongly acidic due to the phosphate group that is ionized. Nucleotides are named by adding the term 5′-monophosphate to the name of the nucleoside. Thus, the RNA nucleotides are adenosine-5′-monophosphate (AMP), guanosine-5′-monophosphate (GMP), cytidine-5′-monophosphate (CMP), and uridine-5′-monophosphate (UMP). The DNA nucleotides are deoxyadenosine-5′-monophosphate (dAMP), deoxyguanosine-5′-monophosphate (dGMP), deoxycytidine-5′-monophosphate (dCMP), and deoxythymidine-5′-monophosphate (dTMP). In practice, the initial of each nitrogenous base (A, G, C, T, and U) is often used to refer to each type of nucleotide.

Features of Nucleotides

  1. Energy: Energy is stored in their phosphodiester bonds. When the phosphodiester bond is hydrolyzed, chemical energy is obtained by the cell for use in anabolic processes.
  2. Coenzymes: Some nucleotides are coenzymes for enzymes called dehydrogenases. For example: A – H2 + FAD → FADH2 + A. Types of nucleotides assuming this function are: FAD/FADH2, NAD+/NADH + H+, FMN/FMNH2, NADP+/NADPH + H+
  3. Monomers: Nucleotides are monomers of nucleic acids. In nucleic acids, the nucleotides are joined by a characteristic 3′-5′ phosphodiester bond, in which the phosphoric acid binds the 3′ carbon of one nucleotide and the 5′ carbon of the next.
  4. Act as second messengers (protein hormones).

Monosaccharides

This is a polyhydroxyaldehyde or polyhydroxyketone with between 3 and 6 carbons. They are classified by the number of carbons:

Classification by Carbon Number

  1. Trioses (monosaccharides with 3 carbons): Examples include dihydroxyacetone (DHA) and glyceraldehyde (G). Both molecules are intermediate metabolites of important biological processes such as glycolysis or fermentation.
  2. Pentoses (monosaccharides with 5 carbons): Examples include ribulose (Ru), ribose (Ri), and 2′-deoxyribose (dRi). Ribulose is involved in important metabolic cycles such as the Calvin cycle (dark phase of photosynthesis). Ribose and 2′-deoxyribose are structural molecules that are part of nucleotides.
  3. Hexoses (monosaccharides with 6 carbons): Examples include ketohexoses (hexoses with a keto group, such as fructose) and aldohexoses (hexoses with an aldehyde group, such as glucose, galactose, and mannose). Aldohexoses function as energy reserves (mainly glucose). Glucose produces ATP through respiration and fermentation. They form disaccharides (maltose). They are energetic reserve polysaccharides (starch) and structural polysaccharides (chitin).

O-Glycosidic Bonds

There are two types: monocarbonitric (links involving a single anomeric carbon) or dicarbonitric (links involving two anomeric carbons). According to their spatial configuration, they can be alpha (links formed from two OH groups in cis position) and beta (links formed from two OH groups in trans position).

Structural Polysaccharides

Characteristics

  1. Glycogen: Polysaccharide of animal origin located in the muscles and the liver. It is constituted of amylopectin with alpha (1-4) and (1-6) links. It is branched. The ramifications appear every 8 or 10 glucose units. Its regulation involves insulin and glucagon.
  2. Cellulose: Plant polysaccharide located in the plant wall, forming part of the primary and secondary walls. Contained in the matrix, it represents 50% of the plant wall. It also appears in cotton (100%). Cotton forms cellobiose chains that join to form micelles and microfibrils, all united by hydrogen bonds, resulting in crystal structures. Cellulose is hydrolyzed by enzymes called cellulases (beta-1,4-glucosidases). These enzymes break the beta 1-4 link, forming cellobiose and glucose.
  3. Chitin: Polymer formed by long chains of N-acetylglucosamine (NAG) attached via beta (1-4) linkages. It is part of the exoskeleton of arthropods and in some cases, contains calcium for increased hardness.