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5.1.4.1. Nucleotide functions

Nucleic acid precurors

Nucleic acid precursors, since, as we have seen, 5'-monophosphate nucleotides, ribonucleotides and deoxyribonucleotides, are the monomers of RNA and DNA .

Non-nucleic nucleotides

Other nucleotides are not part of nucleic acids, such as:

Adenosine phosphatesribonucleotides of adenine linked by link phosphoester one, two or three molecules of phosphoric acid, respectively:

  • AMP: Adenosine monophosphate (Adenine-Ribose-P).
  • ADP: Adenosindiphosphate (Adenine-Ribose-PP).
  • ATP: Adenosine triphosphate (Adenine-Ribose-PPP).

This phosphoester bond that is established between nucleotides is a high-energy, hydrolyzable bond, so they can act as energy transporters. The ATP is considered the universal unit of energy .

ADP, adenosine diphosphate, if it has energy at its disposal, uses it to join a third phosphate group to the other two to obtain adenosine triphosphate (ATP).

ATP

By derivative work: Batterytime (talk)ATP_structure.svg: User:Mysid (ATP_structure.svg) [Public domain], via Wikimedia Commons

To form this bond a lot of energy is needed (8 kcal / mol), and when it breaks, it will release the same amount of energy.

Although energy is not stored as such, energy used to form the bond and released later when the bond is broken by hydrolysis in exothermic reactions, makes ADP and ATP molecules considered  energy carriers.

If there is a lot of energy available, many ATP molecules will be formed from ADP, so the ATP / ADP ratio will be high. If the amount of energy available to the cell is small, it will decrease ATP levels and increase those of ADP, so the ATP / ADP ratio will be lower.

The energy released in exergonic reactions is used to form ATP from ADP and phosphoric acid (phosphorylation), while the energy needed in endergonic reactions comes from that released when ATP is hydrolyzed to ADP and phosphoric acid (dephosphorylation).

Typically, the high-energy bonds that exist between phosphate groups are represented by the virgulilla symbol (~), rather than the usual notation (-) for bonds.

Other ATP- like ribonucleotides such as GTPUTP, and CTP play a more limited role as energy transferors.

Coenzymes

The coenzymes are cofactors nonprotein organic, thermosetting, which attached to a apoenzyme form the holoenzyme, and generally act as electron carriers. Many of the coenzymes are nucleotides.

Unlike enzymes, coenzymes are not specific in terms of the substrate on which they act, but rather each group of coenzymes intervenes in the same type of reaction, regardless of the substrate.

Some enzymes catalyze oxidation and reduction reactions in the cell, in which the transfer of electrons occurs. If one enzyme is oxidized, another will simultaneously be reduced, and vice versa. These coenzymes are responsible for the transfer of these electrons, so they have two forms, reduced (represented with more hydrogens) and another oxidized.

  • Pyridin-nucleotides: consisting of a nucleotide nicotinamide (niacin derivative or vitamin B3) and a ribonucleotide of adenine. Two oxidized forms are known:
    • NAD+nicotinamide-adenine-dinucleotide. Its reduced form is presented as NADH + H+.
    • NADP+nicotinamide-adenine-dinucleotide-phosphate (its formula is the same as that of NAD+, but it has a phosphate group on the 2' carbon of the adenine nucleotide) with two or three phosphoric acid molecules, respectively. Its reduced form is NADPH + H+.

The pyridine-nucleotides are coenzymes redox reactions acting as electron carriers, being the reduced forms NADH and NADPH. They are involved in various metabolic processes, such as cellular respiration.

NAD - NADH

 By NAD_oxidation_reduction.svg: Fvasconcellos 19:44, 9 December 2007 (UTC). w:Image:NAD oxidation reduction.png by Tim Vickers.derivative work: Gustavocarra (NAD_oxidation_reduction.svg) [Public domain], via Wikimedia Commons 

  • Flavin-nucleotides: they are derivatives of riboflavin (vitaminB2), of which two oxidized forms are known:
    • FMNflavin-mononucleotide, which is a phosphoric acid molecule bound to riboflavin.
    • FADflavin-adenine-dinucleotide, which is the former attached to an adenine nucleotide.

They also act as electron transporters in oxidation and reduction reactions, the reduced forms being FMNH2 and FADH2. To pass from one form to another, they capture or give up hydrogen by oxidizing or reducing the substrate.

They differ from pyridine nucleotides (nicotinamide coenzymes), in that flavin nucleotides are strongly bound to enzymes, which is why they are called prosthetic groups.

  • Coenzyme A: it is formed by pantothenic acid (vitamin of group B) joined to an ADP (adenosine-diphosphate). The active part of the molecule is a thiol group (radical –SH), that is why it is also represented as CoA-SH, and acts as a carrier for acetyl groups (two-carbon hydrocarbon chains) and acyl groups (“n” hydrocarbon chains carbons).

Chemical messengers

For example, cyclic adenosine monophosphate (cAMP), which originates when phosphate binds to the 3' and 5' carbons of ribose (two ester bonds) forming a cyclic phosphodiester bond.

The cAMPcyclic adenosine monophosphate, is formed in cells from ATP, through a reaction catalyzed by the enzyme adenylate cyclase located in the cell membrane.

The cAMP acts as a mediator in many hormonal processes, transmitting and amplifying signals that reach the cells of hormones.

Fundamental Ideas About Nucleotide Functions

The main functions of nucleotides are:

  • Structural function: they are the constituents of nucleic acids.
  • Storage function and energy ratio. When energy is obtained in a chemical reaction, an ADP molecule joins another phosphate group to form ATP through a high-energy bond. By breaking this link, energy will be obtained again.
  • Coenzyme function. They participate in cellular catabolism. Coenzymes take up H+ and electrons and remain as NADH, NADPH, and FADH2, respectively. In the reduced state, they can  easily give up H+ and electrons to other molecules.
  • Function of chemical messengers. The molecule of cyclic adenosine monophosphate (cAMP) is involved in many hormonal processes, transmitting and amplifying signals that reach the cells of hormones.