An ester bond is defined as the bond between an alcohol group (-OH) and a carboxylic acid group (-COOH), formed by the elimination of a water molecule (H 2 O) (Futura-Sciences,, SF).
The structure of ethyl acetate is shown in Figure 1. The ester bond is that single bond that forms between the oxygen of the carboxylic acid and the carbon of the ethanol.
Figure 1: structure of ethyl acetate.
R-COOH + R'-OH → R-COO-R '+ H 2 O
In the figure, the blue part corresponds to the part of the compound that comes from ethanol and the yellow part corresponds to acetic acid. The ester bond is marked in the red circle.
Hydrolysis of the ester bond
To understand a little better the nature of the ester bonds, the reaction mechanism of the hydrolysis of these compounds is explained. The ester bond is relatively weak. In an acidic or basic medium, it hydrolyzes to form alcohol and carboxylic acid, respectively. The reaction mechanism of the hydrolysis of esters is well studied.
In basic medium, nucleophilic hydroxides first attack at the electrophilic C of the C = O ester, breaking the π bond and creating the tetrahedral intermediate.
Then the intermediate collapses, reforming the C = O, resulting in the loss of the leaving group, the alkoxide, RO-, which leads to the carboxylic acid.
Finally, an acid / base reaction is a very rapid equilibrium where the alkoxide, RO- functions as a base that deprotonates the carboxylic acid, RCO2H, (an acid treatment would allow the carboxylic acid to be obtained from the reaction).
Figure 2: hydrolysis of the ester bond in a basic medium.
The mechanism of the hydrolysis of the ester bond in an acid medium is a bit more complicated. An acid / base reaction occurs first, since you only have a weak nucleophile and a poor electrophile you need to activate the ester.
The protonation of the carbonyl ester makes it more electrophilic. In the second step, the oxygen in the water functions as the nucleophile by attacking the electrophilic C at the C = O, with the electrons moving toward the hydronium ion, creating the tetrahedral intermediate.
In the third step, an acid / base reaction occurs, deprotating the oxygen that came from the water molecule to neutralize the charge.
In the fourth step, another acid / base reaction occurs. You need to get the -OCH3 out, but you need to make it a good leaving group by protonation.
In the fifth step, they use electrons from an adjacent oxygen to help "push out" the leaving group, producing a neutral alcohol molecule.
In the last step an acid / base reaction occurs. Deprotonation of the hydronium ion reveals the C = O carbonyl in the carboxylic acid product and regenerates the acid catalyst (Dr. Ian Hunt, SF).
Ester types
Carbonic ester
Carbonic esters are the most common of this type of compound. The first carbonic ester was ethyl acetate or also called ethyl ethanoate. Formerly this compound was known as vinegar ether, whose name in German is Essig-Äther whose contraction was derived from the name of this type of compound.
Esters are found in nature and are widely used in industry. Many esters have characteristic fruit odors, and many are naturally present in the essential oils of plants. This has also led to its common use in artificial scents and fragrances when smells try to be imitated.
Several billion kilograms of polyesters are produced industrially annually, important products as they are; polyethylene terephthalate, acrylate esters and cellulose acetate.
The ester bond of carboxylic esters is responsible for the formation of triglycerides in living organisms.
Triglycerides are found in all cells, but mainly in adipose tissue, they are the main energy reserve that the body has. Triacylglycerides (TAGs) are glycerol molecules linked to three fatty acids through an ester bond. The fatty acids present in TAGs are predominantly saturated (Wilkosz, 2013).
Figure 3: triglyceride formed by glycerol and three fatty acids linked by an ester bond.
Triacylglycerides (triglycerides) are synthesized in practically all cells. The main tissues for the synthesis of TAG are the small intestine, liver, and adipocytes. Except for the gut and adipocytes, TAG synthesis begins with glycerol.
Glycerol is phosphorylated first with glycerol kinase and then activated fatty acids (fatty acyl-CoAs) serve as substrates for the addition of phosphatidic acid-generating fatty acids. The phosphate group is removed and the last fatty acid is added.
Figure 4: esterification of glycerol 3 phosphate to form phosphatidic acid.
In the small intestine, dietary TAGs are hydrolyzed to release fatty acids and monoacylglycerides (MAG) prior to uptake by enterocytes. Enterocyte MAGs serve as substrates for acylation in a two-step process that produces a TAG.
Within adipose tissue there is no expression of glycerol kinase so the building block for TAG in this tissue is the glycolytic intermediate, dihydroxyacetone phosphate, DHAP.
DHAP is reduced to glycerol-3-phosphate by cytosolic glycerol-3-phosphate dehydrogenase, and the remaining TAG synthesis reaction is the same as for all other tissues.
Phosphoric ester
Phosphoric esters are produced by the formation of an ester bond between an alcohol and phosphoric acid. Given the structure of the acid, these esters can be mono, di and trisubstituted.
Figure 5: structure of the phosphoric acid triester.
These types of ester bonds are found in compounds such as phospholipids, ATP, DNA, and RNA.
Phospholipids are synthesized by the formation of an ester bond between an alcohol and phosphatidic acid phosphate (1,2-diacylglycerol 3-phosphate). Most phospholipids have a saturated fatty acid on C-1 and an unsaturated fatty acid on C-2 of the glycerol backbone.
The most commonly added alcohols (serine, ethanolamine, and choline) also contain nitrogen that can be positively charged, while glycerol and inositol do not (King, 2017).
Figure 6: structure of a phospholipid. The ester bond is marked in the red circle.
Adenosine triphosphate (ATP) is a molecule that is used as the currency of energy in the cell. This molecule is composed of an adenine molecule linked to the ribose molecule with three phosphate groups (figure 8).
Figure 7: ATP molecule. The ester bond is marked in the red circle.
The three phosphate groups of the molecule are called gamma (γ), beta (β) and Alpha (α), the latter esterifying the C-5 hydroxyl group of ribose.
The bond between ribose and the α-phosphoryl group is a phosphoester bond because it includes a carbon atom and a phosphorous atom, while the β- and γ-phosphoryl groups in ATP are connected by phosphoanhydride bonds that do not involve carbon atoms..
All phosphoanhydro have considerable chemical potential energy, and ATP is no exception. This potential energy can be used directly in biochemical reactions (ATP, 2011).
A phosphodiester bond is a covalent bond in which a phosphate group is attached to adjacent carbons through ester bonds. The bond is the result of a condensation reaction between a hydroxyl group of two sugar groups and a phosphate group.
The diester bond between phosphoric acid and two sugar molecules in DNA and backbone RNA links two nucleotides together to form oligonucleotide polymers. The phosphodiester bond links a 3 'carbon to a 5' carbon in DNA and RNA.
(base1) - (ribose) -OH + HO-P (O) 2-O- (ribose) - (base 2)
(base1) - (ribose) - O - P (O) 2 - O- (ribose) - (base 2) + H 2 O
During the reaction of two of the hydroxyl groups in phosphoric acid with a hydroxyl group in two other molecules, two ester bonds are formed in a phosphodiester group. A condensation reaction in which one molecule of water is lost generates each ester bond.
During the polymerization of nucleotides to form nucleic acids, the hydroxyl group of the phosphate group attaches to the 3 ′ carbon of a sugar of one nucleotide to form an ester bond to the phosphate of another nucleotide.
The reaction forms a phosphodiester bond and removes a water molecule (phosphodiesther bond formation, SF).
Sulfuric ester
Sulfuric esters or thioesters are compounds with the functional group RS-CO-R '. They are the product of the esterification between a carboxylic acid and a thiol or with sulfuric acid (Block, 2016).
Figure 8: generic structure of a thioester. The ester bond is marked in the red circle.
In biochemistry, the best known thioesters are derivatives of coenzyme A, for example, acetyl-CoA.
Acetyl coenzyme A or acetyl-CoA (Figure 8) is a molecule that participates in many biochemical reactions. It is a central molecule in the metabolism of lipids, proteins and carbohydrates.
Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for energy production. It is also the precursor molecule for the synthesis of fatty acids and is a product of the degradation of some amino acids.
Figure 9: structure of acetyl CoA.
The CoA-activated fatty acids mentioned above are other examples of thioesters originating within the muscle cell. The oxidation of fatty acid-CoA thioesters actually occurs in discrete vesicular bodies called mitochondria (Thompson, 2015).
References
- ATP. (2011, August 10). Recovered from learnbiochemistry.wordpress: learnbiochemistry.wordpress.com.
- Block, E. (2016, April 22). Organosulfur compound. Retrieved from britannica: britannica.com.
- Ian Hunt. (SF). Hydrolysis of Esters. Recovered from chem.ucalgary.ca: chem.ucalgary.ca.
- Futura-Sciences,. (SF). Ester bond. Recovered from futura-sciences.us.
- King, MW (2017, March 16). Fatty acid, triglycerides and phospholipid synthesis and metabolism. Recovered from themedicalbiochemistrypage.org.
- phosphodiesther bond formation. (SF). Recovered from biosyn: biosyn.com.
- Thompson, TE (2015, August 19). Lipid. Recovered from britannica: britannica.com.
- Wilkosz, R. (2013, November 6). The Formation of Ester Bonds in the Synthesis of Lipids. Recovered from wisc-online.com.