The interest of this conference is agricultural, environmental, bioenergetics, and sanitary. In that context, domestic, agricultural and industrial environments produce organic waste, which needs to be collected, selected, stored and recycled properly in order to avoid environmental pollution and promote agriculture. The green Industry proposed involves the conversion of natural, non-toxic organic waste in order to efficiently produce organic fertilizers for agriculture. These types of fertilizers from biological origin are suitable because they are not toxic for human and the environment. Enzymatic reactions described in this presentation concern mainly the hydrolysis of proteins, sugars and lipids, the acidification of intermediate products from hydrolysis, the formation of acetate, and the production of methane. In other words, this review is timely as it discusses for the chemical behavior or the reactivity of different functional groups to better understand the enzymatic catalysis in the transformations of residual proteins, carbohydrates, and lipids to generate biomethane and fertilizers. In the same perspective, this review is to enrich the documentation related to organic reactions catalyzed by enzymes, which occur in the anaerobic degradation of residual organic substances, with emphasis on the structures of organic compounds and reaction mechanisms. This will allow understanding the displacement of the electrons of a reactive entity rich in electrons to another reactive entity that is poor in electrons to form new bonds in products.
The green industry is an industry based on green chemistry. The latter can be defined as the design, development and application of chemical processes and products to reduce or eliminate the use and formation of harmful substances for human health and the environment. Bio-catalytic processes fully participate in the concept of green chemistry, which was introduced in the 1990s [
When organic residues are not recycled or processed efficiently, they are broken down by microorganisms and this favors the multiplication of insects, which can prove to be true vectors of disease. Odors from organic decomposition sites also affect quality of life. To solve the problem of organic waste, biomethanation is an ecological and efficient solution for converting organic matter to produce methane as well as organic fertilizers under the action of microorganisms. The valorization of organic waste through biomethanation contributes to the improvement of hygiene, and reduces bad odors and the use of non-natural fertilizers and pesticides.
Biomethanation also helps to avoid incineration and the dumping of organic waste in landfills in order to reduce emissions of greenhouse gases. Indeed, landfills and the incineration of organic waste essentially produce greenhouse gases such as carbon dioxide and nitrogen dioxide [
Biomethanation of organic waste is a collection of organic reactions such as hydrolysis, acidification, synthesis of acetate (ethyl acetate or methyl acetate) and synthesis of methane catalyzed by enzymes. Various substrates (proteins, sugars and lipids) are metabolized or degraded in an anaerobic fashion by specific enzymes and intermediate products serve as starting materials for subsequent enzymatic conversions to produce methane and fertilizers.
The residual organic substances consist mainly of proteins, sugars and lipids. The enzymatic hydrolysis of these macromolecules is the first step in the production of methane and fertilizers. Enzymes are proteins or organic catalysts secreted within the cells of microorganisms. However, enzymes are also capable of acting outside the original cell. Normally, different reactions are catalyzed by different enzymes, despite the fact that a small number of enzymes are capable of catalyzing several substrates. In general, the amino acids in active centre of the enzyme act as acids, bases, nucleophiles or electrophiles and form hydrogen bonds with the substrates or with other amino acid residues in order to stabilize the transition state (
Amino acids are small molecules with an amine group, carboxylic acid group and a prosthetic group (R), which differentiate the amino acids from each other. The amino acids are constituents of proteins. They are generally levorotatory (L) and the amine functional group is on the left (
Nucleophilic N-terminal hydrolases constitute a family of enzymes specialized in the breakdown of amide bonds, for instance, serine protease characterized by the presence of serine, histidine and aspartic acid in the active centre [
hydroxyl group of the serine (Scheme 1, reaction 2). The role of aspartate is to remove the proton bound to the nitrogen atom of histidine, while histidine removes the acid proton from the hydroxyl group of serine (Scheme 1, reaction 1) [
Sugars or carbohydrates are organic compounds that act as an energy reservoir at the level of higher plants. Because humans consume plants, sugars turn out to be essential constituents of human food. In general, carbohydrates are one of the major chemical groups containing carbon, hydrogen and oxygen, such as starch, which is an energy reserve substance in the plant cell [
During the hydrolysis of starch, it was found that the alpha stereochemistry is maintained in the final product and the plausible catalytic mechanism involves the protonation of the glycosidic oxygen by the proton donor glutamate amino acid (Scheme 2, reaction 2) [
Cellulose is an organic polymer formed by several units of glucose molecules linked together by beta-(1-4)-glycosidic bonds (Scheme 3) [
Scheme 1. Serine protease mechanism.
Scheme 2. Catalytic mechanism of starch.
Scheme 3. Enzymatic hydrolysis of cellulose.
the cellulosic waste and the liquid glucose was cleanly obtained [
Cellulase is a group of different enzymes, in particular, the endoglucanase enzymes which hydrolyze beta-(1-4)-glycosidic internal bonds, exoglucanase enzymes that hydrolyze beta-(1-4)-glycosidic external bonds and the cellobiase that hydrolyzes the disaccharide cellobiose to produce two molecules of glucose. Experimental studies showed that two amino acids (glutamate and aspartate) constitute the root of the catalytic activity in the hydrolysis of cellulose. In this regard, the glutamate amino acid yields a proton to glycosidic oxygen and the aspartate amino acid acts as a base by accepting the proton of a water molecule, which acts as a nucleophile (Scheme 4) [
The best known disaccharide is sucrose obtained from sugarcane juice or sugar beet. During enzymatic hydrolysis, sucrose degrades to generate D-(+)-glucose and D-(-)-fructose (Scheme 5, reaction 1) [
Scheme 4. Cellulose enzymatic hydrolysis mechanism.
Scheme 5. (a) Enzymatic hydrolysis of sucrose; (b) Sucrose enzymatic hydrolysis mechanism.
reaction 3). The latter acts also as a nucleophile (Scheme 5, reaction 3) [
Lipids are triglycerides or esters of fatty acids and glycerol containing long (R) chains which are insoluble in water (Scheme 6). From the environmental point of view, lipids are important organic constituents in wastewater, which contribute enormously to environmental pollution. In this regard, if they are not removed, the lipids form a layer on the surface of the water and thus prevent the diffusion of oxygen from the air into the water. Consequently, the result is the death of aquatic living beings. From this perspective, it has been shown that the lipases are potential catalysts used to hydrolyze oils from industrial effluents,
Scheme 6. Triglyceride enzymatic hydrolysis mechanism.
slaughterhouses and frying oils [
The catalytic activity of the lipases is influenced by the water molecules present in the structure of these enzymes. From this point of view, it has been observed that when the lipases are dehydrated, they lose the ability to catalyze esterification reactions involving congested alcohols. This could be explained by the fact that the absence of water affects the mobility necessary to bind the substrates in the active centre [
The products from hydrolysis are used by microorganisms as substrates to produce acids, alcohols and carbon dioxide (Scheme 7). At this level of biomethanation, amino acids could be used as a source of energy for microorganisms. From this point of view, Rupasinghe and his colleagues reported an enzymatic method for producing glucose, ethanol and acetic acid from apple residues (Scheme 7) [
The enzymatic oxidation of ethanol to acetaldehyde is catalyzed by alcohol
Scheme 7. Synthesis of acetic acid.
dehydrogenase (Scheme 7, reaction 2). On the other hand, the oxidation of acetaldehyde to acetic acid is catalyzed by the aldehyde dehydrogenase (Scheme 7, reaction 3). These enzymes use nicotinamide adenine dinucleotide (NAD) as co-enzyme and glutamate plays the role of a base in the active centre of the enzyme (dehydrogenase). Regarding the dehydrogenation of ethanol, the mechanism involves the transfer of the hydride ion to NAD+ (oxidized form of NAD) and this results in the formation of NADH (reduced form of NAD) and the desired product (acetaldehyde) (Scheme 7, reaction 2), while dehydrogenation of acetaldehyde involves nucleophilic attack by water molecule upon the electrophilic carbon of acetaldehyde. This step is followed by the removal of the proton by glutamate and the transfer of the hydride ion at the level of the oxidized form of NAD (Scheme 7, reaction 3) [
It has been observed that the major part of the methane comes from the acetate (Scheme 8) [
Scheme 8. Synthesis of acetate.
molecule of water and forming an intermediate adduct, which in turn reacts with the alcohol (ethanol or methanol) to produce the corresponding acetate (Scheme 8, reaction 3) [
Bacteria degrade ethyl acetate to produce ethanol and acetic acid. The latter is then metabolized to produce methane and carbon dioxide with the assistance of decarboxylase as catalyst. From the mechanism point of view, it has been reported that decarboxylation involves the formation of negatively charged intermediate compounds. In this perspective, the electrophilic substitution of carbon dioxide by a proton involves the loss of carbon dioxide to form a carbanion (methyl anion), which accepts a proton from the catalytic amino acid residue, in this case lysine, to generate the desired product (methane) (Scheme 9) [
This conference paper has showed the importance of recycling residual organic substances using biomethanation as an ecological strategy that minimizes environmental degradation. In fact, energy from biogas is very important especially in regions lacking energy infrastructure, for instance in Democratic Republic of the Congo, because it reduces the use of fossil fuels, limits deforestation and improves people’s livelihoods. In the same regard, natural fertilizers produced from domestic and non hazardous industrial waste are benefic to promote agriculture in order to minimize the use of synthetic fertilizers, which could harm the
Scheme 9. Synthesis of methane
environment. The reaction mechanisms have been also proposed to better understand the formation of products, which occur during different steps of biomethanation.
I declare that I do not have conflict of interest regarding the publication of this conference paper.
Mwene-Mbeja, T.M. and Vaneeckhaute, C. (2019) Conference Paper: Green Industry Adapted to Recycling Needs of Lubumbashi City and Surrounding Areas in Democratic Republic of the Congo. Green and Sustainable Chemistry, 9, 11-25. https://doi.org/10.4236/gsc.2019.91002