Enzymes are catalysts for chemical reactions, providing them with activation energy without participating in the reactions themselves or being broken down. Enzymes consist of proteins known as epo-enzymes and a non-protein group (organic or inorganic) known as the coenzyme or cofactor (Figure 9). Enzyme proteins are often simple, but in some enzymes they are complex, with a molecular weight exceeding one million Daltons. Among the most prominent organic coenzymes are vitamin B in the enzyme nicotinamide adenine phosphate. Similarly, among the most well-known inorganic coenzymes are the zinc ion in cytochrome c and chloride in the enzyme amylase. Zinc is present in the enzyme alcohol dehydrogenase. Enzymes have specific active sites through which they bind to corresponding sites on the reactants. Enzymes differ in their specificity. Some are absolutely specific to a single reactant, such as urease, which breaks down urea into water and carbon dioxide. Others are absolutely specific, requiring an electron acceptor, such as glucose-6-phosphate dehydrogenase, which converts glucose-6-phosphate to phospholactic acid. The resulting electrons are transferred to the compound nictonimide adenine phosphate plus, converting it to nictonimide adenine hydrogen phosphate. Some enzymes are bidirectional, such as lactate dehydrogenase, which converts lactate to pyruvate or vice versa. Other enzymes catalyze chain reactions where the reactants share an identical group. For example, alpha-glucosides can break down several types of alpha-glucosides.
Enzymes are classified into groups based on the similarity of
their reactants, such as proteinases, esterases, carbohydrases, phosphatases,
and others. Initially, enzymes were classified into six main groups. However,
other groups were later added (Figure 10). These groups are:
Transferases: These include enzymes that facilitate the transfer of chemical
groups within the same molecule or between different molecules.
Hydrolase: This group comprises a large number of enzymes that catalyze the
breakdown of certain bonds by adding water. These include enzymes that add
water to ester, peptide, glycosidic, and other bonds.
Oxidoreductases: These transfer electrons from one target substance to another,
oxidizing the former and reducing the latter.
Lyases: This group of enzymes removes a chemical group from the substrate
without adding water. The removed atoms are replaced by a double bond, or they
assist in adding groups to double bonds.
Isomerases: This group of enzymes helps convert a substrate into another
isomer. Ligases: This group of enzymes catalyzes the formation of new bonds
between two different or similar compounds. These enzymes rely on the energy
stored in the adenosine triphosphate (ATP) molecule to catalyze the reaction.
This type of reaction is accompanied by the breaking of the pyrophosphate bond
in adenosine triphosphate
References
https://www.esalq.usp.br/lepse/imgs/conteudo_thumb/Structure-of-the-cell.pdf
https://training.seer.cancer.gov/anatomy/cells_tissues_membranes/cells/structure.html
https://www.pmf.unizg.hr/_download/repository/2_Chemical_Composition.pdf
https://www.sc.chula.ac.th/courseware/2303101j/VIII-Cell.pdf
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