Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

2. Structural Components of the Plant Cell
2.10 The Role of Enzymes in Cellular Activity

METABOLISM refers to the exchange of matter between an Organism and its environment, as well as the transport and Transformation of substances within the organism (internal and Intermediary Metabolism). The Structure of a living Cell is highly complex and unstable. Maintaining its stability requires a continuous expenditure of energy. In addition, Energy is required to perform most of the Functions of a living cell. The source of energy is The breakdown of The Cell's Organic compounds, known as dissimilation. Dissimilation predominantly involves breakdown reactions that are exergonic (energy-releasing). The organic compounds undergoing breakdown must either be continuously supplied from the outside (in animals) or synthesized from inorganic substances using external Energy Sources, such as light (in plants). Organic compounds are also essential for the reproduction of living matter.

Assimilation is the ability of living organisms to take up substances from the environment, modify them, and convert them into compounds specific to a given organism. Assimilation mainly involves biosynthetic (anabolic) reactions and is an endergonic process that proceeds with the consumption of energy. Assimilation creates the material foundation for dissimilation, while at the same time being unable to occur without it. Together, assimilation and dissimilation form the basis that maintains the continuous interaction between the organism and its environment. This metabolic exchange primarily involves Water, dissolved substances, and gases (oxygen, CO2, water vapor).

Metabolism is the fundamental function of living matter, as this process supplies the organism with the necessary substances and energy. Metabolism encompasses Physical and Chemical reactions integrated in space and time into a single, orderly whole. This orderliness can only be achieved through efficient regulatory mechanisms. A crucial role in this regulation is played by Enzymes—specialized cellular compounds and biocatalysts that determine The rate of individual metabolic Chemical Reactions.

Enzymes were discovered by C. S. Kirchhoff in 1914. Chemically, enzymes are Proteins. They are synthesized on Ribosomes and undergo gradual Denaturation during their functioning. Enzymes are highly active even at low concentrations and do not become part of the final products of the reactions they catalyze. Enzymes exhibit Specificity both toward substrates (substrate specificity) and toward specific types of chemical reactions (reaction specificity). As the German chemist E. Fischer put it in 1911, an enzyme fits a substrate like a lock and key. Enzymes differ significantly in their substrate specificity. There are enzymes with 'group specificity,' which can transform structurally similar substrates, albeit at different rates.

Enzyme names end with the suffix '-ase,' with a few trivial exceptions (Pepsin, Papain). The name typically reflects The Nature of the substrate or the function of the enzyme (amylase breaks down starch; aminotransferase transfers amino groups). The systematic names of enzymes, recommended for nomenclature, characterize both the substrate and the function.

Enzymes are classified into simple and conjugated (or complex) enzymes. Simple enzymes consist solely of protein molecules, whereas conjugated enzymes consist of a colloidal protein moiety (apoenzyme) and a non-protein component (prosthetic group).

Apoenzymes determine the specificity of a given enzyme. In conjugated enzymes, the protein component is also referred to as the feron, and the prosthetic group as the agon. The terms 'agon' and 'feron' were proposed by the German researcher R. Willstätter.

The prosthetic group is smaller in size than the apoenzyme. Prosthetic groups can be Metal Ions or low-molecular-weight organic substances. Organic active groups that easily dissociate from the protein part were termed Coenzymes by the French scientist G. Bertrand (NAD, FAD, CoA).

In 1961, the International Union of Biochemistry in New York adopted a new standard Classification of Enzymes, dividing them into six classes:

1. Oxidoreductases catalyze The transfer of a hydrogen atom and e- from one substrate to another, thereby oxidizing the first and reducing the second (dehydrogenases, oxidases, peroxidases, catalase, cytochrome system).

2. Transferases are enzymes that catalyze the transfer of entire groups, such as methyl-, glycosyl-, amino-, or phosphotransferases. Phosphotransferases, for which ATP serves as the phosphate group donor, are called Kinases.

3. Hydrolases catalyze Hydrolysis reactions involving water. These include esterases acting on esters, lipases, carbohydrases, and ATPases.

4. Lyases catalyze the non-hydrolytic Cleavage of specific groups from a substrate with The formation of a double bond, or The addition of groups across a double bond (decarboxylase, aldolase).

5. Isomerases catalyze isomerization reactions.

6. Ligases (synthetases) catalyze endergonic reactions joining two substrate molecules coupled energetically with the cleavage of ATP.

The kinetics of the catalytic action of an enzyme are determined by a specific part of its molecule known as the Active Site.

The Active Site of simple enzymes is formed by a specific spatial arrangement of several amino acid residues. In conjugated enzymes, the active sites also incorporate coenzymes, prosthetic groups, and metal atoms. An enzymatic reaction is characterized by a specific initial velocity, which, depending on the Substrate Concentration, is maintained for a varying duration and decreases as the substrate amount diminishes. Substrate concentration plays a key role in the formation of enzyme-substrate complexes.