Respiration Enzymes: Non-Oxidative and Oxidative| Plant Physiology

The following points highlight the two main types of respiration enzymes. The types are: 1. Non-Oxidative 2. Oxidative.

Type # 1. Non-Oxidative:

Transphosphorylases:

These enzymes catalyze transfer of phosphate group, from one kind of molecule to another, or form one carbon atom to other in the same molecule. The presence of magnesium ions is essential for the activity of some of these enzymes.

Enzymes of this group showing the reactions they catalyze are given as follow:

Desmolases:

Some enzymes catalyse reactions in which carbon chains are broken without hydrolysis, e.g., aldolase splits fructose 1,6-bisphosphate into 3-phosphoglyce-raldehyde and dihydroxyacetone phosphate.

Carboxylases:

They catalyze reactions where CO₂ is removed from a compound. The decarboxylation of pyruvic acid is catalyzed by pyruvic carboxylase and of oxaloacetic acid by oxaloacetic acid carboxylase.

Hydrases:

These enzymes catalyze addition or removal of water to the molecules, without causing their splitting. Enolase which catalyzes one of the reversible reactions of glycolysis removes H₂0 from 2-phospoglyceric acid to produce 2-phosphoenolpyruvic acid. The isoaconitase catalyzes the reversible transformation of aconitic to isocitric acid and fumarase catalyzes the reversible conversion of L-malic acid to fumaric acid.

Type # 2. Oxidative:

Dehydrogenases:

These enzymes cause intracellular oxidation and reduction by the transfer of hydrogen atoms and electrons from one kind of molecules to another kind. Some of them can transfer hydrogen directly to oxygen of air, thus resembling oxidases in their action. There are others which transfer hydrogen and electrons to compounds other than O₂ and can operate under aerobic or anaerobic conditions.

There are “aerobic” or “anaerobic dehydrogenases”. Most of the plant dehydrogenases are of latter type. Dehydrogenases act simultaneously on two substrates, one is oxidized (dehydrogenated) while the other is reduced (hydrogenated).

The former is called the hydrogen and electron donor and the latter the hydrogen and electron acceptor. These enzymes are named after the substrate which acts as the hydrogen donor. Most of the dehydrogenases have coenzymes which serve as hydrogen and electron acceptors.

Two such coenzymes are diphosphopyridine nucleotide (DPN) and triphosphopyridine nucleotide (TPN). The former is also called NAD, (nicotinamide adenine dinucleotide) and the latter as NADP (nicotinamide adenine dinucleotide phosphate). The reduced coenzymes then transfer the hydrogen to the flavin adenine dinucleotide (FAD) which is reduced to FADH₂.

The FAD is the co­enzyme of succinic dehydrogenase and acts as acceptor directly in the dehydrogenation of succinic acid. The FADH₂ is re- oxidized by the enzymes of the cytochrome group. In some reactions flavin mononucleotide (FMN) which consists of part of the FAD molecule, may serve in its place.

Cytochromes:

Cytochromes transfer electrons from the FADH₂ to oxygen of air. Oxygen reacts with the electrons and with hydrogen ions to form a molecule of water.

There are several cytochromes (a, b, c and a₃) and each one is protein molecule to which is bound a heme group similar to the one present in hemoglobin. In the centre of the heme group is an atom of iron, which is alternately oxidized and reduced and is converted from Fe++ to Fe+++ and back by giving off and taking up an electron.

Fe⁺⁺DFe⁺⁺⁺ + e⁻

The cytochromes b, c, a and a₃ get reduced and oxidized alternately in that order after receiving electron from the FADH₂. The reduced cyt.a₃ or the cytochrome oxidase passes on the electrons to the oxygen molecule. The oxygen, the electron and the hydrogen ions from the medium react to form water.

Oxidases:

They catalyze the transfer of electrons from substrate to molecular oxygen. Thus H₂O₂ is produced as an end product. They operate only under aerobic conditions. The three major oxidases known to occur in plants are cytochrome oxidase, phenolase, and ascorbic acid oxidase. These enzymes catalyze the final step in those reactions of the Krebs cycle in which oxygen is a reactant and H₂O is an end product.

Cytochrome oxidase (cyt. a₃) is present in the tissue of a number of plants and catalyzes the oxidation of cytochrome a which has previously been reduced, as described above.

Phenolase (polyphenol oxidase, catechol oxidase) is found in potato tubers and other plant parts. It catalyzes the oxidation of phenolic compounds such as guaicol, catechol, pyrogallol and tyrosine, to the corresponding hydroquinone, e.g.,

It is observed that cut potato tubers or cut banana fruits turn brown or blackish, and exposed juices of tissues also get darkened in air.

Ascorbic acid oxidase occurs in many plant tissues, and catalyzes the oxidation of ascorbic acid, or vitamin C to dehydro-ascorbic acid as follow:

Both phenolase and ascorbic acid oxidase are copper-containing enzymes. The metal group in these enzymes is both prosthetic and catalytically active group. The substrate is present in the living cells.

Peroxidases:

Enzymes of this group cause oxidation of phenolic compounds such as pyrogallol, catechol, in the presence of hydrogen peroxide or in the presence of organic peroxides formed in plant cells. In other words, hydrogen peroxide serves as the hydrogen acceptor instead of oxygen.

Peroxidase from horse-radish root is an iron protein compound with the iron in a hematin group and this acts as the prosthetic group.

Plant tissues also contain enzymes which can oxidise IAA and regulate the level of IAA endogenously. IAA-oxidase is one such enzyme which belongs to a category of peroxidase.

Catalase:

It brings about decomposition of H₂O₂ into water and molecular oxygen.

It also decomposes peroxide in the same way:

This enzyme is widely distributed in living organisms and its presence in the cells prevents the accumulation of H₂O₃ which may be produced as a by-product of metabolism.