Plant Physiology - Musiyenko M. M. 2001

Respiration
Main pathways of carbohydrate dissimilation. Anaerobic phase of respiration (glycolysis)

There are three distinct stages in the Respiration process. The first occurs under anaerobic conditions and is known as Glycolysis. It serves as the common initial pathway for both aerobic respiration and all Types of Fermentation.

During the Second Stage, the products of glycolysis enter the Tricarboxylic Acid Cycle, also known as the Krebs cycle. Notably, molecular oxygen is not involved in the first two stages; it is only required in the final, Third Stage. This is because a significant portion of the respiratory substrate's energy, following the initial two steps, remains stored in reduced carriers. These carriers become oxidized by transferring their electrons via the Mitochondrial Electron Transport chain to O₂.

Glycolysis is the primary pathway of Carbohydrate Catabolism, consisting of a sequence of reactions in which glucose is converted into pyruvic acid with the simultaneous production of ATP (Fig. 88).

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Fig. 88. The Glycolytic Pathway of carbohydrate breakdown

In glycolysis, hexoses are broken down and partially oxidized to form pyruvic acid. Although glycolysis is fundamentally an anaerobic process, this definition is not absolute, as it frequently occurs in the presence of oxygen as well. Since strict anaerobiosis is rare in higher plants, glycolysis in their Cells typically takes place in the presence of oxygen.

Glycolysis is a crucial metabolic pathway in respiration because it generates energy in the form of ATP in cells that lack Photosynthesis. Its role is especially vital in non-photosynthetic Organs and germinating seeds.

The substrates for glycolysis (starch and sucrose) are initially hydrolyzed: starch yields glucose, while sucrose yields fructose. Starch is stored in METABOLISM/14.html">Chloroplasts or seeds, whereas sucrose is transported via the phloem from photosynthetic organs or storage Tissues and is hydrolyzed by invertase.

Glycolysis takes place in the Cytosol. The name itself derives from the Greek words glykys (sweet) and lysis (dissolution). The complete glycolytic pathway of carbohydrate breakdown was elucidated by 1940, largely due to the work of Gustav Embden, Otto Meyerhof, and Jakub Parnas, which is why it is sometimes referred to as the EMP pathway. The intermediates of glycolysis consist of either six or three carbon atoms.

Compounds with a six-carbon Skeleton are derivatives of glucose and fructose, while those containing three carbon atoms are derivatives of dihydroxyacetone, glyceraldehyde, glycerate, and Pyruvate:

All intermediate products of glycolysis are phosphorylated, with the phosphate group attached via an ester bond:

It is important to emphasize that while glycolysis ultimately generates energy, its initial reactions actually consume ATP. ATP is used to phosphorylate hexoses with the help of hexokinase. Hexose activation occurs through the phosphorylation of the sixth carbon atom via interaction with ATP. This initial Energy Expenditure is subsequently offset in later steps. Thus, in The First stage, 2 ATP molecules are consumed to convert hexose into hexose monophosphate. Glucose enters The Cell via a specific transporter or through Active Transport systems.

The conversion of glucose into fructose-1,6-bisphosphate (FBP) occurs in three steps: phosphorylation, isomerization, and a second phosphorylation.

The strategy of this initial phase is to produce compounds that can be readily cleaved into phosphorylated three-carbon fragments, from which energy is extracted in subsequent steps.

During the isomerization of glucose-6-phosphate to fructose-6-phosphate, the six-membered pyranose ring of glucose is converted into a five-membered furanose ring, meaning that an aldose is transformed into a ketose:

Fructose-6-phosphate is phosphorylated by a second ATP molecule to yield fructose-1,6-bisphosphate, catalyzed by the enzyme Phosphofructokinase. Notably, The rate of glycolysis is limited precisely by The activity of this enzyme, which is regulated by the availability of ATP:

The second stage of Glycolysis consists of four steps and begins with the Cleavage of fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate:

Subsequent glycolytic reactions involve only three-carbon compounds, with glyceraldehyde-3-phosphate playing a leading role. As for dihydroxyacetone phosphate (DHAP), it is readily converted into glyceraldehyde-3-phosphate (GAP) by the enzyme Triosephosphate isomerase:

Thus, the consecutive action of aldolase and triosephosphate isomerase yields two molecules of glyceraldehyde 3-phosphate (GAP):

No energy is released at the stages of glycolysis considered so far, but the subsequent reactions involve the utilization of energy stored within the glyceraldehyde 3-phosphate (GAP) molecules.

Glyceraldehyde 3-phosphate is first converted into 1,3-bisphosphoglycerate (1,3-BPG) through the action of a complex enzyme, glyceraldehyde 3-phosphate dehydrogenase (an NAD-dependent SH-enzyme); each enzyme molecule consists of four identical subunits. Each subunit is a polypeptide chain composed of approximately 220 amino acid residues. The enzyme contains SH-groups and the NAD coenzyme.

The Essence of the process lies in The oxidation of the aldehyde group of phosphoglyceraldehyde to the carboxyl group of 1,3-bisphosphoglyceric acid. This oxidation is accompanied by energy release. Simultaneously, the NAD coenzyme is reduced:

This is the only oxidation-reduction reaction in glycolysis during which substrate oxidation takes place along with The transfer of electrons and protons to NAD⁺.

During the oxidation of GAP to PGA (phosphoglyceric acid) within the enzyme-substrate complex (the NAD-dependent SH-enzyme and phosphoglyceraldehyde) in the presence of inorganic phosphate, a high-energy bond is formed in the 1,3-bisphosphoglyceric acid molecule, characterized by a high Free energy of Hydrolysis. This occurs due to the Conversion of the aldehyde group at the first carbon atom of PGA into an acyl phosphate—a mixed anhydride of phosphoric and carboxylic acids:

The energy for its formation is released during the oxidation of the aldehyde group; the ΔG of hydrolysis of the acyl phosphate bond of 1,3-bisphosphoglyceric acid is 49.3 kJ×mol^-1. In the next reaction, this molecule of phosphoglyceric acid (PGA) is used to generate ATP:

The transfer of the phosphate residue from the acyl phosphate group to ADP is catalyzed by phosphoglycerate kinase. The ΔG for this reaction is (-49.3) + (+30.05) = -18.8 kJ×mol^-1.

Finally, the last stage of glycolysis takes place, which involves the reactions of pyruvic acid formation and the generation of one ATP molecule:

The conversion of 3-phosphoglyceric acid (3-PGA) into 2-phosphoglyceric acid (2-PGA) is catalyzed by phosphoglyceromutase. Subsequently, dehydration of 2-PGA yields high-energy phosphoenolpyruvate (the ΔG of phosphoenolpyruvate hydrolysis is -61.9 kJ×mol^-1). The reaction is catalyzed by the enzyme enolase in the presence of Mg²⁺ or Mn²⁺ ions:

Dehydration is accompanied by energy redistribution within the molecule, resulting in The formation of a high-energy (macroergic) bond in the phosphoenolpyruvic acid molecule.

Because in this case the high-energy covalent phosphate bond is formed directly on the oxidized substrate, this process is known as substrate-level phosphorylation.

Phosphoenolpyruvate (PEP) is used in the final reaction of glycolysis to produce ATP with the participation of pyruvate kinase:

The phosphate residue of the high-energy enol phosphate is transferred to ADP to form ATP and enolpyruvic acid, which spontaneously tautomerizes into pyruvic acid (ΔG = -61.9 + (+30.5) = -31.4 kJ×mol^-1):

Since The breakdown of a single glucose molecule at THE START OF glycolysis produces two molecules of phosphoglyceraldehyde, all the reactions we have examined occur twice. Therefore, the overall reaction of glycolysis can be written as follows: