Glycolysis is the process of catabolizing sugars (6-carbon compounds) to generate ATP and 3-carbon compounds (pyruvate) that can then be fed into the citric acid cycle (also known as the Krebs cycle, or tricarboxylic acid (TCA) cycle) and further metabolized to CO<sub>2</sub>, FADH<sub>2</sub> and NADH in the mitochondrion. The NADH and FADH<sub>2</sub> generated by the mitochondria is used to reduce O<sub>2</sub> to water, and create a proton (H<sup>+</sup>) gradient from which more ATP is generated from ADP + Pi via ATP synthase. I will talk more about the citric acid cycle and the mitochondrial electron transport respiratory chain and oxygen-dependent ATP generation in a subsequent post. **Here I would like to focus on glycolysis which is not oxygen-dependent, and thus can also operate under anaerobic conditions.** This is a cytosolic pathway in plants, as in animals.  

Glycolysis has been extensively investigated in mammalian cells over the last several decades and many textbooks have detailed chapters on this metabolic pathway and its intricate control (see for example: 'Biochemistry', by *C.K. Mathews and K.E. van Holde*. The Benjamin/Cummings Publishing Company, Inc. (1990); 'Biochemistry 2nd Edition', by *L. Stryer*. W.H. Freeman and Company. (1981); 'Biochemistry, 5th Edition', by *J.M. Berg, J.L. Tymoczko and L. Stryer*.  W.H. Freeman and Company. (2002)). The plant glycolytic pathway is fully described in [Jones, R.L., Buchanan, B.B., Guissem, W. Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists (2002)](https://www.bookdepository.com/Biochemistry-Molecular-Biology-Plants-Russell-L-Jones/9780943088396) and the following images and discussion summarize some of the key points about the plant glycolysis pathway -- its structure, function, and control -- derived from the latter text, but presented in my own words and illustrations. Please also consult [Glycolysis - Wikipedia](https://en.wikipedia.org/wiki/Glycolysis) for further details.

During glycolysis we see that some investment of ATP is needed to phosphorylate glucose and generate F1,6BP (2 ATP's per glucose are required). F1,6BP is then cleaved into 2 x triose-phosphates (glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP)). For each pair of triose-phosphates subsequently processed to pyruvate (PYR), 2 x NADH and 4 x ATP are formed. ATP is produced by both phosphoglycerate kinase (PGK) and pyruvate kinase (PK). After subtracting the initial ATP invested in F1,6BP formation, the net result is 2 x ATP and 2 x NADH produced for every glucose molecule processed via glycolysis! Under aerobic conditions, the 2 x NADH generated in glycolysis at the glyceraldehyde-3-phosphate dehydrogenase (G3PDH) step can be processed by the respiratory chain in the mitochondrion to generate 6 ATP. **Note that triose-phosphates (derived from the Calvin cycle or starch catabolism in the chloroplast/plastid) and exiting the chloroplast via the triose-phosphate/phosphate translocator in exchange for Pi, do not require any further investment of ATP when metabolized to pyruvate via glycolysis!** 

![Glycolysis1.jpg](https://steemitimages.com/DQmZJsHruKkzoTDREDDhKKQAAMBSEf96PatRsVbMgFrGVx3/Glycolysis1.jpg)
Control, Stoichiometry & Energetics of Glycolysis (D. Rhodes)
Abbreviations used: AcCoA = acetyl-coenzyme A, ADP = adenonsine diphosphate + Pi, ATP = adenonsine triphosphate,  Ala = alanine, AAT = alanine aminotransferase, ALD = aldolase, 1,3BPG = 1,3-bisphosphoglycerate, DAHP = dihydroxyacetone phosphate (G3P + DHAP = triose-phosphates), ENO = enolase, FAD = flavin adenine dinucleotide, F6P = fructose-6-phosphate, F1,6BP = fructose-1,6-bisphosphate,  G6P = glucose-6-phosphate,  G3P = glyceraldehyde-3-phosphate,  G3PDH = glyceraldehyde-3-phosphate dehydrogenase, GDP = guanosine diphosphate + Pi, GTP = guanosine triphosphate, HK = hexokinase, Pi = phosphate, PEP = phospho*enol*pyruvate, PFK = phosphofructokinase,  3PGA = 3-phosphoglycerate, 2PGA = 2-phosphoglycerate, PGK = phosphoglycerate kinase, PGI =  phosphoglucose isomerase, PGM = phosphoglycerate mutase, PYR = pyruvate, PDH = pyruvate dehydrogenase, NAD<sup>+</sup> = nicotinamide adenine dinucleotide, TPI = triose-phosphate isomerase.

Pyruvate (PYR) generated in glycolysis can then be converted to acetyl-CoA (AcCoA) and enter the citric acid cycle to generate CO<sub>2</sub>, NADH, FADH<sub>2</sub>, and ultimately ATP via the mitochondrial respiratory electron transport chain that is O<sub>2</sub>-dependent (to be discussed more thoroughly in a subsequent post). 

The principal control of plant glycolysis is at the phosphofructokinase (PFK) step (F6P --> F1,6BP). This step is activated by inorganic phosphate (Pi) (Green Arrow in the image above) and inhibited by phospho*enol*pyruvate (PEP) (Red Arrow in the image above) in plants. As noted in a previous article, the level of Pi is a measure of the energy status of the cytoplasm (low ATP = high Pi; high ATP = low Pi) ([Carbohydrate Metabolism in Plants - Overview](https://steemit.com/steemstem/@davidrhodes124/carbohydrate-metabolism-in-plants-overview)) **and so high Pi signals make more ATP!**

**Anaerobic Metabolism**

![Glycolysis2.jpg](https://steemitimages.com/DQmcrkUZM7LRZX1GL4BDXwHLTazBBPrj5zq7RxTME5SqKFC/Glycolysis2.jpg)
Anaerobic metabolism (D. Rhodes)

Under anaerobic conditions (i.e. lack of oxygen) the respiratory electron transport chain can no longer function. NADH and FADH<sub>2</sub> rapidly build up under these conditions, as ATP levels fall. This creates a severe shortage of NAD<sup>+</sup> and ATP both in the mitochondrion and the cytoplasm. The citric acid cycle cannot operate without a supply of NAD<sup>+</sup>, and so shuts down. Pyruvate catabolism to acetyl-CoA  via PDH is also NAD<sup>+</sup>   dependent and ceases under anaerobic conditions. Furthermore, the shortage of NAD<sup>+</sup> begins to restrict flux via glycolysis at the G3PDH step.

Anaerobic stress (hypoxia leading to anoxia) in plants is typically caused by flooding of roots, which inhibits oxygen diffusion into the root. In response to these conditions plants turn on the expression of genes encoding enzymes that catabolize pyruvate to lactate and ethanol (lactate dehydrogenase (LDH), pyruvate decarboxylase (PDC) and alcohol dehydrogenase (ADH)). These anaerobic stress-induced pathways consume NADH and generate NAD<sup>+</sup> that allows glycolysis to continue and maintain some ATP production at the PGK and PK steps of glycolysis. Lactate is toxic to plant cells. As it is very acidic it can cause cytoplasmic acidosis. Initial acidification of the cytoplasm by lactate production results in activation of PDC. The activated and anaerobically-induced PDC/ADH pathway generates ethanol (diverting flux away from lactate). Ethanol can be more readily excreted from anaerobic root cells. Plants that are deficient in ADH are more susceptible to flooding stress injury. They die more rapidly as a consequence of lactate fermentation and cytoplasmic acidosis because they are unable to produce ethanol.  

*When you exercise vigorously your muscles can become oxygen deficient, and the LDH pathway can be induced in your muscle tissues, leading to lactic acid accumulation, which can then cause leg cramps! However, muscle tissue does not express the PDC/ADH pathway so your muscles do not produce ethanol, and you do not become ethanol intoxicated!*

The PDC/ADH pathway is used by yeasts to ferment pyruvate to ethanol in the beer/ale brewing industry. Barley starch, catabolized to maltose and glucose during the malting process, provides the carbon skeletons for ethanol production  by yeast under anaerobic conditions (see my previous article on [What is the difference between ale and beer?](https://steemit.com/beer/@davidrhodes124/what-is-the-difference-between-ale-and-beer)).

During anaerobic stress in plants there is often a build up of pyruvate which is then transaminated to alanine (Ala)  via the reaction catalyzed by alanine aminotransferase (AAT). Thus, alanine accumulation often occurs in anaerobically stressed plant tissues. It is thought that alanine accumulation might serve to control the flow of PYR into lactate and ethanol. Acidification of the cytoplasm via lactate accumulation may also trigger gamma-aminobutyrate (GABA) accumulation via the catalytic action of glutamate decarboxylase that has an acid pH optimum and is calcium/calmodulin activated in plants (not shown in the image above). I will return to discuss GABA in my next post which will focus on the citric acid cycle in plants.

*Please feel free to comment or ask questions below. I will try to respond as soon as possible.*