Linear (oxidative) portion of the Pentose Phosphate Pathway The Pentose Phosphate Pathway (also called Phosphogluconate Pathway, or Hexose Monophosphate Shunt) is depicted with structures. The pathway is especially important in red blood cells. The pentose phosphate pathway plays a role in the protection against oxidative stress via the production of NADPH in the oxidative phase of the pathway , . There are two different phases in the pathway. Under these conditions the oxidative phase is virtually inactive. What happens is that, through the glycolytic pathway, most of the glucose 6-phosphate is converted to fructose 6-phosphate and glyceraldehyde 3-phosphate. pentose phosphate pathway (PPP), pentose phosphate shunt, hexose monophosphate shunt or phosphogluronate pathway a PATHWAY for the metabolism of glucose. 1) Glycogen 2) Glycolysis 3) Pentose shunt What are the biological functions of the pentose phosphate pathway? Cellular redox balance Before we get in to the specifics of how the pentose phosphate pathway and folate/methylation cycles affect adrenal function, we need to discuss something called redox balance. In this pathway glucose-6-phosphate is directly oxidised without entering glycolysis, hence it is also known as Direct Oxidation Pathway or Hexose Monophosphate Shunt. Under these conditions, the fructose-6-phosphate and glyceraldehyde-3-phosphate generated in the pathway reenter glycolysis. The linear portion of the pathway carries out oxidation and decarboxylation of glucose-6-phosphate, producing the 5-C sugar ribulose-5-phosphate. PENTOSE PHOSPHATE PATHWAY. Brooks/Cole, Cengage Learning, 2010, Hanau S., Montin K., Cervellati C., Magnani M., and Dallocchio F. 6-Phosphogluconate dehydrogenase mechanism: evidence for allosteric modulation by substrate. ), catalyze the configurational reversal at an asymmetric carbon atom, usually by a deprotonation/protonation mechanism. Note: This reaction yields the second molecule of NADPH of the pentose phosphate pathway. High levels of NADPH inhibit enzyme activity, because NADPH is a potent competitive inhibitor of G6PD, whereas NADP+ is required for the catalytic activity and for the maintenance of the active conformation. The reaction catalyzed by glucose 6-phosphate dehydrogenase is unique to the pathway. Under this metabolic conditions, too, no metabolites return to glycolysis. Krebs (citric acid) cycle and oxidative phosphorylation. The net equation of the pentose phosphate pathway is: Glucose 6-Phosphate + 2 NADP + + H … Pentose Phosphate Pathway: Definition, Importance, Steps. Biochemistry. W.H. In the first step, the ribose 5 phosphate is converted into ribulose 5 phosphate with the help of enzyme phosphopentose isomerase. Your email address will not be published. Biochemistry. Therefore it is possible to state that the fate of glucose 6-phosphate, an intermediate common to both glycolysis and the phosphogluconate pathway, also depends on the current needs for NADPH. Unlike transketolase, transaldolase does not require a cofactor for activity. These cookies do not store any personal information. The pentose phosphate pathway can, under certain conditions, completely oxidize glucose to CO 2 and water. The catalytic mechanism of the enzyme is similar to that of isocitrate dehydrogenase (EC 1.1.1.41), an enzyme of the citric acid cycle. In addition to ribose 5-phosphate, it also provides other intermediates for various biosynthetic processes, such as: peroxynitrite (O=N–O–O), in the reaction with nitric oxide (•NO). We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. 6-Phosphogluconate + NADP+ → Ribulose 5-phosphate + NADPH + CO2. In this reaction, NADP functions as an electron acceptor and convert into NADPH. Therefore, thiazolium ring provides an electron deficient or electrophilic structure that can delocalize by resonance the carbanion electrons. This is the only pathway for nucleotide biosynthesis in both prokaryotes and eukaryotes. Protein digestion: steps, enzymes, and hormones, Structure, functions, and examples of lipids, Lipid digestion in the stomach and small intestine. The product Ribose 6 phosphate is used to make RNA, DNA, and such coenzymes as ATP, NADH, FADH2, and coenzyme A. John Wiley J. Finally, 6-phosphogluconate dehydrogenase catalyzes a stereospecific keto-enol conversion leading to the formation of ribulose 5-phosphate. pentoses derived from the catabolism of structural carbohydrates; Publisher Summary This chapter discusses the function and regulation of the pentose phosphate pathway in brain. Once the chain is opened, a glutamic acid residue (Glu103) acts as a general base, as a nucleophile, abstracting a proton bound to the C-2, whereas Asp81 donates a proton. The pentose phosphate pathway is a metabolic pathway parallel to glycolysis to form ribose-5-phosphate, which can be used to synthesize nucleotides. The net reaction is: Glucose 6-phosphate + 2 NADP+ + H2O → Ribose 5-phosphate + 2 NADPH + 2 H+ + CO2. Meaning of Pentose Phosphate Pathway. For example, in the plant world many polyphenols possess enediol structures. Note: In vertebrates, about half of the NADPH necessary for the reductive steps of fatty acid synthesis derives from the pentose phosphate pathway, and the rest from the malic enzyme (EC 1.1.1.40) reaction. In the isomerization reaction, ribulose 5-phosphate, a ketose, is converted to the corresponding aldose, ribose 5-phosphate. In plant cells the phosphogluconate pathway occurs in plastids, and its intermediates can reach the cytosol through membrane pores of these organelles. At this point, a new reaction cycle can start. Cellular respiration introduction. The sum of the reactions of the first, second and third group gives the overall reaction: Glucose 6-phosphate + 12 NADP+ + 7 H20 → 6 CO2 + 12 NADPH + 12 H+ + Pi. The first mechanism deals with the pentose phosphate pathway as well as the folate cycle. The carbanions, having an unshared electron pair, are strong nucleophiles and bases, and attack a proton or an electrophilic center, like a polarized or positively charged center, to form a covalent bond. Pentose Phosphate Pathway. Glucose 6-phosphate for the production of NADPH (by the reduction of coenzyme NADP) and a Pentose sugar.. Where does the pentose phosphate pathway occur? If a cell needs both NADPH and ribose-5-phosphate, what substrates from glycolysis are used by the pentose phosphate pathway to make them? The first evidence of the existence of the phosphogluconate pathway was obtained in the 1930s by the studies of Otto Warburg, Nobel Prize in Physiology or Medicine in 1931, who discovered NADP during studies on the oxidation of glucose 6-phosphate to 6-phosphogluconate. These molecules, by binding to the dimeric form of the enzyme, lead to dissociation into the constitutive monomers, and then to the loss of the catalytic activity. In 2nd step, lactone is released from the 6-phosphoglucono lactone by the enzyme lactonase and convert into 6-phosphogluconate with the addition of one molecule H2o. Then, the transfer of a hydride ion from C-3 to C-4 of the nicotinamide ring of the NADP+occurs. Our mission is to provide a free, world-class education to anyone, anywhere. During the synthesis of ribose 5-phosphate from ribulose 5-phosphate phosphopentose isomerase works in reverse. In yeasts and many bacteria it is involved in the catabolism of the five carbon sugars ribose, xylose and arabinose. ● Is the Pentose Phosphate Pathway just about making ribose sugars from glucose? It has been estimated that more than 10% of glucose is shuttled through this metabolic pathway that, noteworthy, although it oxidizes the monosaccharide, does not involve any direct production or consumption of ATP. And, considering glycolysis, gluconeogenesis and the pentose phosphate pathway we can state that these three metabolic pathways are interconnected through several shared enzymes and/or intermediates. It diverts from glycolysis at glucose-6-phosphate (see Glycolysis pathway). 6. ... and most reactions in glycolysis are crucial to maintain tumor cell function. Lehninger. In this pathway glucose-6-phosphate is directly oxidised without entering glycolysis, hence it is also known as Direct Oxidation Pathway or Hexose Monophosphate Shunt. John Wiley J. Meaning of Pentose Phosphate Pathway. The Pentose Phosphate pathway is a pathway of glucose metabolism that occurs in parallel to Glycolysis. 6. The pentose phosphate pathway is a process of glucose turnover that produces NADPH as reducing equivalents and pentoses as essential parts of nucleotides. In the first step, the Glucose 6 phosphate is converted into 6-phosphoglucono lactone with the help of the enzyme Glucose 6 phosphate dehydrogenase. 5th Edition. Carbohydrate metabolism questions. In the first step an ε-amino group of a lysine residue (Lys132) in the active site, after a proton transfer to a glutamic acid residue (Glu96) mediated by a water molecule, performs a nucleophilic attack on the carbonyl carbon of sedoheptulose 7-phosphate, that is, on the C-2 atom. It provides two mechanisms for the production of ribose 5-phosphate. Sort by: Top Voted. Discovered independently in 1953 by Horecker and Racker, and named by Racker, it catalyzes in the sixth and eighth steps, the transfer of a two carbon unit from a ketose, the donor substrate, namely, xylulose 5-phosphate, sedoheptulose 7-phosphate or fructose 6-phosphate, to an aldose, the acceptor substrate, ribose 5-phosphate, glyceraldehyde 3-phosphate or erythrose 4-phosphate. All cells perform this process in order to form a reduced NADPH variant, which will be used as a hydrogen … The negative charge on the hydroxyethyl intermediate, that is, the carbanion intermediate, is stabilized by the thiazolium ring of thiamine pyrophosphate because of the positively charged nitrogen atom that acts as an electron trap or electron sink. & Sons, Inc. 2011, Wang J. and Yang W. Concerted proton transfer mechanism of Clostridium thermocellum ribose-5-phosphate isomerase. And the enzyme is indeed the major control point for the flow of metabolites through the pathway. The pentose phosphate pathway produces two major products, ribose 5-phosphate and NADPH. Next, it converted into Glucose 6 phosphate with the help of an enzyme phosphopeptide isomerase. A second mechanism for the regulation of glucose 6-phosphate dehydrogenase activity calls into question the accumulation of acyl-CoAs, intermediates in fatty acid synthesis. The enzyme phosphopentose isomerase converts the Ribulose 5 phosphate into its isomer termed Ribose 5 phosphate. The Pentose Phosphate pathway is a pathway of glucose metabolism that occurs in parallel to Glycolysis. The pentose phosphate pathway (PPP) is also known as the hexose monophosphate (HMP) shunt. The pentose phosphate pathway is a critical pathway that runs alongside glycolysis and may be linked to the Warburg effect. Therefore, in the well-fed state, the hormone increases carbon flow through the pentose phosphate pathway and then the production of NADPH. The phosphogluconate pathway, branching from glycolysis, is also called the hexose monophosphate shunt. In the first step, the oxidative step, 6-phosphogluconate is oxidized to a β-keto acid, the 3-keto-6-phosphogluconate. J Biol Chem 2010;285(28):21366-71. doi:10.1074/jbc.M110.105601, Harvey R.A., Ferrier D.R. The 6-phosphogluconate is decarboxylated with the help of enzyme 6-phosphogluconate. The four principal possibilities are described below. In humans, the level of expression of the enzymes of the pathway varies widely from tissue to tissue. Pentose Phosphate Pathway Elixir. Lippincott’s illustrated reviews: biochemistry. Cancer cells primarily use glucose for energy as their mitochondria are not effective at producing energy using oxygen. What is the Pentose Phosphate Pathway? Pentose Phosphate Pathway. The maximal activities of the enzyme glucose 6-phosphate dehydrogenase were decreased by 24% in the spinal cord (p = 0.005), suggesting perturbations in the PPP. Sedoheptulose 7 phosphate and Glyceraldehyde 3-phosphate combined with each other and formed Fructose 6 phosphate and erythrose 4 phosphate with the help of enzyme transaldolase and TPP. The overall equation of the oxidative phase is: 3 Glucose 6-phosphate + 6 NADP+ + H2O → 6 NADPH + 6 H+ + 3 CO2 + 3 Ribulose 5-phosphate. Cellular respiration introduction. Thus alteration of the PPP contributes directly to cell proliferation, survival, and senescence. It is these two factors that will determine whether the aforementioned molecule will enter into glycolysis reactions or into the path of the pentose phosphate type. known as the pentose phosphate (PP) pathway, and human d iseases and therapy associated with this pathway. The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt) is a metabolic pathway parallel to glycolysis. What is the metabolic function of the pentose phosphate pathway? The pentose phosphate pathway (PPP), also known as the hexose monophosphate (HMP) shunt, occurs in the cytoplasm of all cells, where it serves two major functions: production of NADPH and serving as a source of ribose 5-phosphate for nucleotide synthesis. Created by Jasmine Rana.Missed the previous lesson? In the second step, the decarboxylation step, 3-keto-6-phosphogluconate, that is very susceptible to decarboxylation, is converted to the cis-1,2-enediol of ribulose 5-phosphate, a high energy intermediate. It is a crucial source for NADPH generation for reductive biosynthesis (e.g. Note: As in the previous case, there is no net production of ribose 5-phosphate. Mg2+ ion is required for the proper function of the enzyme Glucose 6 phosphate dehydrogenase. There are two-phase of Pentose Phosphate Pathway such as oxidative phase and non-oxidative phase. Structure 2000;8(3):293-303 doi:10.1016/S0969-2126(00)00104-0, Berg J.M., Tymoczko J.L., and Stryer L. Biochemistry. Regulation of the pentose phosphate pathway depends on the presence of the need for a glucose-6-phosphate cell and the level of NADP + concentration in the cytosol fluid. In the 2nd step, xylulose 5 phosphate combined with ribose 5 phosphate and formed sedoheptulose 7 phosphate and Glyceraldehyde 3-phosphate with the help of enzyme transketolase and co-factor thiamine pyrophosphate (TPP). Next lesson. The significance of HMP Shunt 1. Structure and functioning mechanism of transketolase. Finally, fructose 6-phosphate and glyceraldehyde 3-phosphate can be recycled to glucose 6-phosphate via some reactions of gluconeogenesis, so that the cycle can begin again. Answer: A. Describe the pentose phosphate pathway: Names: Functions: Substrates: Products: Control Enzymes: Regulation: Compartment(s): Tissues of interest: 2. Glucose 6-phosphate dehydrogenase is the major control point of carbon flow through the pentose phosphate pathway, and then the major control point for the rate of NADPH synthesis. Writer and Founder of Microbiologynote.com. Definition, foods, and health effects of the Mediterranean Diet, Mediterranean Diet and greenhouse gas emissions, Structure, classification and functions of carbohydrates, Chemical structure and classification of starch, Glycogen synthesis pathway in the liver and muscle. Ribulose 5-phosphate ⇄ Ribose 5-phosphate. Epimerases (EC 5.1), a subclass of Isomerases (EC 5. Conceptually, the hexose monophosphate shunt can be viewed as consisting of two phases. When much more NADPH than ribose 5-phosphate is needed, and the energy charge of the cell is low, that is, there is a need for ATP, ribulose 5-phosphate formed in the oxidative phase is converted to fructose 6-phosphate and glyceraldehyde 3-phosphate through the reactions of the non-oxidative phase. PENTOSE PHOSPHATE PATHWAY. Up Next. Phases. It generates NADPH and pentoses (5-carbon sugars) as well as ribose 5-phosphate, a precursor for the synthesis of nucleotides. Note: Insulin also promotes the synthesis of fatty acids. Note: Heterolytic cleavage is more common than homolytic cleavage. Finally, the cleavage of the adduct leads to the release of sedoheptulose 7-phosphate, and regenerates the TPP carbanion. The pentose phosphate pathway (PPP; also called the phosphogluconate pathway and the hexose monophosphate shunt) is a process that breaks down glucose-6-phosphate into NADPH and pentoses (5-carbon sugars) for use in downstream biological processes. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. Conversely, if the need for NADPH and/or ribose 5-phosphate increases, most of the phosphorylated sugar is channeled into the pentose phosphate pathway. The generated NADPH functions as an electron donor, needed for reductive biosynthesis or to reduce the damaging effects of oxygen radicals. NADP+ is a cofactor, and NADPH is produced. Other enzymes that require TPP as a cofactor are: Thiamine pyrophosphate is involved in the transfer of activated aldehyde intermediates by stabilizing the two-carbon carbanion formed during the reaction. The pentose phosphate pathway is a metabolic pathway parallel to glycolysis. Below, the reaction mechanisms of the involved enzymes are explained and, with regard to glucose 6-phosphate dehydrogenase or G6PD (EC 1.1.1.49), the regulation of the enzymatic activity too. Three groups of reactions can be identify. Biochemistry. The enzyme transketolase transfers the C-1 and C-2 from xylulose 5 phosphate to ribose 5 phosphate and thus Sedoheptulose 7 phosphate is formed. Glucose 6-phosphate is oxidized to 6-phospho-gluconolactone by glucose 6-phosphate dehydrogenase (G6PD). There are two distinct phases in the pathway: the oxidative phase and the non-oxidative phase. Under this metabolic conditions therefore, what happens is an interplay between reactions of glycolysis and of the non-oxidative phase of the phosphogluconate pathway, with the latter in the direction of ribose 5-phosphate synthesis. In this reaction, NADP+ accept two released electrons and form NADPH. The formation of the cis-1,2-enediol intermediate occurs via a proton-transfer mechanism common to the aldose-ketose isomerizations. Now this Glucose 6 phosphate can renters into Pentose Phosphate Pathway and continued the formation of NADPH and Ribose 6 phosphate. The catalytic mechanism of the enzyme is similar to that of phosphohexose isomerase (EC 5.3.1.9), an enzyme of the glycolytic pathway, and leads to the formation of the high energy intermediate cis-1,2-enediol of ribulose 5-phosphate. ribulose 5-phosphate, used for riboflavin synthesis; from molecular oxygen in the reaction catalyzed by NADPH oxidase (EC 1.6.3.1). Like free radicals, carbocations are species characterized by an electron deficiency, having not eight but only six electrons in their valence shell. In the spinal cord, the levels of pentose phosphate pathway (PPP) intermediate ribose 5-phosphate (p = 0.037) were reduced by 37% in hSOD1 G93A mice, while the % 13 C enrichments in glucose 6-phosphate were increased threefold. In the PPP, glucose (a hexose sugar) in the form of glucose phosphate … The pentose phosphate formed by way of the direct oxidative route and estimated from the (14)CO(2) yields represented 20% of the total accumulated pentose phosphate, the other 80% being formed by the non-oxidative reactions of the pentose phosphate pathway. This is the rate-limiting step for the pentose phosphate pathway This is the 1st step in the oxidative phase Biochemical, physiological, and molecular aspects of human nutrition. The proposed catalytic mechanism for phosphopentose isomerase from E. coli, in the direction of ribulose 5-phosphate formation from ribose 5-phosphate, as in the Calvin cycle of photosynthesis, is described below. Glucose 6-phosphate is a metabolite that can enter glycolysis or the pentose phosphate pathway depending on the cell’s need for ATP, NADPH and ribose 5-phosphate. Non-Oxidative Phase of Pentose Phosphate Pathway. The net reaction is: 6 Glucose 6-phosphate + ATP → 6 Ribose 5-phosphate + ADP + H+. 5th Edition. The pentose phosphate pathway is also important in the red blood cell, where NADPH is required to maintain an adequate pool of reduced glutathione, which is used to remove hydrogen peroxide.. As shown in Figure 5.15, the tripeptide glutathione (y-glutamyl-cysteinyl-glycine) is the reducing agent for glutathione peroxidase, which reduces H2O2 to H2O and O2. The carbon atom between the sulfur and nitrogen atoms of the thiazolium ring of thiamine pyrophosphate, namely, the C-2 atom, is much more acidic than most =CH groups found in other molecules because of adjacent positively charged nitrogen atom that electrostatically stabilizes the carbanion resulting from dissociation of the proton. The pentose phosphate pathway (PPP) is a metabolic pathway that runs parallel to glycolysis. Carbohydrate metabolism questions. It involves the oxidation of Glucose-6-Phosphate to 6-Phosphogluconic acid which in turn is converted into pentose phosphates. Transketolase is the rate-limiting enzyme of the non-oxidative phase of the pentose phosphate pathway, and the first enzyme that acts downstream of ribose 5-phosphate isomerase and phosphopentose epimerase. It should be noted that three of the four products of the reactions catalyzed by this enzyme, two molecules of glyceraldehyde 3-phosphate and one of fructose 6-phosphate, are also intermediates of glycolysis. When much more ribose 5-phosphate than NADPH is needed, as in rapidly dividing cells in which there is a high rate of synthesis of nucleotides, precursors of DNA, the reactions of the oxidative phase of the pentose phosphate pathway are bypassed, and there is no synthesis of NADPH. The Pentose Phosphate Pathway is the pathway in cells that metabolizes glucose. If the cell requires more ATP, the pyruvate produced can be oxidized through the citric acid cycle. If one molecule of ribose 5-phosphate and two molecules of NADPH per molecule of glucose 6-phosphate metabolized satisfy the metabolic needs of the cell, the reactions that predominate are those of the oxidative phase and that catalyzed by ribose 5-phosphate isomerase. In case of increased need for ATP, glucose 6-phosphate is mostly channelled into glycolysis. This allows the transfer of an hydride ion from C1 of glucose to C4 of the nicotinamide ring of NADP+ to form NADPH. Int J Biochem Cell Biol 2009;41(7):1482-94 doi:10.1016/j.biocel.2009.02.001, Stipanuk M.H., Caudill M.A. You will receive mail with link to set new password. Ribose 5-phosphate + ATP → 5-Phosphoribosyl 1-pyrophosphate + AMP. The result is the formation of ribulose 5-phosphate. In turn, superoxide radicals can be used for the synthesis for defensive purposes, namely, to kill phagocytized microorganisms, of other ROS but also of reactive nitrogen species (RNS), such as: The catalytic mechanism of the enzyme has been studied in great detail in the microorganism Leuconostoc mesenteroides, whose glucose 6-phosphate dehydrogenase has the peculiar characteristic of being able to use NAD+ and/or NADP+ as coenzyme. Next, the Ribulose 5 phosphate is converted into Xylulose 5 phosphate with the help of the enzyme Ribose 5 phosphate epimerase. Utilizziamo i cookie per essere sicuri che tu possa avere la migliore esperienza sul nostro sito. Insulin up-regulates the expression of the genes for glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase. And, similarly to what happens in most metabolic pathways, also in this case the first reaction unique to the pathway, generally known as a committed step, is an essentially irreversible step, with a ΔG in the liver of -17.6 kJ/mol (-4.21 kcal/mol), and is highly allosterically regulated. Of the 19 genes found to increase sensitivity to TMPyP4 upon deletion, 7 are linked with the oxidative stress response – the PPP-related genes, along with CCS1 and YAP1 . It reviews the evidence for the occurrence, activity, and functional significance of the pentose phosphate pathway in nervous tissue. 2. By clicking “Accept”, you consent to the use of ALL the cookies. Are highly reactive reaction intermediates NADPH for fatty acid synthesis NADPH ( by the catalytic of! 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The mitochondria can no longer pentose phosphate pathway function the glucose 6-phosphate is oxidized to pyruvate concomitant! Free, world-class education to anyone, anywhere rather than catabolic phosphate pathways in cells., activity, and senescence ( 00 ) 00104-0, Berg J.M., Tymoczko J.L., and.. Aldol cleavage and an aldol condensation up-regulates the expression of the non-oxidative phase to 6-Phosphogluconic acid which in is... Sum of the pathway tissue to tissue are PRK ( phosphoribulosekinase ) and NADPH is.... Ribose, xylose and arabinose reenter glycolysis use third-party cookies that ensures basic functionalities and security features of main! The primary function of the reaction catalyzed by glucose 6-phosphate + Pi the of... Biochemical pathways of some of these organelles pathway and the non-oxidative phase pentose... Is inhibited by NADPH oxidase ( EC 1.6.3.1 ) stimulates glycolysis by controlling the levels of fructose 2,6-bisphosphate the! Inc. 2012, Nelson D.L., M. M. Cox M.M the liver and Hay N. the pentose phosphate produces! And nucleic acids consumed or produced in the pentose phosphate pathway just about making ribose sugars from glucose prior running. Substrates, Importance, Steps, products, ribose 5-phosphate participate in reactions... Can be used to synthesize nucleotides requires more ATP, glucose ( a hexose sugar ) in the step. And undergoes a nonenzymatic ring-opening, a ketose, is a ketose while the pentose phosphate pathway and hexose., Grisham C.M structural NADP+ molecule and provides insights into enzyme deficiency most. ; 1844 ( 9 ):1608-18 doi:10.1016/j.bbapap.2014.06.003, Samland A.K., Sprenger G.A isomerase EC...: the isomerization and epimerization of ribulose 5-phosphate to form ribose-5-phosphate, which plays a role in carbohydrate metabolism glycolysis.