14.2.1: Metabolism - Sugars

 

Glycolysis

Carbohydrates, whether synthesized by photosynthetic organisms, stored in cells as glycogen, or ingested by heterotrophs, must be broken down to obtain energy for the cell’s activities as well as to synthesize other molecules required by the cell. Starch and glycogen, polymers of glucose, are the main energy storage forms of carbohydrates in plants and animals, respectively. To use these sources of energy, cells must first break down the polymers to yield glucose. The glucose is then taken up by cells through transporters in cell membranes. The metabolism of glucose, as well as other six carbon sugars (hexoses) begins with the catabolic pathway called glycolysis. In this pathway, sugars are oxidized and broken down into pyruvate molecules. The corresponding anabolic pathway by which glucose is synthesized is termed gluconeogenesis. Neither glycolysis nor gluconeogenesis is a major oxidative/reductive process, with one step in each one involving loss/gain of electrons, but the product of glycolysis, pyruvate, can be completely oxidized to carbon dioxide (TCA cycle). Indeed, without production of pyruvate from glucose in glycolysis, a major energy source for the cell would not be available.

Glucose is the most abundant hexose in nature and is traditionally used to illustrate the reactions of glycolysis, but fructose (in the form of fructose-6- phosphate) is also readily metabolized, while galactose can easily be converted into glucose for catabolism in the pathway as well. The end metabolic products of glycolysis are two molecules of ATP, two molecules of NADH and two molecules of pyruvate (Figure 1), which, in turn, can be oxidized further in the citric acid cycle.

Other pathways

Intermediates of glycolysis and gluconeogenesis that are common to other pathways include glucose-6-phosphate (PPP, glycogen metabolism), Fructose-6-phosphate (PPP), Glyceraldehyde-3- phosphate (PPP), dihydroxyacetone phosphate (PPP, glycerol metabolism), 3- phosphoglycerate (PPP), and pyruvate (amino acid metabolism). It is worth noting that glycerol from the breakdown of fat can readily be metabolized to dihydroxyacetone phosphate (DHAP) and thus enter the glycolysis pathway. It is the only part of a fat that is used in these pathways.

Reaction 1

Glucose gets a phosphate from ATP to make glucose-6-phosphate (G6P) in a reaction catalyzed by the enzyme hexokinase, a transferase enzyme.

Glucose + ATP ⇄ G6P + ADP

Hexokinase is one of three regulated enzymes in glycolysis and is inhibited by one of the products of its action - G6P. Hexokinase has flexibility in its substrate binding and is able to phosphorylate a variety of hexoses, including fructose, mannose, and galactose.

Why phosphorylate glucose?

Phosphorylation of glucose serves two important purposes. First, the addition of a phosphate group to glucose effectively traps it in the cell, as G6P cannot diffuse across the lipid bilayer. Second, the reaction decreases the concentration of free glucose, favoring additional import of the molecule. G6P is a substrate for the pentose phosphate pathway and can also be converted to glucose-1-phosphate (G1P) for use in glycogen synthesis and galactose metabolism.

It is worth noting that the liver has an enzyme like hexokinase called glucokinase, which Reaction #1 - Phosphorylation of glucose - catalyzed by hexokinase has a much higher Km (lower affinity) for glucose. This is important, because the liver is a site of glucose synthesis (gluconeogenesis) where cellular concentrations of glucose can be relatively high. With a lower affinity glucose phosphorylating enzyme, glucose is not converted to G6P unless glucose concentrations get high, so the liver is able to release the glucose it makes into the bloodstream for the rest of the body to use.

Reaction 2

Next, G6P is converted to fructose-6-phosphate (F6P), in a reaction catalyzed by the enzyme phosphoglucose isomerase.

G6P⇄F6P

The reaction has a low ΔG°’, so it is readily favorable in either direction.

Reaction 3

F6P+ATP⇄F1,6BP+ADP

The second input of energy occurs when F6P gets another phosphate from ATP in a reaction catalyzed by the enzyme phosphofructokinase (PFK) to make fructose-1,6- bisphosphate (F1,6BP). PFK is a very important enzyme regulating glycolysis, with several allosteric activators and inhibitors.

Like the hexokinase reaction the energy from ATP is needed to make the reaction energetically favorable. PFK is the most important regulatory enzyme in the pathway and this reaction is the rate limiting step. It is also one of three essentially irreversible reactions in glycolysis.

Reaction 4

F1,6BP⇄Ga-3P+DHAP

With the glycolysis pump thus primed, the pathway proceeds to split the F1,6BP into two 3-carbon intermediates. This reaction catalyzed by the lyase known as aldolase is energetically a “hump” to overcome in the glycolysis direction (∆G°’ = +24 kJ/mol) so to get over the energy hump, cells must increase the concentration the reactant (F1,6BP) and decrease the concentration of the products, which are glyceraldehyde- 3-phosphate (Ga-3P) and dihydroxyacetone phosphate (DHAP).

Reaction 5

DHAP⇄Ga-3P

In the next step, DHAP is converted to Ga-3P in a reaction catalyzed by the enzyme triosephosphate isomerase. At this point, the six carbon glucose molecule has been broken down to two units of three carbons each - Ga-3P. From this point forward each reaction of glycolysis contains two of each molecule. Reaction #5 is fairly readily reversible in cells.

Reaction 6

Ga-3P+NAD++Pi ⇄ 1,3BPG+NADH+H+

In this reaction, Ga-3P is oxidized in the only oxidation step of glycolysis catalyzed by the enzyme glyceraldehyde-3- phosphate dehydrogenase, an oxidoreductase. The aldehyde in this reaction is oxidized, then linked to a phosphate to make an ester - 1,3-bisphospho-glycerate (1,3BPG). Electrons from the oxidation are donated to NAD+, creating NADH.

NAD+ is a critical constituent in this reaction and can be regenerated anaerobically at the end of the pathway .

Note here that ATP energy was not required to put the phosphate onto the oxidized Ga-3P. The reason for this is because the energy provided by the oxidation reaction is sufficient for adding the phosphate.

Reaction 7

1,3-BPG + ADP ⇄ 3-PGA + ATP

The two phosphates in the tiny 1,3-BPG molecule repel each other and give the molecule high potential energy. This energy is utilized by the enzyme phosphoglycerate kinase (another transferase) to phosphorylate ADP and make ATP, as well as the product, 3-phosphoglycerate (3-PGA). This is an example of a substrate-level phosphorylation. Such mechanisms for making ATP require an intermediate with a high enough energy to phosphorylate ADP to make ATP.

Though there are a few substrate level phosphorylations in cells (including another one at the end of glycolysis), the vast major of ATP is made by oxidative phosphorylation in the mitochondria (in animals). Since there are two 1-3 BPGs produced for every glucose, the two ATPs produced in this reaction replenish the two ATPs used to start the cycle and the net ATP count at this point of the pathway is zero.

Reaction 8

3−PGA ⇄ 2−PGA

Conversion of the 3-PGA intermediate to 2-PGA (2- phosphoglycerate).

Reaction 9

2−PGA ⇄ PEP + H2O

2-PGA is converted by enolase (a lyase) to phosphoenolpyruvate (PEP) by removal of water, creating a very high energy intermediate. The reaction is readily reversible, but with PEP, the cell has one of its highest energy molecules and that is important for the next reaction.

Reaction 10

PEP + ADP ⇄ PYR + ATP

Conversion of PEP to pyruvate by pyruvate kinase is the second substrate level phosphorylation of glycolysis, creating ATP. This reaction is what some refer to as the “Big Bang” of glycolysis because there is almost enough energy in PEP to stimulate production of a second ATP (ΔG°’ = 31.6 kJ/ mol), but it is not used. Consequently, this energy is lost as heat. If you wonder why you get hot when you exercise, the heat produced in the breakdown of glucose is a prime contributor and the pyruvate kinase reaction is a major source.

Pyruvate kinase is the third and last enzyme of glycolysis that is regulated (Figure 3). The primary reason this is the case is to be able to prevent this reaction from occurring when cells are making PEP while going through gluconeogenesis (Figure 3).

Catabolism of other sugars

Though glycolysis is a pathway focused on the metabolism of glucose and fructose, the fact that other sugars can be readily metabolized into glucose means that glycolysis can be used for extracting energy from them as well. Galactose is a good example. It is commonly produced in the produced in the body as a result of hydrolysis of lactose, catalyzed by the enzyme known as lactase. Deficiency of lactase is the cause of lactose intolerance.

Galactose begins preparation for entry into glycolysis by being converted to galactose-1- phosphate.  Each turn of the cycle takes in one galactose-1-phosphate and releases one glucose-1-phosphate.

Deficiency of galactose conversion enzymes results in accumulation of galactose (from breakdown of lactose). Excess galactose is converted to galactitol, a sugar alcohol. Galactitol in the human eye lens causes it to absorb water and this may be a factor in formation of cataracts.

Free fructose can also enter glycolysis by two mechanisms. First, it can be phosphorylated to fructose-6-phosphate by hexokinase. Or, phosphorylation of fructose by fructokinase produces fructose-1-phosphate and cleavage of that by fructose-1- phosphate aldolase yields DHAP and glyceraldehyde.

Phosphorylation of glyceraldehyde by triose kinase yields Ga-3P. This alternative entry means for fructose may have important implications because DHAP and Ga-3P are introduced into the glycolysis pathway while bypassing PFK regulation. Some have proposed this may be important when considering metabolism of high fructose corn syrup, since it forces production of pyruvate, a precursor of acetyl-CoA, which is itself a precursor of fatty acids when ATP levels are high.

Glycerol metabolism

Glycerol is an important molecule for the synthesis of fats, glycerophospholipids, and other membrane lipids.

All of the intermediates of the citric acid cycle can be converted ultimately to oxaloacetate, which is a gluconeogenesis intermediate, as well.

It is worth noting that animals are unable to use fatty acids as materials for gluconeogenesis in net amounts, but they can, in fact, use glycerol in both glycolysis and gluconeogenesis. It is the only part of the fat molecule that can be so used.

Pyruvate metabolism

As noted, pyruvate produced in glycolysis can be oxidized to acetyl-CoA, which is itself oxidized in the citric acid cycle to carbon dioxide. That is not the only metabolic fate of pyruvate, though.

Pyruvate is a “starting” point for gluconeogenesis, being converted to oxaloacetate in the mitochondrion in the first step. Pyruvate in animals can also be reduced to lactate by adding electrons from NADH. This reaction produces NAD⁺ and is critical for generating the latter molecule to keep the glyceraldehyde-3-phosphate dehydrogenase reaction of glycolysis (reaction #6) going under conditions when there is no oxygen.

This is because oxygen is necessary for the electron transport system (ETS) to operate and it performs the important function of converting NADH back to NAD⁺. When the ETS is running, NADH donates electrons to Complex I and is oxidized to NAD⁺ in the process, generating the intermediate needed for oxidizing Ga-3P. In the absence of oxygen, however, NADH cannot be converted to NAD⁺ by the ETS, so an alternative means of making NAD⁺ is necessary for keeping glycolysis running under low oxygen conditions (fermentation).

Bacteria and yeast generate NAD⁺ under oxygen deprived conditions by doing fermentation in a different way. They use NADH-requiring reactions that regenerate NAD⁺ while producing ethanol from pyruvate instead of making lactate. Thus, fermentation of pyruvate is essential to keep glycolysis operating when oxygen is limiting. It is also for these reasons that brewing of beer (using yeast) involves depletion of oxygen and muscles low in oxygen produce lactic acid (animals).

Pyruvate is a precursor of alanine which can be easily synthesized by transfer of a nitrogen from an amine donor, such as glutamic acid. Pyruvate can also be converted into oxaloacetate by carboxylation in the process of gluconeogenesis.

The enzymes involved in pyruvate metabolism include pyruvate dehydrogenase (makes acetyl-CoA), lactate dehydrogenase (makes lactate), transaminases (makes alanine), pyruvate carboxylase (makes OAA). When oxygen is absent, pyruvate is converted to lactate (animals) or ethanol (bacteria and yeast). When oxygen is present, pyruvate is converted to acetyl-CoA.

Gluconeogenesis

The anabolic counterpart to glycolysis is gluconeogenesis, which occurs mostly in the cells of the liver and kidney and virtually no other cells in the body. In seven of the eleven reactions of gluconeogenesis (starting from pyruvate), the same enzymes are used as in glycolysis, but the reaction directions are reversed. Notably, the ∆G values of these reactions in the cell are typically near zero, meaning their direction can be readily controlled by changing substrate and product concentrations by small amounts.

The three regulated enzymes of glycolysis all catalyze reactions whose cellular ∆G values are not close to zero, making manipulation of reaction direction for their reactions non-trivial. Consequently, cells employ “work-around” reactions catalyzed by four different enzymes to favor gluconeogenesis, when appropriate.

Bypassing pyruvate kinase

Two of the enzymes (pyruvate carboxylase and PEP carboxykinase - PEPCK) catalyze reactions that bypass pyruvate kinase. F1,6BPase bypasses PFK and G6Pase bypasses hexokinase. Notably, pyruvate carboxylase and G6Pase are found in the mitochondria and endoplasmic reticulum, respectively, whereas the other two are found in the cytoplasm along with all of the enzymes of glycolysis.

Biotin An important coenzyme used by pyruvate carboxylase is biotin. Biotin is commonly used by carboxylases to carry CO₂ to incorporate into the substrate.

Also known as vitamin H, biotin is a water soluble B vitamin (B7) needed for many metabolic processes, including fatty acid synthesis, gluconeogenesis, and amino acid metabolism. Deficiency of the vitamin is rare, since it is readily produced by gut bacteria. 

Reciprocal regulation

All of the enzymes of glycolysis and nine of the eleven enzymes of gluconeogenesis are all in the cytoplasm, necessitating a coordinated means of controlling them. Cells generally need to minimize the extent to which paired anabolic and catabolic pathways are occurring simultaneously, lest they produce a futile cycle, resulting in wasted energy with no tangible product except heat. The mechanisms of controlling these pathways have opposite effects on catabolic and anabolic processes. This method of control is called reciprocal regulation.

Allosteric Regulation of Glycolysis & Gluconeogenesis

Reciprocal Regulation

AMP - Activates PFK, Inhibits F1,6BPase

Citrate - Activates PFK, Inhibits F1,6BPase

Glycolysis Only

ATP - Inhibits PFK and Pyruvate Kinase

Alanine - Inhibits Pyruvate Kinase

Reciprocal allosteric effects For example, in glycolysis, the enzyme known as phosphofructokinase (PFK) is allosterically activated by AMP . The corresponding enzyme from gluconeogenesis catalyzing a reversal of the glycolysis reaction is known as F1,6BPase. F1,6BPase is inhibited by AMP.

Reciprocal covalent effects

In glycogen metabolism, the enzymes phosphorylase kinase and glycogen phosphorylase catalyze reactions important for the breakdown of glycogen. The enzyme glycogen synthase catalyzes the synthesis of glycogen.  

Phosphorylation of glycogen phosphorylase has the effect of activation, whereas phosphorylation of glycogen synthase makes it less active. Conversely, dephosphorylation has the reverse effects on these enzymes - glycogen phosphorylase become less active and glycogen synthase becomes more active.

Simple and efficient

The advantage of reciprocal regulation schemes is that they are very efficient. It doesn’t take separate molecules or separate treatments to control two pathways simultaneously. Further, its simplicity ensures that when one pathway is turned on, the other is turned off.

This is especially important with catabolic/ anabolic regulation, because having both pathways going on simultaneously in a cell is not very productive, leading only to production of heat in a futile cycle. 

Specific gluconeogenesis controls

Besides reciprocal regulation, other mechanisms help control gluconeogenesis. First, PEP carboxykinase (PEPCK) is controlled largely at the level of synthesis. 

Pyruvate carboxylase is sequestered in the mitochondrion (one means of regulation) and is sensitive to acetyl-CoA, which is an allosteric activator. Acetyl-CoA concentrations increase as the citric acid cycle activity decreases. Glucose-6-phosphatase is present in low concentrations in many tissues, but is found most abundantly and importantly in the major gluconeogenic organs – the liver and kidney cortex.

Specific glycolysis controls

Control of glycolysis and gluconeogenesis is unusual for metabolic pathways, in that regulation occurs at multiple points. For glycolysis, this involves three enzymes:

1. Hexokinase (Glucose ⇄ G6P)

2. Phosphofructokinase-1 (F6P ⇄ F1,6BP)

3. Pyruvate kinase (PEP ⇄ Pyruvate).

Regulation of hexokinase is the simplest of these. The enzyme is unusual in being inhibited by its product, glucose-6-phosphate. This ensures when glycolysis is slowing down hexokinase is also slowing down to reduce feeding the pathway.

Pyruvate kinase

It might also seem odd that pyruvate kinase, the last enzyme in the pathway, is regulated, but the reason is simple. Pyruvate kinase catalyzes the most energetically rich reaction of glycolysis. The reaction is favored so strongly in the forward direction that cells must do a ‘two-step’ around it in the reverse direction when making glucose in the gluconeogenesis pathway. In other words, it takes two enzymes, two reactions, and two triphosphates (ATP and GTP) to go from one pyruvate back to one PEP in gluconeogenesis. When cells are needing to make glucose, they can’t be sidetracked by having the PEP they have made in gluconeogenesis be converted directly back to pyruvate by pyruvate kinase. Consequently, pyruvate kinase must be inhibited during gluconeogenesis or a futile cycle will occur and no glucose will be made.

Reactions pulled

As noted above, the aldolase reaction is energetically unfavorable (high positive ∆G°’), thus allowing F1,6BP to accumulate. When this happens, some of the excess F1,6BP binds to pyruvate kinase, which activates and jump starts the conversion of PEP to pyruvate. The resulting drop in PEP levels has the effect of “pulling” on the reactions preceding pyruvate kinase. As a consequence, the concentrations of Ga-3P and DHAP fall, helping to pull the aldolase reaction forward.

PFK regulation

PFK has a complex regulation scheme.  PFK is allosterically activated by AMP.

Citrate inhibits PFK.  PFK is also inhibited by ATP.  PFK’s inhibition by ATP is noteworthy and odd at first glance because ATP is also a substrate whose increasing concentration should favor the reaction instead of inhibit it. The root of this conundrum is that PFK has two ATP binding sites - one at an allosteric site that binds ATP relatively inefficiently and one that the active site that binds ATP with high affinity. Thus, only when ATP concentration is high is binding at the allosteric site favored and only then can ATP turn off the enzyme.

Cori cycle

With respect to energy, the liver and muscles act complementarily. The liver is the major organ in the body for the synthesis of glucose. Muscles are major users of glucose to make ATP.  Actively exercising muscles use oxygen faster than the blood can deliver it. As a consequence, the muscles go anaerobic and produce lactate. This lactate is of no use to muscle cells, so they dump it into the blood. Lactate travels in the blood to the liver, which takes it up and reoxidizes it back to pyruvate, catalyzed by the enzyme lactate dehydrogenase.

Pyruvate in the liver is then converted to glucose by gluconeogenesis. The glucose thus made by the liver is dumped into the bloodstream where it is taken up by muscles and used for energy, completing the important intercellular pathway known as the Cori cycle.

Polysaccharide metabolism

Sugars are metabolized rapidly in the body and that is one of the primary reasons they are used. Managing levels of glucose in the body is very important - too much leads to complications related to diabetes and too little gives rise to hypoglycemia (low blood sugar). Sugars in the body are maintained by three processes - 1) diet; 2) synthesis (gluconeogenesis); and 3) storage. The storage forms of sugars are, of course, the polysaccharides and their metabolism is our next topic of discussion.

Glycogen

Animals store glucose primarily in liver and muscle in the form of a compound related to amylopectin known as glycogen. The structural differences between glycogen and amylopectin are solely due to the frequency of the α-1,6 branches of glucoses. In glycogen they occur about every 10 residues instead of every 30-50, as in amylopectin.

Glycogen provides an additional source of glucose besides that produced via gluconeogenesis. Because glycogen contains so many glucoses, it acts like a battery backup for the body, providing a quick source of glucose when needed and providing a place to store excess glucose when glucose concentrations in the blood rise.

The branching of glycogen is an important feature of the molecule metabolically as well. Since glycogen is broken down from the "ends" of the molecule, more branches translate to more ends, and more glucose that can be released at once.

Just as in gluconeogenesis, the cell has a separate mechanism for glycogen synthesis that is distinct from glycogen breakdown. As noted previously, this allows the cell to separately control the reactions, avoiding futile cycles, and enabling a process to occur efficiently (synthesis of glycogen) that would not occur if it were simply the reversal of glycogen breakdown.

Glycogen breakdown

Breakdown of glycogen involves 1) release of glucose-1-phosphate (G1P), 2) rearranging the remaining glycogen (as necessary) to permit continued breakdown, and 3) conversion of G1P to G6P for further metabolism. G6P can be 1) used in glycolysis, 2) converted to glucose by gluconeogenesis, or 3) oxidized in the pentose phosphate pathway.

Glycogen phosphorylase (sometimes simply called phosphorylase) catalyzes breakdown of glycogen into glucose-1- Phosphate (G1P). The reaction that produces G1P from glycogen is a phosphorolysis, not a hydrolysis reaction. The distinction is that hydrolysis reactions use water to cleave bigger molecules into smaller ones, but phosphorolysis reactions use phosphate instead for the same purpose. Note that the phosphate is just that - it does NOT come from ATP. Since ATP is not used to put phosphate on G1P, the reaction saves the cell energy.

G1P can be converted to G6P by action of an enzyme called phosphoglucomutase. This reaction is readily reversible, allowing G6P and G1P to be interconverted as the concentration of one or the other increases. This is important, because phosphoglucomutase is needed to form G1P for glycogen synthesis.

Regulation of glycogen metabolism

Regulation of glycogen metabolism is complex, occurring both allosterically and via hormone-receptor controlled events that result in protein phosphorylation or dephosphorylation. In order to avoid a futile cycle of glycogen synthesis and breakdown simultaneously, cells have evolved an elaborate set of controls that ensure only one pathway is primarily active at a time.

Regulation of glycogen metabolism is managed by the enzymes glycogen phosphorylase and glycogen synthase. Glycogen phosphorylase is regulated by covalent modification (phosphorylation / dephosphorylation). Its regulation is consistent with the energy needs of the cell.

PKA and cAMP

PKA is activated by cAMP, which is, in turn, produced by adenylate cyclase after activation by a G-protein. Epinephrine exerts its greatest effects on muscle and glucagon works preferentially on the liver. Thus, epinephrine and glucagon can activate glycogen breakdown by stimulating synthesis of cAMP followed by the cascade of events.

Glycogen synthesis

The anabolic pathway opposing glycogen breakdown is that of glycogen synthesis.  Synthesis of glycogen starts with G1P, which is converted to an 'activated' intermediate, UDPglucose. This activated intermediate is what 'adds' the glucose to the growing glycogen chain in a reaction catalyzed by the enzyme known as glycogen synthase.

Steps

Let us first consider the steps in glycogen synthesis. 1) Glycogen synthesis from glucose involves phosphorylation to form G6P, and isomerization to form G1P (using phospho-glucomutase, common to glycogen breakdown). G1P is reacted with UTP to form UDP-glucose in a reaction catalyzed by UDP-glucose pyrophosphorylase. Glycogen synthase catalyzes synthesis of glycogen by joining carbon #1 of the UDP-derived glucose onto the carbon #4 of the non-reducing end of a glycogen chain, to form the familiar α(1,4) glycogen links. Another product of the reaction is UDP.

Effect of phosphorylation

In glycogen synthesis, protein kinase A phosphorylates the active form of glycogen synthase, and converts it into the usually inactive b form.

Glycogen synthase, glycogen phosphorylase can all be dephosphorylated by the same enzyme and it is activated when insulin binds to its receptor in the cell membrane.

Big picture

In the big picture, binding of epinephrine or glucagon to appropriate cell receptors stimulates a phosphorylation cascade which simultaneously activates breakdown of glycogen by glycogen phosphorylase and inhibits synthesis of glycogen by glycogen synthase. Epinephrine, is also known as adrenalin, and the properties that adrenalin gives arise from a large temporary increase of blood glucose, which powers muscles.

On the other hand, insulin stimulates dephosphorylation by activating phosphoprotein phosphatase. Dephosphorylation reduces action of glycogen phosphorylase (less glycogen breakdown) and activates glycogen synthase (starts glycogen synthesis). Our bodies make glycogen when blood glucose levels rise. Since high blood glucose levels are harmful, insulin stimulates cells to take up glucose. In the liver and in muscle cells, the uptaken glucose is made into glycogen.

Pentose phosphate pathway

The pentose phosphate pathway (PPP) is an oxidative pathway involving sugars that is sometimes described as a parallel pathway to glycolysis. It is, in fact, a pathway with multiple inputs and outputs. PPP is also a major source of NADPH for biosynthetic reactions and can provide ribose-5-phosphate for nucleotide synthesis.

Oxidation 

Beginning with G6P, PPP proceeds through its oxidative phase as follows:

The enzyme catalyzing the reaction is G6P dehydrogenase. It is the rate limiting step of the pathway and the enzyme is inhibited both by NADPH and acetyl-CoA. NADPH is important for anabolic pathways, such as fatty acid synthesis and also for protection against damage from reactive oxygen species.

In the reversible reactions of the pentose phosphate pathway, one can see how glycolysis intermediates can easily be rearranged and made into other sugars. Thus, Ga-3-P and F6P can be readily made into Ribose-5- phosphate for nucleotide synthesis.

Involvement of F6P in the pathway permits cells to continue making nucleotides (by making R-5-P).