1. The pentose pathway (hexose monophosphate shunt)
2. Fructose metabolism
3. Galactose metabolism
4. Glucose conversion to glucuronic acid and its use
The pentose pathway (the hexose monophosphate shunt)
The pentose pathway is capable of direct glucose oxidation to CO2 without involving the TCA cycle and the ETC. Oxidation requires dehydrogenases. Dehydrogenases require co-factors. In the pentose pathway as co-factors act NADP+. NADP+ accepts reducing equivalents and NADPH is thus produced. NADPH has many functions. NADPH for example (1) is source of reducing equivalents in some biosynthetic pathways (fatty acids synthesis, steroids synthesis, etc…), (2) acts in antioxidant defence of the cells (e.g. glutathione (GSH) synthesis), or (3) acts in biotransformation reactions.
The pentose pathway also produces (in further steps) ribose-5-P (ribose-5-P is precursor in nucleic acids synthesis) or other monosaccharides.
There is nowhere mentioned ATP generation, in deed the pentose pathway is not intended for energy production because NADPH cannot be oxidized in the ETC.
Now it is possible to summarize main functions of the pentose pathway:
1) NADPH production – The pentose pathway is the most important source of NADPH in cells.
2) Ribose-5-P (Rib-5-P) production
3) Reciprocal conversion of monosaccharides – it is important e.g. in glycoproteins synthesis
The pentose pathway is localized in cytosol. Predominantly in hepatocytes, adipocytes, testicle cells, adrenal cells, erythrocytes, lactating mammary gland. Enzymes for this metabolic pathway however are in every tissue. The pentose pathway could be divided into two phases: (1) oxidative phase, (2) non-oxidative phase.
In oxidative phase of the pentose pathway is glucose-6-P oxidized to ribulose-5-P, CO2 is released, 2 NADPH are produced. Overall equation:
Glucose-6-P + 2 NADP+ → CO2 + 2 NADPH + ribulose-5-P
It is important to stress the very first step catalysed by the enzyme glucose-6-phosphate dehydrogenase. This reaction is irreversible and it is the most important regulatory step. Velocity of whole pathway depends on velocities of two dehydrogenase reactions. Their velocities depend on availability of NADP+ (i.e. oxidized form of the coenzyme). Velocity of the pentose pathway decreases when the availability of NADP+ decreases. In other words excess of NADPH (i.e. reduced form) slows the pentose pathway.
In the non-oxidative phase reciprocal conversions of the phosphorylated monosaccharides take place. These reactions are freely reversible. Basic scheme of the non-oxidative phase of the pentose pathway is as follows:
3 C5 → 2 C6 + C3
3 ribulose-5-P → 2 fructose-6-P + glyceraldehyde-3-P
1) Ribulose-5-P is converted to (1) ribose-5-P (ketose is changed by isomerase to aldose) or (2) xylulose-5-P (catalysed by epimerase)
2) Next steps could be expressed like this:
C5 + C5 ↔ C3 + C7 ↔ C6 + C4
Xylulose-5-P + ribose-5-P ↔ glyceraldehyde-3-P + sedoheptulose-7-P ↔ Fru-6-P + erythrose-4-P
These reactions are catalysed by two transferases – transketolase and transaldolase.
Transketolase transfers two-carbon units from xylulose-5-P (ketose) to ribose-5-P. This yields glyceraldehyde-3-P (xylulose-5-P minus two-carbon) and sedoheptulose-7-P (ribose-5-P plus two-carbon). This reaction requires B1 vitamin – thiamine diphosphate.
Transaldolase transfers three-carbon units from sedoheptulose-7-P (ketose) to aldehyde group of glyceraldehyde-3-P.
In general is valid this statement: two- and three-carbon units are generated from ketoses and are accepted by aldoses. Thus ketose is converted to shorter aldose and aldose is converted to longer ketose.
3) Fru-6-P can be used e.g. in glycolysis, but erythrose-4-P cannot. It should not cumulate, hence next reaction occur:
C4 + C5 ↔ C3 + C6
Erythrose-4-P + xylulose-5-P ↔ glyceraldehyde-3-P + fructose-6-P
Fructose-6-P and glyceraldehyde-3-P are final products of the non-oxidative phase of the pentose pathway. They can be used in glycolysis or gluconeogenesis (take place in cytoplasm as well) and oxidized or converted to glucose-6-P. Glucose-6-P can again enter the pentose pathway. This is why the pentose pathway has alternative name: the pentose cycle.
Glycolysis and gluconeogenesis are interwoven with the pentose pathway. Some say that the pentose pathway is diversion of gluconeogenesis/glycolysis. This is the reason why there is another alternative name: the hexose monophosphate shunt.
If we would like to describe the pentose pathway as the alternative glucose oxidation pathway, overall equation follows:
6 glucose-6-P → 6 CO2 + 6 ribulose-5-P + 12 NADPH+H+
6 ribulose-5-P →→→ non-oxidative phase and gluconeogenesis→→→5 glukóza-6-P
This takes place when cell needs maximize NADPH proceeds.
The pentose pathway could serve as the source of ribose-5-P or other monosaccharides. In situations when cell needs monosaccharides and does not require NADPH inverse course of the non-oxidative phase can occur. Glyceraldehyde-3-P and fructose-6-P are drained from glycolysis and converted to ribose-5-P and other monosaccharides.
The pentose pathway regulation
As mentioned above the pentose pathway is regulated by availability of the coenzyme NADP+. When NADPH (reduced form) is not used in other metabolic pathways it accumulates and concentration of the oxidized form (NADP+) decreases. Hence the velocity of the pentose pathway reactions catalysed by the enzymes requiring NADP+ decreases. These enzymes are glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase. Synthesis of the key enzymes is induced by insulin and prolactin (only in women and during lactation).
Glucose-6-phosphate dehydrogenase deficiency is considered as the most widespread enzyme defect. There is probably 400 million persons affected by this defect (predominantly in the Africa, the Mediterranean, the Middle East, and the Asia). One of the consequences is haemolytic anaemia (because of the disturbing of the antioxidant systems in the erythrocytes.) More in Subchapter 5/4.
Fructose could be taken in either free (fruit, honey), or in disaccharide sucrose. Sucrose is split in the gut by sucrase to fructose and glucose. Fructose is absorbed to enterocytes by facilitated diffusion using a specific transporter (GLUT-5). Minor fraction of the fructose is converted to glucose (through glucose-6-P) in enterocytes, majority however is released to the portal blood. Fructose is used in the glycolysis. It undergoes two different pathways in different tissues.
Fate of fructose in the liver
Fructose is very rapidly absorbed by the liver and is metabolised by the enzyme fructokinase. Fructokinase is specific for fructose phosphorylation. Equation follows::
Fructose + ATP → fructose-1-P + ADP
(catalysed by fructokinase)
Fructose-1-P is not intermediate of glycolysis. It is converted by the enzyme aldolase B (different from the aldolase in the glycolysis which is denoted as aldolase A). Fru-1-P is split by the enzyme aldolase B and two trioses are produced – glyceraldehyde and dihydroxyacetone phosphate (DHAP). DHAP is an intermediate of glycolysis and enters it. Glyceraldehyde fate is little bit more complicated. Glyceraldehyde could be either (1) phosphorylated by specific kinase to glyceraldehyde-3-P, or (2) reduced to glycerol. It is quite obvious that phosphorylation is much more important because glyceraldehyde-3-P is an intermediate of the glycolysis.
Fru-1-P → dihydroxyacetone phosphate + D-glyceraldehyde
(catalysed by specific aldolase B)
Dihydroxyacetone phosphate → glyceraldehyde-3-P → glycolysis
(catalysed by triosephosphate isomerase – enzyme in regular glycolysis!)
D-glyceraldehyde → glyceraldehyde-3-P → glycolysis
D-glyceraldehyde → glycerol → glycerol-3-phosphate → triacylglycerol
Fructose metabolism in the liver includes connection: (1) to TAG synthesis, (2) glycolysis, and (3) gluconeogenesis.
Rarely there is congenital defect of the enzyme aldolase B. This condition is called fructose intolerance. Accumulation of Fru-1-P is present and disturbances in saccharides metabolism occur.
In fructose metabolism is bypassed reaction of glycolysis catalysed by the enzyme phosphofructokinase. This reaction is the slowest. Therefore fructose metabolism is faster than glucose metabolism. This could lead to increased lipogenesis in the liver (there is great abundance of the pyruvate (and thus AcCoA)) and excessive fatty acids and glycerol-3-phosphate are synthesized. Thus excessive TAG are produced.
Alternative fate of fructose
To a lesser extent and also in other tissues (e.g. in muscles) is fructose phosphorylated by hexokinase:
Fructose + ATP → fructose-6-P + ADP
Fructose-6-P is direct intermediate of the glycolysis hence the connection to the glycolysis is faster. Hexokinase has however for fructose high KM value, thus low affinity.
Significance of the fructose for sperm cells
Sperm cells use fructose as the main source of energy. It is not surprising that in seminal fluid there is very high fructose concentration (5-10 mmol/l). Fructose in the seminal fluid is produced by seminal glands from glucose by reducing glucose to sorbitol, than sorbitol is oxidized to fructose.
Galactose is 4-epimer of glucose. Galactose is part of lactose. (disaccharide). Lactose is split in the gut by the enzyme lactase. Glucose and galactose are thus produced. Both glucose and galactose are absorbed to the enterocytes by secondary active transport. Galactose is phosphorylated to galactose-1-P by galactokinase in the liver. Galactose-1-P is converted to glucose:
Galactose-1-P + UDP-glucose → UDP-galactose + glucose-1-P
(catalysed by the enzyme hexose-1-phosphate uridyltransferase)
UDP-galactose → UDP-glucose
(catalysed by the enzyme 4-epimerase)
Hexose-1-phosphate uridyltransferase defect causes rare disease called galactosemia. In this condition body is not capable of conversion of galactose to glucose. Galactose accumulates in organs – liver, erythrocytes, gut, eye lens, kidney, heart, brain. Galactose is converted to galactitol (or dulcitol; it is alcohol produced by galactose reduction). Galactitol causes cataract. Therapy includes absolute exclusion of milk products.
Glucose conversion to galactose
Galactose is used for (1) lactose synthesis in lactating mammary gland, or (2) in synthesis of glycoproteins, proteoglycans and glycolipids.
As mentioned above glucose-galactose interconversion needs activated forms. Glucose is activated to UDP-1-glucose and it is isomerized to UDP-galactose:
UDP-1-glucose ↔ UDP-galactose
(catalysed by 4-epimerase)
UDP-galactose is macroergic compound therefore it can be used for synthesis of compounds mentioned above.
Lactose synthesis take place only in lactating mammary gland. UDP-galactose is combined with glucose. This yields lactose. Reaction is catalysed by galactosyltransferase. Lactation is promoted by prolactin – peptide hormone from adenohypophysis.
Glucose conversion to glucuronic acid and its use
Glucuronic acid is one of the uronic acids. The uronic acids are parts of many important compounds – e.g. glycosaminoglycans, proteoglycans. Uronic acids are used as conjugation agent in biotransformation reactions. It promotes excretion of many endogenous and exogenous compounds. The uronic acids include glucuronic acid, L-iduronic acid and galacturonic acid.
Glucuronic acid is synthesized from glucose. Primary hydroxyl group on C6 in glucose molecule is oxidized to carboxylic group.
Oxidation is preceded by activation of glucose to UDP-glucose. Individual steps take place as follows:
1) Glucose phosphorylation (catalysed by the enzyme hexokinase/glucokinase)
Glucose + ATP → glucose-6-P + ADP
2) Glucose isomerization (catalysed by the enzyme glucosephosphate isomerase)
Glucose-6-P → glucose-1-P
3) Glucose activation, i.e. UDP bonding (catalysed by the enzyme UDP-glucose pyrophosphorylase)
Glucose-1-P + UTP → UDP-1-glucose + PPi
4) Oxidation to glucuronate
UDP-1-glucose + 2 NAD+ → UDP-glucuronate + 2 NADH
UDP-glucuronate is active form of the glucuronic acid and it can enter many reactions.
Use of the glucuronic acid
1) Proteoglycans synthesis
2) Conjugating agent
The glucuronic acid is conjugated with compounds that are poorly soluble in water (bilirubin, steroid hormones, xenobiotics). The goal is excrete them from the body. When glucuronic acid is connected these compounds get more soluble in water and they can be more easily eliminated in the urine or in the bile. The key enzyme is UDP-glucuronyltransferase. More info: Subchapter 9/6.
Subchapter Authors: Josef Fontana and Petra Lavríková