this chapter covers several aspects of carbohydrate digestion. first it goes over some basics about carbohydrates and digestion in general, then talks about the specific enzymes at work in the brush border of the small intestine, then a brief section about lactose intolerance, and finally, a section about the transport of glucose from the intestine into the blood.
the normal american diet is made of 40-50% carbohydrates, and of this, 50-60% is made of the starch molecules (10,000 to 1 million glucosyl units long) amylose and amylopectin. amylose consists of glucose molecules bonded with alpha 1-4 bonds, while amylopectin is the same, but with alpha 1-6 bonds (and therefore branches) as well. digestion of these starches begins in the mouth with alpha amylase, which is an example of an endoglucosidase-- an enzyme which cleaves alpha 1-4 bonds at random intervals. this initial digestion leaves chunks of polysaccharides called alpha-dextrins. stomach acid deactivates the amylase, and carbohydrate digestion continues in the small intestine.
in the small intestine, carbohydrate digestion takes place mainly on the "brush border" of the intestinal mucosa, which have enzymes that are embedded in the intestinal membrane that poke into the intestinal lumen. the four major "glucosidases" are glucoamylase, trehalase, beta-glucosidase, and sucrase-isomaltase. each of these enzymes has a specific structure and catalytic sites that are specific to certain types of carbohydrates. glucoamylase breaks down alpha 1-4 bonds from the non reducing end ("tail end") of the sugar, until isomaltase remains-- isomaltase is basically just two sugar units branched together in an alpha 1-6 bond. the action of glucoamylase on polysaccharides seems similar to the action of glycogen phosphorylase in the breakdown of glycogen in that both are removing glucose residues one at a time from the tail end, and both can not remove the last glucose unit (or in glucose phosphorylase's case, the last 4).
sucrase-isomaltase is another brush border glucosidase with two catalytic sites. one is specific to isomaltose and maltose (and thus can break down the isomaltose from glucoamylase's activity) and one is specific to sucrose and maltose. beta glucosidase also has two catalytic sites. one is specific to breaking the beta 1-4 bond between glucose and galactose in lactose, and the other is specific to cleaving beta 1-4 bonds in glycolipids. the last glucosidase in the brush border is trehalase, which has only one catalytic site which is specific to trehalose, a sugar found only in some insects, mushrooms, and algae. these four enzymes work in tandem to break down the different types of carbohydrates that are dumped into the duodenum. the relative concentration of these enzymes change depending on the location in the gut. for example, pancreatic alpha amylase is secreted mainly in the duodenum, sucrase-maltase and beta-glucosidase is mainly in the jejunum, and glucoamylases is most common in the ileum.
a couple other notes about carbohydrate digestion. lactose intolerance occurs with either a lactase deficiency or intestinal damage. most adults only have 10% of the lactase activity that they had as children. lactose intolerance can ultimately lead to malabsorption of nutrients: lactose is undigested in the gut, and is instead metabolized by the bacterial flora in the colon, producing gases and lactic acid. the lactic acid can increase the intestinal lumen's osmolarity and cause water to distend the abdomen, which increases peristalsis and potentially causes diarrhea and malabsorption of other nutrients.
the last section of the chapter focused on the membrane channels that transport glucose across the intestinal lining into the blood. there are two types of such transport proteins, regular facilitated glucose channels and Na+ facilitated channels. Na+ facilitated channels use an ATPase Na+/K+ pump to establish a low concentration of Na+ inside the intestinal cell. the concentration gradient that is formed from the higher Na+ concentration in the intestinal lumen is then coupled with glucose transport in these membrane proteins. facilitative glucose transporters (also called "GLUT" transporters) do not require ATP and simply allow glucose to flow down its concentration gradient from the intestinal lumen, into the epithelium, and out into the serosa side ("blood side").
a few interesting notes relating to these glucose transporting membrane proteins. the liver's GLUT transporters have a higher than usual Km (the concentration at which the substrate is half-saturated with enzyme-- generally representing the enzyme's affinity for substrate and in this case the GLUT for glucose) for glucose, because it will only accept glucose into its cells (to be converted into glycogen for storage) when the blood glucose concentration is very high, like after a high carb meal. also, in muscle and fat cells, insulin stimulates glucose absorption by means of recruiting intracellular vesicles of glucose transport proteins to the cell membrane, where they can facilitate glucose transport into the cell.
questions
1. what percentage of a normal american diet consists of carbohydrates?
2. what percentage of the carbohydrate calories consists of amylose and amylopectin?
3. how many glucosyl units do amylose and amylopectin have?
4. what types of bonds do amylose and amylopectin have?
5. what are the major natural sweeteners found in fruit, honey, and vegetables?
6. what is the major dietary carb found from animal sources?
7. how much liquid do the salivary glands secrete per day?
8. what is an "endoglucosidase" and what is an example of one?
9. what is salivary amylase inactivated by?
10. how much digestive enzyme is secreted by the pancreas per day?
11. what are in the pancreatic secretions?
12. what are oligosaccharides?
13. what are the glucosidases found in the brush border of the small intestine?
14. describe the digestion of lactose and sucrose in the small intestine.
15. describe the structure and activity of glucoamylase?
16. what are alpha-dextrins vs. limit-dextrins?
17. describe the structure and activity of the sucrase-isomaltase complex.
18. what percentage of maltase activity can be attributed to the sucrose-isomaltose complex?
19. describe the structure and activity of trehalase.
20. describe the structure and activity of the beta-glucosidase complex
21. pancreatic alpha-amylase activity is highest in...
22. sucrase-isomaltase activity is highest in...
23. beta-glucosidase activity is highest in...
24. glucoamylase activity is highest in...
25. what type of carbohydrates enter the colon?
26. what are the fatty acids that result from bacterial starch digestion in the colon?
27. what are the gases that result from bacterial starch digestion in the colon?
28. lactose intolerance can be caused by...
29. what are normal lactase levels of an adult as compared to a child?
30. what happens when lactose is ingested by a lactose intolerant person?
31. what does the glycemic index represent?
32. which sugars have the highest glycemic index?
33. what are the two types of glucose transport proteins?
34. describe the mechanism of the Na+ dependent glucose transporter.
35. what are facilitative glucose transporters?
36. compare the digestion of glucose with that of galactose and fructose.
37. in body tissues, why is glucose transport across membranes not the rate limiting step of glucose metabolism?
38. how does the high Km of glucose transport proteins in the liver relate to the liver's blood glucose regulation?
39. how is insulin related to glucose transport proteins in the liver?
answers
1. 40-45%
2. 50-60%
3. 10,000 to 1 million
4. amylose has alpha 1-4 bonds between glucosyl residues. amylopectin has alpha 1-4 bonds between glucosyl units as well as alpha 1-6 bonds between branches.
5. fructose, sucrose, glucose.
6. lactose, which is made of glucose and galactose.
7. ~1 liter a day
8. an enzyme that breaks internal alpha 1-4 bonds in a polysaccharide at random intervals, such as amylase.
9. acidity of the stomach
10. ~1.5 liters a day
11. trypsinogen, chymotrypsinogen, carboxypeptidase (for digestion of proteins), alpha-amylase (for carbohydrates), lipase (fat), and bicarbonate (neutralizing gastric acidity)
12. 4-9 glucosyl units long, contain one or more alpha-1,6 branches.
13. beta-glucoamylase, sucrase-isomaltase, beta-glycosidase, trehalase.
14. converted to monosaccharides by glucosidases attached to the brush border lining.
15. it has two domains that have different substrate specificity, and acts as an exoglucosidase by breaking alpha 1-4 bonds on the non reducing ("tail end") of the saccharides, releasing glucose units until only isomaltose remains.
16. alpha dextrins are the pieces of polysaccharides that result from salivary alpha-amylase's endoglucosidase activity. limit dextrins are oligosaccharides that have been formed from the further breakdown of polysaccharides by pancreatic alpha-amylase.
17. made of two subunits: sucrose-maltose subunit cleaves alpha 1-4 bonds in sucrose, maltose. isomaltose-maltose unit cleaves alpha 1-6 bonds in isomaltose, and also breaks down maltose.
18. 80%
19. trehalose is a smaller dissaccharidase that only has one catalytic site with specificity for trehalose, which is a relatively rare source of carbohydrate found in some insects, algae, and mushrooms.
20. has two catalytic sites: glucosyl-ceramidase site, which cleaves beta bonds in glycolipids, and lactase site, which breaks beta 1-4 bonds between glucose and galactose in lactose.
21. duodenum
22. jejunum
23. jejunum
24. ileum
25. any undigested starches: starches high in amylose, poorly hydrated starches (like in dried beans), dietary fiber.
26. acetic acid, propionic acid, butyric acid. 2,3,4 carbon.
27. hydrogen gas, CO2, methane
28. low lactase levels or intestinal injury
29. 10% of the level of a child
30. lactose is undigested by the lactase in the small intestine and is therefore metabolized by the colonic bacteria, which produces lactic acid, methane, and H2. the increased lactic acid increases osmolarity of the intestinal lumen, causing more water to be dumped into the lumen, causing excess peristalsis, causing malabsorption of other nutrients.
31. how quickly blood glucose levels rise after consumption of a food.
32. glucose and maltose.
33. sodium dependent glucose transporters and facilitative glucose transporters
34. a Na+ / K+ ATPase pump pumps Na+ out of the intestinal epithelium cells so that there is a low Na+ concentration within. the Na+ dependent transporter channels then use the resulting Na+ concentration gradient to power movement of glucose within the cell.
35. these are glucose channels that exist on both the luminal and serosal side of the intestinal epithelium that allow glucose to move down its concentration gradient without expenditure of energy.
36. galactose passes through the intestinal membrane in a similar way to glucose- via both Na+ facilitated channels and facilitative glucose transporters. fructose passes through by facilitated diffusion only.
37. because the transport proteins have a high affinity for glucose (a low Km) or are present in high numbers.
38. the liver will only transport glucose into its cells (and therefore convert glucose to glycogen for storage) when the blood glucose level is high, such as right after a high carb meal.
39. binding of insulin recruits GLUT proteins from intracellular vescicles onto the membrane.
Showing posts with label galactose. Show all posts
Showing posts with label galactose. Show all posts
Tuesday, December 9, 2008
Sunday, December 7, 2008
biochem: mark's medical biochem chapter 29- fructose, galactose, pentose pathway
this chapter looked at the metabolic pathways for two other common dietary carbohydrates, fructose and galactose, as well as the pentose pathway, which produces NADPH and ribose 5-phosphate (used in nucleotide synthesis). fructose and galactose metabolism are similar in that they are basically phosphorylated and then converted into intermediates of the glycolytic pathway.
fructose metabolism:
1. fructose is phosphorylated to fructose 1-phosphate via fructokinase
2. fructose 1-phosphate is cleaved to DHAP and glyceraldehyde via aldolase B
3. glyceraldehyde phosphorylated to G3P via triose kinase
4. G3P and DHAP can be used in the glycolytic pathway
galactose metabolism:
1. galactose is phosphorylated to galactose 1-phosphate via galactokinase
2. galactose 1-phosphate plus UDP-glucose yields glucose 1-phosphate plus UDP-galactose
3. glucose 1-phosphate is converted to glucose 6-phosphate, which can be used in the glycolytic pathway.
production of fructose from glucose was also looked at in the "polyol" pathway:
1. glucose is reduced to sorbitol (polyol) via aldolase reductase, using NADPH as the reducing equivalents.
2. sorbitol is oxidized to fructose via sorbital dehydrogenase, producing NADH.
this conversion of glucose to fructose is used mainly in the seminiferous tubules, where spermatozoans use fructose instead of glucose in order to maintain the integrity of their plasma membrane (apparently glucose causes "acrosomal damage")
the pentose pathway is introduced as an alternate use of glucose 6-phosphate: instead of oxidizing for ATP, glucose 6-phosphate can taken through the pentose pathway to create NADPH (provides reducing equivalents, aids in catabolic reactions and helps protect against reactive oxygen species) and ribose 5-phosphate (used in nucleotide synthesis). the pentose pathway has two parts, an oxidative and non oxidative pathway. the oxidative pathway takes glucose 6-phosphate and creates 2 NADPH and ribulose 5-phosphate:
1. glucose 6-phosphate is oxidized to 6-phosphoglucanolactone via glucose 6 phosphate dehydrogenase, producing NADPH
2. 6-phosphoglucanolactone is hydrolyzed to 6-phosphoglucanolactate via glucanolactase
3. 6-phosphoglucanolactate undergoes oxidative decarboxylation to ribulose 5-phosphate, releasing CO2 and producing another NADPH
the non-oxidative portion of the pentose pathway takes three molecules of 5-ribulose phosphate, the product of the oxidative portion, and rearranges the molecules using transferases to eventually form 2 molecules of fructose 6-phosphate and 1 molecule of G3P. the reactions are as follows:
1. out of 3 molecules of 5-ribulose phosphate, 2 are converted to xylulose 5-phosphate via epimerases
2. the 3rd molecule of 5-ribulose phosphate is converted to ribose 5-phosphate (which can be used for nucleotide synthesis if necessary)
3. xylulose 5-P and ribose 5-P are rearranged to G3P and sedoheptulose 7-P via transketolase
4. sedoheptulose 7-P and G3P are rearranged to fructose 6-P and erythrose 4-P via transaldolase
5. erythrose 4-P and xylulose 5-P are rearranged to fructose 6-P and G3P
thus, 3 molecules of ribulose 5-phosphate are converted to 2 molecules of fructose 6-phosphate and one molecule of G3P.
we then look at how the pentose pathway can be used to accommodate and balance cellular needs for ATP, ribose 5-phosphate, and NADPH. several scenarios are considered-- when only NADPH is needed, only the oxidative portion of the pathway is activated, producing two molecules of NADPH per glucose molecule, and the nonoxidative pathway converts ribulose 5-phosphate to glucose 6-phosphate (presumably by isomerization of fructose 6-phosphate) which is then put back into the oxidative portion to create even more NADPH. when NADPH and ribose 5-phosphate are both needed, the oxidative pathway is activated to produce NADPH, and ribose 5-phosphate is produced from the non-oxidative pathway via isomerase. when only ribose 5-phosphate is needed, this implies that NADPH levels are relatively high-- which actually inhibits the first enzyme in the oxidative pathway. thus the oxidative pathway is inhibited, but the nonoxidative pathway produces ribose 5-phosphate from ribulose 5-phosphate. finally, in the case when NADPH and pyruvate are needed, both the oxidative and non oxidative portions are stimulated.
questions
1. describe the mechanism for the metabolism of fructose.
2. where does metabolism of fructose occur?
3. describe the mechanism for the polyol pathway.
4. where is the polyol pathway mainly used and why?
5. what is the galactose metabolism pathway?
6. what does the pentose pathway produce and why are the products useful?
7. why is NADP+ used instead of NAD+?
8. describe the mechanism for the oxidative branch of the pentose pathway.
9. describe the mechanism for the non-oxidative branch of the pentose pathway.
10. describe how the glycolytic intermediates can be used to produce ribose 5-phosphate for nucleotide synthesis.
11. what are some uses of NADPH in the body?
12. the entry of glucose 6-phosphate into the pentose phosphate pathway is regulated by...
describe how the pentose phosphate pathway is used to respond to cellular needs for:
13. NADPH
14. NADPH and ribose 5-phosphate
15. ribose 5-P only
16. NADPH and pyruvate
answers
1. fructose is phosphorylated by fructokinase to fructose 1-phosphate. fructose 1-phosphate is cleaved by aldosase B into DHAP and glyceraldehyde. glyceraldehyde is phosphorylated by triose kinase into G3P. G3P can then be converted to 1,3 bisphosphoglycerate in the glycolytic pathway or combined with DHAP to form fructose 1,6 bisphosphate in the gluconeogenic / glycolytic pathway.
2. mainly in the liver, but also in the mucosa of the small intestine.
3. glucose is reduced to sorbitol (polyol) by aldolase reductase, forming NADPH. sorbitol is oxidized to fructose via sorbitol dehydrogenase.
4. glucose conversion into fructose is mainly used in the seminal vescicles, which store the spermatozoans. spermatozoans use fructose rather than glucose to avoid plasma membrane damage while in the male reproductive system.
5. galactose is phosphorylate by galactokinase to galactose 1-phosphate. galactose 1-phosphate is converted to glucose 1-phosphate via galactose 1-phosphate uridylyltransferase. glucose 1-phosphate is then isomerized to glucose 6 phosphate for use in other metabolic pathways.
6. an alternative pathway to the first few steps of glycolysis that produces ribose 5-phosphate, which is used in nucleotide synthesis, and NADPH, which is used in reductive detoxification.
7. NADPH is used in reactions that have need for reducing equivalents because the ratio of NADPH to NADP+ is much higher than that of NADH to NAD (because NADH is immediately used in the electron transport chain)
8. glucose 6-phosphate is oxidized to 6-phosphogluconolactone via glucose 6-phosphate dehydrogenase (producing NADPH). 6-phosphogluconolactone is hydrolyzed to 6-phosphogluconolactate via gluconolactase. 6-phosphate gluconolactate undergoes oxidative decarboxylation to ribulose 5 phosphate, releasing CO2 and forming NADPH.
9. 3 molecules of ribulose 5-phosphate are converted via isomerases, epimerases, transketolases, and transaldolases into two molecules of fructose 6-phosphate and one molecule of G3P, which can be used in glycolysis.
10. ribose 5-phosphate can be synthesized using the non-oxidative portion of the pentose pathway because the reactions are reversible. 2 molecules of fructose 6-phosphate and 1 molecule of G3P can ultimately produce 3 molecules of ribose 5-phosphate.
11. NADPH is mainly produced from the oxidative portion of the pentose pathway and is used to provide reducing equivalents and is involved in protecting the body against reactive oxygen species. it is also involved in anabolic processes such as cholesterol synthesis, fatty acid synthesis and chain elongation.
12. the concentration of NADPH in the cell. high NADPH inhibits glucose 6-phosphate dehydrogenase.
13. the oxidative portion of the pentose phosphate pathway is utilized to produced 2 moles of NADPH per molecule of glucose. the non oxidative portion of the pentose pathway is used to convert ribulose 5-phosphate back to glucose 6-phosphate, where it can be used to generated more NADPH.
14. the oxidative portion creates NADPH and ribulose 5-phosphate. isomerases convert ribulose 5 phosphate into ribose 5-phosphate.
15. high levels of NADPH will inhibit the oxidative portion; just the non-oxidative portion will create ribose 5-phosphate from fructose 6-phosphate and G3P.
16. the oxidative portion will create NADPH and the nonoxidative will create fructose 6-phosphate and G3P, which can be used in glycolysis to produce pyruvate.
fructose metabolism:
1. fructose is phosphorylated to fructose 1-phosphate via fructokinase
2. fructose 1-phosphate is cleaved to DHAP and glyceraldehyde via aldolase B
3. glyceraldehyde phosphorylated to G3P via triose kinase
4. G3P and DHAP can be used in the glycolytic pathway
galactose metabolism:
1. galactose is phosphorylated to galactose 1-phosphate via galactokinase
2. galactose 1-phosphate plus UDP-glucose yields glucose 1-phosphate plus UDP-galactose
3. glucose 1-phosphate is converted to glucose 6-phosphate, which can be used in the glycolytic pathway.
production of fructose from glucose was also looked at in the "polyol" pathway:
1. glucose is reduced to sorbitol (polyol) via aldolase reductase, using NADPH as the reducing equivalents.
2. sorbitol is oxidized to fructose via sorbital dehydrogenase, producing NADH.
this conversion of glucose to fructose is used mainly in the seminiferous tubules, where spermatozoans use fructose instead of glucose in order to maintain the integrity of their plasma membrane (apparently glucose causes "acrosomal damage")
the pentose pathway is introduced as an alternate use of glucose 6-phosphate: instead of oxidizing for ATP, glucose 6-phosphate can taken through the pentose pathway to create NADPH (provides reducing equivalents, aids in catabolic reactions and helps protect against reactive oxygen species) and ribose 5-phosphate (used in nucleotide synthesis). the pentose pathway has two parts, an oxidative and non oxidative pathway. the oxidative pathway takes glucose 6-phosphate and creates 2 NADPH and ribulose 5-phosphate:
1. glucose 6-phosphate is oxidized to 6-phosphoglucanolactone via glucose 6 phosphate dehydrogenase, producing NADPH
2. 6-phosphoglucanolactone is hydrolyzed to 6-phosphoglucanolactate via glucanolactase
3. 6-phosphoglucanolactate undergoes oxidative decarboxylation to ribulose 5-phosphate, releasing CO2 and producing another NADPH
the non-oxidative portion of the pentose pathway takes three molecules of 5-ribulose phosphate, the product of the oxidative portion, and rearranges the molecules using transferases to eventually form 2 molecules of fructose 6-phosphate and 1 molecule of G3P. the reactions are as follows:
1. out of 3 molecules of 5-ribulose phosphate, 2 are converted to xylulose 5-phosphate via epimerases
2. the 3rd molecule of 5-ribulose phosphate is converted to ribose 5-phosphate (which can be used for nucleotide synthesis if necessary)
3. xylulose 5-P and ribose 5-P are rearranged to G3P and sedoheptulose 7-P via transketolase
4. sedoheptulose 7-P and G3P are rearranged to fructose 6-P and erythrose 4-P via transaldolase
5. erythrose 4-P and xylulose 5-P are rearranged to fructose 6-P and G3P
thus, 3 molecules of ribulose 5-phosphate are converted to 2 molecules of fructose 6-phosphate and one molecule of G3P.
we then look at how the pentose pathway can be used to accommodate and balance cellular needs for ATP, ribose 5-phosphate, and NADPH. several scenarios are considered-- when only NADPH is needed, only the oxidative portion of the pathway is activated, producing two molecules of NADPH per glucose molecule, and the nonoxidative pathway converts ribulose 5-phosphate to glucose 6-phosphate (presumably by isomerization of fructose 6-phosphate) which is then put back into the oxidative portion to create even more NADPH. when NADPH and ribose 5-phosphate are both needed, the oxidative pathway is activated to produce NADPH, and ribose 5-phosphate is produced from the non-oxidative pathway via isomerase. when only ribose 5-phosphate is needed, this implies that NADPH levels are relatively high-- which actually inhibits the first enzyme in the oxidative pathway. thus the oxidative pathway is inhibited, but the nonoxidative pathway produces ribose 5-phosphate from ribulose 5-phosphate. finally, in the case when NADPH and pyruvate are needed, both the oxidative and non oxidative portions are stimulated.
questions
1. describe the mechanism for the metabolism of fructose.
2. where does metabolism of fructose occur?
3. describe the mechanism for the polyol pathway.
4. where is the polyol pathway mainly used and why?
5. what is the galactose metabolism pathway?
6. what does the pentose pathway produce and why are the products useful?
7. why is NADP+ used instead of NAD+?
8. describe the mechanism for the oxidative branch of the pentose pathway.
9. describe the mechanism for the non-oxidative branch of the pentose pathway.
10. describe how the glycolytic intermediates can be used to produce ribose 5-phosphate for nucleotide synthesis.
11. what are some uses of NADPH in the body?
12. the entry of glucose 6-phosphate into the pentose phosphate pathway is regulated by...
describe how the pentose phosphate pathway is used to respond to cellular needs for:
13. NADPH
14. NADPH and ribose 5-phosphate
15. ribose 5-P only
16. NADPH and pyruvate
answers
1. fructose is phosphorylated by fructokinase to fructose 1-phosphate. fructose 1-phosphate is cleaved by aldosase B into DHAP and glyceraldehyde. glyceraldehyde is phosphorylated by triose kinase into G3P. G3P can then be converted to 1,3 bisphosphoglycerate in the glycolytic pathway or combined with DHAP to form fructose 1,6 bisphosphate in the gluconeogenic / glycolytic pathway.
2. mainly in the liver, but also in the mucosa of the small intestine.
3. glucose is reduced to sorbitol (polyol) by aldolase reductase, forming NADPH. sorbitol is oxidized to fructose via sorbitol dehydrogenase.
4. glucose conversion into fructose is mainly used in the seminal vescicles, which store the spermatozoans. spermatozoans use fructose rather than glucose to avoid plasma membrane damage while in the male reproductive system.
5. galactose is phosphorylate by galactokinase to galactose 1-phosphate. galactose 1-phosphate is converted to glucose 1-phosphate via galactose 1-phosphate uridylyltransferase. glucose 1-phosphate is then isomerized to glucose 6 phosphate for use in other metabolic pathways.
6. an alternative pathway to the first few steps of glycolysis that produces ribose 5-phosphate, which is used in nucleotide synthesis, and NADPH, which is used in reductive detoxification.
7. NADPH is used in reactions that have need for reducing equivalents because the ratio of NADPH to NADP+ is much higher than that of NADH to NAD (because NADH is immediately used in the electron transport chain)
8. glucose 6-phosphate is oxidized to 6-phosphogluconolactone via glucose 6-phosphate dehydrogenase (producing NADPH). 6-phosphogluconolactone is hydrolyzed to 6-phosphogluconolactate via gluconolactase. 6-phosphate gluconolactate undergoes oxidative decarboxylation to ribulose 5 phosphate, releasing CO2 and forming NADPH.
9. 3 molecules of ribulose 5-phosphate are converted via isomerases, epimerases, transketolases, and transaldolases into two molecules of fructose 6-phosphate and one molecule of G3P, which can be used in glycolysis.
10. ribose 5-phosphate can be synthesized using the non-oxidative portion of the pentose pathway because the reactions are reversible. 2 molecules of fructose 6-phosphate and 1 molecule of G3P can ultimately produce 3 molecules of ribose 5-phosphate.
11. NADPH is mainly produced from the oxidative portion of the pentose pathway and is used to provide reducing equivalents and is involved in protecting the body against reactive oxygen species. it is also involved in anabolic processes such as cholesterol synthesis, fatty acid synthesis and chain elongation.
12. the concentration of NADPH in the cell. high NADPH inhibits glucose 6-phosphate dehydrogenase.
13. the oxidative portion of the pentose phosphate pathway is utilized to produced 2 moles of NADPH per molecule of glucose. the non oxidative portion of the pentose pathway is used to convert ribulose 5-phosphate back to glucose 6-phosphate, where it can be used to generated more NADPH.
14. the oxidative portion creates NADPH and ribulose 5-phosphate. isomerases convert ribulose 5 phosphate into ribose 5-phosphate.
15. high levels of NADPH will inhibit the oxidative portion; just the non-oxidative portion will create ribose 5-phosphate from fructose 6-phosphate and G3P.
16. the oxidative portion will create NADPH and the nonoxidative will create fructose 6-phosphate and G3P, which can be used in glycolysis to produce pyruvate.
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