Term
1. Generation of NADPH for reductive biosynthesis
2. Generation of ribose (5-C) sugar for nucleotide synthesis
are primary functions of what pathway? |
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Definition
| Pentose Phosphate Pathway |
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Term
NAD(H) – electron (proton) carrier in biological oxidations
NADP(H) – electron (proton) donor in reductive biosynthesis
True or False: Enzymes can participate in both reactions. |
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Definition
| False, enzymes that catalyze NADH (biological oxidations) wont be seen in reductive biosynthesis |
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Term
The pentose phosphate pathway has two phases:
(A) Oxidative phase (non-reversible):
Glucose 6-phosphate (6C) is converted to ribulose-5 phosphate (5C), generating ??? molecules for each molecule of glucose 6-phosphate oxidized
(B) non-oxidative phase (reversible):
3 ribulose 5-phosphate is converted by a series of rearrangements to produce:
2 fructose 6-phosphate + glyceraldehyde 3-phosphate |
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Definition
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Term
During the oxidative phase of the pentose phosphate pathway, glucose 6-phosphate is oxidized to 6-phospho-gluconolactone by what enzyme? NADP+ is a cofactor, and NADPH is produced. This is the ??? for the pentose phosphate pathway.
6-phospho-gluconolactone is hydrolyzed to 6-phosphogluconate by what enzyme? |
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Definition
Glucose 6-phosphate dehydrogenase
Committed Step (rate-limiting step)
Lactonase |
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Term
During the oxidative phase of the PPP,
6-phosphogluconate is oxidized and decarboxylated by ??? yielding ribulose 5-phosphate. Both NADPH and CO2 are produced. |
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Definition
| 6-phosphogluconate dehydrogenase |
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Term
- pentose phosphate isomerase catalyzes rearrangement of ribulose 5-phosphate to form?
- pentose phosphate epimerase catalyzes rearrangement of ribulose 5-phosphate to form |
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Definition
Ribose 5-phosphate
Xyluose 5-phosphate. |
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Term
| True or False: Ribose 5-P is the precursor to all nucleotides |
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Definition
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Term
| Transketolase catalyzes the 1st reaction between ribose-5-P and xylulose-5-P, but it requires what cofactor? |
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Definition
| Thiamine Pyrophosphate (TPP) |
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Term
| True or False: During the nonoxidative phase of PPP, transketolase transfers 2-C fragments, while transaldolase transfers 3-C fragments. |
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Definition
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Term
True or False:
The actions of enzymes in the non-oxidative phase of the PPP take 3 ribulose 5-P -->
2 fructose 6-phosphate + glyceraldehyde 3-phosphate. |
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Definition
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Term
Balanced equation for the pentose phosphate pathway:
3 Glucose 6-phosphate + 6 NADP+ + 3 H2O ----->
2 fructose 6-phosphate + glyceraldehyde 3-phosphate + 6 NADPH + 6 H+ + 3CO2 |
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Definition
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Term
True or False:
Regulation of pentose phosphate pathway:
- regulated by hormones
- Ratio of NADPH to NADP+
- Need for ribose 5-phosphate |
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Definition
| False, NOT Rregulated by hormones |
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Term
Regulation of pentose phosphate pathway
(there are 3 possible scenarios):
Cell needs ribose 5-phospate but not NADPH ―
fructose 6-phosphate and glyceraldehyde 3-phosphate are diverted from glycolysis, converted to ribose 5-phosphate.
Need for NADPH and ribose 5-phospate are balanced ―
oxidative branch and pentose isomerase are active,
little flow through non-oxidative branch.
Cell needs NADPH but not ribose 5-phosphate ―
oxidative and non-oxidative branch make fructose 6-phosphate and glyceraldehyde 3-phosphate, which are converted by gluconeogenesis to glucose 6-phosphate. Thus, pentose phosphate path and gluconeogenesis function as a cycle to make NADPH and ribose 5-phosphate. |
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Definition
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Term
NADPH is required for reduction of oxidized glutathione by ???
Reduced glutathione protects cellular proteins from oxidative damage (glutathione peroxidase is used)
Therefore, deficiency of NADPH in RBCs leads to |
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Definition
Glutathione reductase
Hemolysis |
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Term
In metabolism of dietary sugars, the principal disaccharide sugars in the human diet are?
These disaccharides are hydrolyzed in the small intestine, yielding what three monosaccharides? |
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Definition
- maltose (a-D-glucosyl-D-glucose) grains, starchy vegetables
- sucrose (a-D-glucosyl-D-fructose) cane sugar, beet sugar
- lactose (b-D-galactosyl-D-glucose) milk, dairy products
Glucose, fructose and galactose |
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Term
Fructose Metabolism-1:
- fructose is derived from sucrose (a-D-glucosyl-D-fructose)
- sucrose hydrolysis (sucrase) in small intestine liberates glucose and fructose
- fructose enters cells via
(expressed in small intestine, kidney medulla, liver) |
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Definition
| The fructose transporter, also known as GLUT-5 |
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Term
During fructose metabolism, fructose is phosphorylated by ???
- this enzyme is found primarily in liver, but is also present in small intestine and kidney medulla |
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Definition
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Term
| True or False: Fructose is regulated by insulin. |
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Definition
| FALSE, fructose is NOT regulated by insulin |
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Term
During fructose metabolism,fructose-1-phosphate is cleaved by ??? to yeild dihydroxyacetone phosphate and D-glyceraldehyde
- this enzyme is also known as fructose aldolase
- expressed in mammalian liver (cytosol)
D-Glycraldehyde is phosphorylated by ??? to yield Glyceraldehyde-3-P.
Fructose thus yields the glycolytic triose intermediates glyceraldehyde 3-phosphate and dihydroxyacetone phosphate
- therefore, fructose can enters either mostly the glycolytic or gluconeogenic pathways, depending on cellular needs |
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Definition
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Term
| Activity of ??? usually exceeds the activity of aldolase B (therefore aldolase cleavage is rate-limiting step) |
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Definition
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Term
Key points regarding fructose metabolism:
- Not regulated by Insulin
- Starts at the three-carbon level
- Severe aldolase B deficiency = fructose intolerance and can lead to intracellular accumulation of fructose-1-phosphate (reason why fructose can not be used by diabetic patients: majoring of inorganic phosphate would be tied up as fructose-1-P) |
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Definition
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Term
Clinical Correlation: Hereditary fructose intolerance
- In general, fructose is metabolized less efficiently than glucose
- Activity of liver aldolase B that cleaves fructose is the rate limiting factor
- Severe deficiency of aldolase B leads to “trapping” of Pi as fructose-1-P
- In this condition (very low aldolase), depletion of ATP in the liver cells may occur |
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Definition
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Term
During galactose Metabolism:
- Lactose is hydrolyzed by B-GALACTOSIDASE (LACTASE), releasing galactose and glucose (many adults are deficient in lactase, or lactose intolerant)
- Galactose is phosphorylated by LACTOKINASE to form galactose-1-phosphate
- UDP is added to galactose-1-P by galactose-1-P uridyltransferase (UDP-glucose is donor) |
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Definition
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Term
True or False: UDP-galactose is a substrate for
UDP-glucose:galactose 4-epimerase, which regenerates UDP-glucose
True or False: Glucose-1-phosphate is a substrate for hexokinase, which yields glucose-6-phosphate
Note:
GALACTOSE can enter both the glycolytic and synthesis of glycogen pathways.
FRUCTOSE can only enter glycolytic pathways |
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Definition
True
FALSE : Phosphoglucomutase |
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Term
Clinical Correlation: Galactosemia
- Results from deficiency of either galactokinase or galactose uridyl transferase
- Accumulation of galactose-1-P and fructose-1-P causes liver failure (lack of transferase)
- Accumulated galactose (lack of kinase) can be reduced to galactitol, cause cataract formation. |
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Definition
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Term
| Fructose can be synthesized in many different tissues through the sorbitol pathway (polyol) pathway. Sorbitol corresponds to the sugar alcohol of glucose. |
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Definition
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Term
Glucuronic acid pathway:
- Glucuronic acid is produced by an alternate oxidation pathway for glucose
- Glucuronate synthesis leads to formation of:
Glycoproteins
Glycolipids
Proteoglycans |
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Definition
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Term
Amino sugars and sugar acids:
- Constituents of glycoproteins, glycolipids, and proteoglycans
- Start with fructose 6-phosphate; amino group is donated by aminotransferase (glutamine is donor)
- Glucosamine 6-phosphate is acetylated, isomerized
- Activation of amino sugar 1-phosphate by UTP
- Glycosyl nucleotide transferases are used (as in glycogen synthesis)
- In addition to UTP, GTP or CTP can be used to activate sugar derivatives |
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Definition
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Term
Regulation of Carbohydrate Metabolism Occurs in Three Ways:
(1) Allosteric modification – ratio [ATP]/[AMP] is critical, and concentration of metabolites such as acetyl CoA and citrate are important
(this is the most rapid form of regulation)
(2) Covalent modification – kinases and phosphatases modify enzyme activity (hormone-regulated)
(3) Enzyme synthesis – receptor-mediated hormone-sensitive regulation of mRNA synthesis
We can observe these regulatory themes:
Coordinate regulation within a pathway (e.g., glycolysis)
Reciprocal regulation of opposing pathways (e.g., glycolysis vs. gluconeogenesis |
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Definition
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