Pentose PO4 pathway, Fructose, - PowerPoint Presentation

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Pentose PO4 pathway, Fructose, galactose metabolism

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The

Entner

Doudoroff

pathway, which is felt to have given rise through evolution to both the Pentose PO4 Pathway and Glycolysis, begins with ATP dependent glucose

phosphotansferase

producing Glucose 6 PO

4

, but produce only one ATP. This pathway prevalent in anaerobes such as

Pseudomonas

, they do not have a Phosphofructokinase (PFK-1). This enzyme may the the last enzyme to evolve forming the glycolytic pathway.

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For each mole of

glucose 6 PO

4

metabolized to

ribulose

5 PO

4

, 2 moles of

NADPH

are produced. 6-Phosphogluconate dh is not only an oxidation step but it’s also a decarboxylation reaction.

The

pentose phosphate pathway

(also called the

phosphogluconate

pathway

and the

hexose

monophosphate

shunt

) is a biochemical pathway parallel to glycolysis that generates NADPH and

pentoses

. While it does involve oxidation of glucose, its primary role is anabolic rather than catabolic. There are two distinct phases in the pathway. The first is the oxidative phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 5-carbon sugars. For most organisms, the pentose phosphate pathway takes place in the cytosol.

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The primary results of the pathway are:

The generation of reducing equivalents, in the form of NADPH, used in reductive biosynthesis reactions within cells (e.g. fatty acid synthesis).

Production of ribose-5-phosphate (R5P), used in the synthesis of nucleotides and nucleic acids. Production of erythrose-4-phosphate (E4P), used in the synthesis of aromatic amino acids

.

Transketolase

and

transaldolase

reactions are similar in that they transfer

between carbon chains,

transketolases 2 carbon units or

transaldolases

3 carbon units.

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Regulation;

Glucose-6-phosphate dehydrogenase is the rate-controlling enzyme of this pathway. It is allosterically stimulated by NADP

+

. The ratio of NADPH:NADP

+

is normally about 100:1 in liver

cytosol

.

This

makes the cytosol a highly-reducing environment. An NADPH-utilizing pathway forms NADP+, which stimulates Glucose-6-phosphate dehydrogenase to produce more NADPH. This step is also inhibited by acetyl-CoA.

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Epimerase

interconverts the stereoisomers ribulose-5-phosphate and xylulose-5-phosphate. 

Isomerase

converts the

ketose

ribulose-5-phosphate to the

aldose

ribose-5-phosphate.  Both reactions involve

deprotonation

to form an endiolate intermediate, followed by specific reprotonation to yield the product. Both reactions are reversible.A major product of this pathway is PRPP, but more about that when we discuss

Purine-Pyrimidine Synthesis

.

Ribose-phosphate

diphosphokinase

(or

phosphoribosyl

pyrophosphate

synthetase

)

ATP

AMP

PRPP

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Role of NADPH and glutathione in protecting cells against highly reactive oxygen derivatives. Reduced glutathione (GSH) protects the cell by destroying hydrogen peroxide and hydroxyl free radicals. Regeneration of GSH from its oxidized form (GSSG) requires the NADPH produced in the glucose 6-phosphate

dehydrogenase

reactio

n.

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Muscle Metabolism of

Fructose (

Anaerobic

Glycolysis

) Large

Amounts of

Hexokinase

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Two important kinetic properties distinguish

glucokinase

from the other hexokinases, allowing it to function in a special role as glucose sensor.

Glucokinase

has a lower affinity for glucose than the other hexokinases.

Glucokinase

changes conformation and/or function in parallel with rising glucose concentrations in the physiologically important range of 4–10

mmol

/L (72–180 mg/dl). It is half-saturated at a glucose concentration of about 8

mmol/L (144 mg/dl).Glucokinase is not inhibited by its product, glucose-6-phosphate

.

This

allows continued signal output (e.g., to trigger insulin release) amid significant amounts of its

product

.

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In

hepatocytes of various mammals, GKRP has always been found in molar excess of the amount of GK, but the GKRP:GK ratio varies according to diet, insulin sufficiency, and other factors. Free GKRP shuttles between the nucleus and the cytoplasm. It may be attached to the microfilament cytoskeleton

.

GKRP competes with glucose to bind with GK, but inactivates it when bound. In conditions of low glucose, GKRP then pulls the GK into the nucleus. Rising amounts of glucose coming into the hepatocyte prompt the GKRP to rapidly release GK to return to the cytoplasm.

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Fructose is metabolized almost completely in the liver in humans, where it is directed toward replenishment of liver glycogen and triglyceride synthesis. Under one percent of ingested fructose is directly converted to plasma triglyceride. 29% - 54% of fructose is converted in liver to glucose, and about quarter of fructose is converted to lactate. 15% - 18% is converted to glycogen. Glucose and lactate are then used normally as energy to fuel cells all over the body

.

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In the liver,

aldolase

B can utilize both F-1,6-BP and F1P as substrates. Therefore, when presented with F1P the enzyme generates DHAP and glyceraldehyde. The DHAP is converted, by triose phosphate

isomerase

(TPI), to G3P and enters glycolysis. The glyceraldehyde can be phosphorylated to G3P by glyceraldehyde kinase or converted to DHAP through the concerted actions of alcohol dehydrogenase, glycerol kinase and glycerol phosphate dehydrogenase.

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The protein encoded by this gene is a glycolytic enzyme that catalyzes the reversible conversion of fructose-1,6-bisphosphate to glyceraldehyde 3-phosphate and

dihydroxyacetone

phosphate. Three

aldolase

isozymes

(A, B, and C), encoded by three different genes, are differentially expressed during development.

Aldolase

A is found in the developing embryo and is produced in even greater amounts in adult muscle.

Aldolase A expression is repressed in adult liver, kidney and intestine and similar to aldolase C levels in brain and other nervous tissue. Aldolase A deficiency has been associated with myopathy and hemolytic anemia. Alternative splicing and alternative promoter usage results in multiple transcript variants.

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Aldolase B also known as fructose-bisphosphate

aldolase

B or liver-type

aldolase

B is one of three

isoenzymes

(A,

B, and C) of the class I fructose 1,6-bp aldolase enzyme, and plays a key role in both glycolysis and gluconeogenesis. The generic fructose 1,6-bp aldolase enzyme catalyzes the reversible cleavage of fructose 1,6-BP into GAP & DHAP as well as the reversible cleavage of fructose 1-p into glyceraldehyde

and DHAP. In mammals,

aldolase

B

is preferentially expressed in the liver, while

aldolase

A is expressed in muscle and erythrocytes and

aldolase

C

is expressed in the brain. Slight differences in

isozyme

structure result in different activities for the two substrate molecules: FBP and fructose 1-p.

Aldolase B exhibits no preference and thus catalyzes both reactions, while aldolases A and C prefer F 1,6bp.

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The synthesis of glycogen in the liver proceeds from

gluconeogenic precursors. Fructose is initially converted to DHAP and

glyceraldehyde

by

fructokinase

and

aldolase

B. The resultant

glyceraldehyde

then undergoes phosphorylation to glyceraldehyde-3-phosphate. Increased conc of DHAP and glyceraldehyde-3-phosphate in the liver drive the gluconeogenic pathway toward glucose-6-phosphate, glucose-1-phosphate and glycogen formation. It appears that fructose is a better substrate for glycogen synthesis than glucose and that glycogen replenishment takes precedence over triglyceride formation. Once liver glycogen is replenished, the intermediates of fructose metabolism are primarily directed toward triglyceride synthesis.

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Fructose results in the insulin-independent induction of several important hepatic

lipogenic enzymes including PK, NADP

+

-dependent

malate

dh, citrate lyase, acetyl CoA carboxylase, FA synthase, as well as

pyruvate

dh.

Fr-1-PO4 then undergoes hydrolysis by Fr-1-PO4 aldolase (aldolase B) to form DHAP and

glyceraldehyde; DHAP can either be isomerized to glyceraldehyde 3-PO4 by TIM or undergo reduction to glycerol 3-PO4 by glycerol 3-PO4 dh. The

glyceraldehyde

produced may also be converted to

glyceraldehyde

3-PO4 by glyceraldehyde kinase or converted to glycerol 3-phosphate by

glyceraldehyde

3-PO4 dh. The metabolism of fructose at this point yields intermediates in

gluconeogenic

pathway leading to glycogen synthesis, or can be oxidized to

pyruvate

and reduced to lactate, or be

decarboxylated

to acetyl CoA in the mitochondria and directed toward the synthesis of free FA, resulting finally in TG synthesis.

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Lactose, which is converted to

galactose

and glucose, is the primary carbohydrate source for developing mammals, and in humans it constitutes 40 percent of the energy consumed during the nursing period. Why lactose evolved as the unique carbohydrate of milk is unclear, especially since most individuals can meet their

galactose

need by biosynthesis from glucose. Whatever the rationale for lactose in milk, the occurrence of

galactose

in

glyco

-proteins, complex polysaccharides, and lipids, particularly in nervous tissue, has suggested specific functions. The

organoleptic and physical properties of galactose and, more specifically, the simultaneous occurrence of calcium and lactose in milk, may be significant evolutionary determinants.

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Galactose-1-phosphate uridylyltransferase

responsible

for converting ingested

galactose

to glucose

.

Galactose-1-phosphate

uridylyltransferase

(GALT) catalyzes the second step of the Leloir pathway of galactose metabolism. UDP-glucose + galactose 1-

phosphate= glucose

1-phosphate + UDP-

galactose

The expression of GALT is controlled by the actions of the FOXO3 gene.

The enzyme UDP-glucose 4-epimerase

also

known as UDP-

galactose

4-epimerase or GALE, is a

homodimeric

epimerase found in bacterial, fungal, plant, and mammalian cells. This enzyme performs the final step in the Leloir pathway of galactose metabolism, catalyzing the reversible conversion of UDP-

galactose to UDP-glucose. GALE tightly binds

nicotinamide adenine dinucleotide (NAD+), a co-factor required for catalytic activity.

The Leloir pathway is a metabolic pathway for the catabolism of D-galactose. It is named after Luis Federico

Leloir. In

the first step,

galactose

mutarotase

facilitates the conversion of β-D-

galactose

to α-D-

galactose

since this is the active form in the pathway. Next, α-D-

galactose

is phosphorylated by

galactokinase

to

galactose

1-phosphate. In the third step, D-galactose-1-phosphate

uridylyltransferase

converts

galactose

1-phosphate to UDP-

galactose

using UDP-glucose as the

uridine

diphosphate

source. Finally, UDP-

galactose

4-epimerase recycles the UDP-

galactose

to UDP-glucose for the

transferase

reaction. Additionally,

phosphoglucomutase

converts the D-glucose 1-phosphate to D-glucose 6-phosphate.

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In the hepatocyte, insulin stimulates the utilization and storage of glucose as lipid and glycogen, while repressing glucose synthesis and release. This is accomplished through a coordinated regulation of enzyme synthesis and activity. Insulin stimulates the expression of genes encoding glycolytic and fatty-acid synthetic enzymes (in blue), while inhibiting the expression of those encoding

gluconeogenic

enzymes (in red). These effects are mediated by a series of transcription factors and co-factors, including sterol regulatory element-binding protein (SREBP)-1, hepatic nuclear factor (HNF)-4, the

forkhead

protein family (Fox) and

PPARγ

co-activator 1 (PGC1). The hormone also regulates the activities of some enzymes, such as glycogen synthase and citrate

lyase

(in green), through changes in phosphorylation state. GK,

glucokinase; Glucose-6-P, glucose-6-phosphate; G-6-Pase, glucose-6-phosphatase; F-1,6-Pase, fructose-1,6-bisphosphatase; PEPCK, phosphoenolpyruvate carboxykinase; PFK, phosphofructokinase; PK, pyruvate kinase; ACC, acetyl-CoA carboxylase; FAS, fatty-acid synthase

. Ins signaling & regulation of glucose metabolism Nature 2001

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