galactose metabolism 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 ID: 915908
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Slide1
Pentose PO4 pathway, Fructose, galactose metabolism
Slide2Slide3The
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.
Slide4Slide5For 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.
Slide6The 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.
Slide7Regulation;
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.
Slide8Epimerase
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
Slide9Role 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.
Slide10Slide11Muscle Metabolism of
Fructose (
Anaerobic
Glycolysis
) Large
Amounts of
Hexokinase
Slide12Two 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
.
Slide13In
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.
Slide14Fructose 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
.
Slide15In 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.
Slide16The 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.
Slide17Aldolase 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.
Slide18Slide19The 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.
Slide20Fructose 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.
Slide21Lactose, 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.
Slide22Slide23Galactose-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.
Slide24In 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
Slide25Slide26