Carbohydrate Metabolism Flashcards by Tori Whalen

Once Glucose is converted to Glucose-6-P what pathways can it enter?

Glycolysis
Pentose Phosphate Shunt
Glycogenesis

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Which enzyme is needed to commit glucose 6-P to glycolysis?

PFK-1

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What are the enzymes of the three irreversible reactions are also the main regulated enzymes of glycolysis

hexokinase and glucokinase
PFK-1
pyruvate kinase

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What reaction does Hexokinase catalyze?

glucose to glucose-6-phosphate

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Hexokinase is found in the

muscle

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When cellular concentrations of glucose-6-P rise above ordinary, Hexokinase is temporarily _____________ to bring the rate of glucose-6-P into balance with its rate of utilization

inhibited

IT IS SENSITIVE TO NEED - DO WE NEED THAT GLUCOSE?

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What reaction does PFK-1 catalyze?

phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate

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what binds to the PFK-1 allosteric site to inhibit it?

ATP

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what happens when ATP inhibits PFK-1 by binding to an allosteric site?

Lowers the affinity of the enzyme for fructose-6-P
AMP and ADP relieve the inhibition of PFK-1

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why do AMP and ADP relieve the inhibition of PFK-1

As ATP is used, AMP and ADP begin to build up - therefore high levels of these tell the cell that more ATP needs to be made

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high levels of what also inhibit PFK-1?

High citrate levels inhibit PFK-1
High citrate serves a signal that the cell is meeting its current needs for energy

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what are the three ways that PFK is allosterically regulated?

ATP - inhibit
Citrate - inhibit
Fructose 2,6-biphosphate - activate

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what is the most significant allosteric regulator of PFK-1?

Fructose 2,6-bisphosphate

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Fructose-2,6-bisphosphate enhances the ______ of PFK-1 for fructose-6-P. What does this stimulate?

affinity
Stimulates glycolysis

Inhibits FBP-1, slowing gluconeogenesis

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what is a key intermediate in the CAC?

Citrate

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Fructose 2,6-bisphosphate is formed by phosphorylating Fructose-6-P, catalyzed by ____________________

phosphofructokinase-2 (PFK-2)

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What reaction does pyruvate kinase catalyze?

the direct transfer of phosphate from phosphoenolpyruvate (PEP) to ADP to produce ATP and pyruvate.

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Pyruvate kinase is allosterically regulated by: Inihibtion (3)

ATP
Acetyl-CoA
Fatty acids

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Pyruvate kinase is allosterically regulated by: Activation (1)

Fructose 1,6-bisphosphate

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Glycolysis is regulated hormonally by ______ and _________

insulin and glucagon

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_________ promotes the transcription of glucokinase, phosphofructokinase-1, and pyruvate kinase

Insulin

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________ reduces the expression
of glucokinase, PFK-1, and pyruvate kinase

Glucagon

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______, _________, and ________ are converted into
glycolytic intermediates

Fructose, Mannose, and galactose are converted into
glycolytic intermediates

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Which monosaccharide is metabolized faster,
glucose or fructose, why?

fructose
- can be more readily made into fat

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Pyruvate can be converted either to: (3)

A) acetyl CoA - enter into CAC B) Oxaloacetate - enter into GNG C) Lactate (Cori cycle)

_________ is an allosteric activator of pyruvate carboxylase

Acetyl CoA

What reaction does FBP-1 catalyze?

catalyzing the hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate and inorganic phosphate

FBP-1 is allosterically inhibited by (2)

- Allosterically inhibited by AMP - Allosterically inhibited by Fructose 2,6-bisphosphate

FBP1 is found in the

liver

FBP2 is found in the

muscle

Gluconeogenesis is regulated hormonally by

insulin and glucagon

Glucagon promotes _________ of FBP-2, lowering levels of Fructose-2,6bisP

activation

Glucagon can also induce the transcription of

PEP carboxykinase

Once Glucose is converted to Glucose-6-P which pathways can it enter

1. Glycolysis 2. Pentose Phosphate Shunt 3. Glycogenesis

Which enzyme is needed to commit glucose-6-P to glycolysis?

hexokinase

The pentose phosphate shunt generates two main products:

1. NADPH 2. Ribose-5-Phosphate (“pentose phosphates) Also generates Fructose-6-Phosphate and glyceraldehyde3-Phosphate, which can feed back into glycolysis

what are the two phases of the Pentose Phosphate Shunt?

Oxidative - Irreversible reactions Non-oxidative - Reversible reactions

During the oxidative phase, the reaction Glucose-6-phosphate -> Ribulose-5-phosphate generates...

Generates 2 NADPH

what are the functions of the oxidative phase? (2)

Functions: - Fatty acid synthesis – more to come next week - Reduces glutathione (antioxidant)

Glutathione is an __________. It is a 3 amino acid peptide made up of _______ - _______- ________

Glutathione is an antioxidant Glycine-cystine-glutamate

_________ regenerates glutathione by replacing the donated H’s

NADPH

Pentose Phosphate Shunt is important when converting carbs to fat

what neutralizes (reduces) hydrogen peroxide to water by donating H’s. NADPH will then regenerate _________ by replacing the donated H's

Glutathione

what enzyme is involved in Glucose-6-phosphate -> Ribulose-5-Phosphate

glucose-6-Phosphate dehydrogenase (G6PD)

what is the rate-limiting step of the oxidative phase?

Glucose-6-phosphate -> Ribulose-5-Phosphate

how is the oxidative phase regulated?

Regulated by ratio of NADPH:NADP+ - High NADPH: NADP+ ratio inhibits G6PD - Also inhibited by high levels of Acyl CoAs - Upregulated by insulin

G6PD deficiency is an X-linked trait. Is it more common in men or women?

men

What do you suppose Ribose-5-P can be used for?

R5P and its derivatives serve as precursors to many biomolecules, including DNA, RNA, ATP, coenzyme A, FAD (Flavin adenine dinucleotide), and histidine

During the non-oxidative phase, what is ribulose-5-P converted to?

Ribose-5-P OR into glycolytic intermediates

Glucose is stored in polymeric form as glycogen mostly in the _____ and ___________.

Glucose is stored in polymeric form as glycogen mostly in the liver and skeletal muscle muscle cells can use glucose directly BUT the liver does not use this glucose

High glucose/energy levels will trigger _______ synthesis

glycogen - Glycogenesis

Glucose is transferred onto a growing chain of glycogen as ____-glucose. Is this reversible or irreversible?

UDP irreversible

UDP-glucose is added to an existing strand of glycogen by the enzyme ________________

glycogen synthase

Glucose is added to the non-reducing end in what type of link?

alpha 1-4

_________ catalyzes the transfer of 4-8 glucose residues to a branch point

Branching enzyme The branching enzyme is also called amylo-α(1,4->1,6) glucosyl transferase

Glycogen synthase cannot initiate a new glycogen chain de novo, it requires a

primer

Primer contains pre-formed (alpha1-4) polyglucose chain with at least ___ glucose residues

4-8

what enzyme contains the primer and the enzyme needed to build the primer for glycogen synthesis (glycogenesis)

glycogenin

Glycogenolysis is a ________ progress. It is the _______ of glycogen into glucose units. Where does this occur?

catabolic Breakdown of glycogen into glucose units Occurs in the liver and muscle

Glucose is removed from the non-reducing ends of glycogen by the enzyme

Enzyme: glycogen phosphorylase

A de-branching enzyme has two functions:

1) transfers the outer 3 glucose residues from the branch to another non-reducing end (leaving only 1 reside behind at the branch point) - Aka Oligo-α(1,4)-α(1,4)-glucotransferase 2) Removes the final glucose residue in the alpha(1à6) linkage Aka Amylo-α(1,6)-glucosidase

Glucose-1-P is converted to glucose-6-P by the enzyme. Hint this is the same enzyme we saw in the firs step of glycogenesis

Enzyme: phosphoglucomutase

The liver can then convert glucose-6-P into glucose with the enzyme _____? Hint: This enzyme catalyzes the third bypass reaction of gluconeogenesis

glucose-6 phosphatase (G6Pase) which is expressed only in the liver, kidneys, and intestine

Muscles do not have this enzyme, why? What does the muscle do with glucose-6-P instead?

because they are not going to be releasing glucose

glycogen phosphorylase

Glycogen _______ and glycogen __________ are the two regulated enzymes in glycogen metabolism

Glycogen synthase and glycogen phosphorylase

Glycogen synthase regulation - allosterically activaited by

glucose-6-P

Glycogen phosphorylase regulation - Allosterically inhibited by? (2) and activated by _____ in the muscle only

inhibited by: * Glucose-6-P * ATP * Free glucose (in the liver only) activated by AMP (muscle only)

Glycogen synthase and glycogen phosphorylase can also be regulated by ________ modification

covalent

Glycogen synthase is _________ by phosphorylation

de-activated

Glycogen phosphorylase is _________ by phosphorylation. What protein catalyzes this phosphorylation?

activated Phosphorylation is catalyzed initially by the same protein, protein kinase A (PKA)