Gluconeogenesis Flashcards
Brain requires ______ grams of glucose per day
120 grams
glycogen stores can supply about half that amount
“New glucose synthesis”
gluconeogenesis
gluconeogenesis is the net synthesis of glucose from non-carbohydrate sources, including:
- pyruvate
- lactate
- glycerol
- glucogenic amino acids (especially alanine and glutamine from muscle)
- TCA cycle intermediates (citrate, a-KG, succinyl-CoA, succinate, malate)
- propionyl-CoA
Role of gluconeogenesis
to provide glucose for extrahepatic tissues (brain and nervous system, renal medulla, erythrocytes, testes, embryonic tissues) that require glucose as the main or only energy source
Gluconeogenesis equation
2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 4 H2O yields Glucose + 4 ADP + 2 GDP + 2 NAD+ + 6 Pi
Gluconeogenesis occurs where in the body?
- liver
- renal cortex
- epithelial cells of small intestinal lining
Compounds Whose Metabolism Cannot Yield Glucose
These include:
- (1) Acetyl-CoA, and
- (2) Even-chain fatty acids and strictly “ketogenic” amino acids lysine and leucine, which are metabolized exclusively to acetyl-CoA
Note that Acetyl-CoA loses its carbons as CO2 as it moves through the TCA cycle and therefore cannot be “recycled” back to oxaloacetate via the cycle and cannot be converted back to pyruvate because the reaction catalyzed by pyruvate dehydrogenase is irreversible in vivo
How is the lactate that is produced during vigorous exercise dealt with?
It is converted back to glucose via gluconeogenesis in the liver as part of the Cori Cycle.
In this way, the body repays the “oxygen debt” incurred when glucose was broken down anaerobically because oxygen was limiting.
The Three “Bypass” Steps of Gluconeogenesis where irreversible steps of glycolysis are being replaced
- Pyruvate carboxylase and phosphoenolpyruvate carboxykinase (replace pyruvate kinase of glycolysis)
- Fructose 1,6-bisphosphatase (replaces PFK-1)
- Glucose 6-phosphatase (replaces hexokinase)
Bypass #3 (G6Pase) occurs only in certain tissues
Liver, kidney and epithelial cells of small intestine
1st step occurs in _______ but rest occur in _______
mitochondrion; cytosol.
Located in mitochondrion; a biotin-dependent enzyme
Pyruvate Carboxylase
Pyruvate carboxylase is a heterotetramer, each subunit contains 4 domains:
- a biotin carboxylation domain (phase 1)
- a pyruvate carboxylase domain (phase 2)
- a domain that binds acetyl-CoA
- the domain to which biotin is attached via lysine residue
Biotin group swings from _____ on one subunit to _____ on different subunit
site 1 ; site 2
Two different isozymes exist for PEPCK, one in the ________ and one in the ________.
mitochondrion; cytosol
Carries out decarboxylation and phosphorylation of oxaloacetate to give PEP
PEP Carboxykinase (PEPCK)
Formation of unstable enol compound (PEP) is driven by _______ and is trapped by _______
decarboxylation; phosphorylation
carboxylation-decarboxylation sequence is a way of _______ pyruvate
“activating”
Bypass #2: Fructose 1,6-bisphosphatase replaces
PFK-1 of glycolysis
Energy Charge regulation on pathways
Energy Charge
- Low E.C.
- High [AMP]
- More energy needed
- Glycolysis stimulated
- High E.C.
- Lots of ATP present
- Biosynthetic reactions promoted
- Gluconeogenesis stimulated, e.g., in order to store glucose as glycogen
Fructose 2,6-bisphosphate regulation on pathways
Fructose 2,6-bisphosphate
- Acts mainly in liver
- Under starvation conditions
- Glucagon released
- F2,6BP levels decrease
- Gluconeogenesis stimulated (glucose is synthesized)
- In fed state
- Insulin released F2,6BP levels increase
- Glycolysis stimulated (need to break down glucose from food for energy storage)
Reciprocal Control of Glycolysis and Gluconeogenesis occurs at
- PFK-1/Fru-1,6-BPase
- PK/PC-PEPCK
The two pathways are responsive to enery charge
•Citrate and acetyl-CoA indicate cell is “flush” with biosynthetic precursors and energy
–gluconeogenesis activated
Bypass #3: Glucose 6-phosphatase
- Five protein components
- Large negative DG; irreversible; enzyme is Mg2+-activated
- Operative only in certain tissues (liver, kidney, small intestine lining); in other tissues, pathway stops at glucose 6-phosphate
- Lack enzyme; “want” to keep glucose as Glu-6-P, which cannot exit cell
- Phosphatase activity located on inner face of ER
- Glucose generated is transported out of cell via transporter GLUT2
Glycogen storage disease due to defect in Glu6phosphatase
Von Gierke’s (a GSD)
Produces ribose-5-phosphate, needed for synthesis of DNA and RNA, as well as the coenzymes ATP, NADH, FADH2 and coenzyme-A
Especially important in rapidly dividing cells such as bone marrow, skin, and intestinal mucosa—also tumors
The pentose phosphate pathway
Two phase of pentose phosphate pathway
oxidative and non-oxidative
Location of pentose phosphate pathway
cytosol
Glutathione exists in two interconvertible forms:
2 GSH (red.) = GSSG (ox.)
GSH protects against
ROSs; it keeps RBCs in reduced state (2 GSH=GSSG)
GSSG converted to GSH via Glutathione Reductase, which uses_____ as a cofactor
NADPH
Gluconeogenesis
Glucose CAN be made from
odd-chain fatty acids acids (from propionyl-CoA (succinyl-CoA)) and from the glycerol of triglycerides
Glycerol conversion
The first of the bypass steps occurs in __________ but the rest occurs in the _________
mitochondria; cytosol
The pyruvate carboxylase reaction occurs in two phases:
- carboxylation of biotin
- tranfer of CO2 group from carboxybiotin to pyruvate
CO2 (HCO3-) must be “activated” through phosphorylation first
Absolute requirement for acetyl-CoA, a potent allosteric activator
Oxaloacetate + GTP —> (yields)
PEP + CO2 + GDP
–Although DG⁰′ for pyruvate → PEP is small, DG in cell is large and negative because [PEP] is very low; therefore the 2-step process is __________
“irreversible”
Stimulates transcription of the PEPCK gene (and others)
glucagon
Glucagon also induces expression of other enzymes:
And represses transcription of:
- Fru-1,6-BPase
- Glu-6-PPase
- various aminotransferases
- It represses transcription of glycolytic enzymes PFK1, PK and glucokinase
- Insulin has opposite effect
The Malate-Aspartate Shuttle
By means of malate shuttle, _________ produced in mitochondria is made available for a later step o of gluconeogenesis in cytosol where [] is low
NADH
•A different route is followed when lactate is the precursor
–Since NADH is produced in cytosol by LDH, shuttle unnecessary
Fructose 1,6-bisphosphate + H2O→
→ Fructose 6-phosphate + Pi
Mg2+ -dependent; essentially irreversible
Bypass #2 is tightly and reciprocally controlled _______and _______
allosterically and hormonally
–When one pathway is “on,” the other is “off”
–Prevents uncontrolled “futile cycling,” which wastes energy (6 – 2 = 4 NTPs)
–But futile cycling can play role in fine-tuning regulation of flux through the two pathways
The Pentose Phosphate Pathway meets the needs of all organisms for NADPH, which is used for reductive biosynthesis of a number of important biomolecules like:
- Fatty acids
- Cholesterol annd the steroid hormones
- nucleotides
- Neurotransmitters
–Protection against reactive oxygen species (ROSs) arising from oxidative stress, which is especially important in erythrocytes and lens and cornea of the eye
Where does the Pentose Phosphate Pathway Occur?
Oxidative Generative of NADPH
- Steps of oxidative phase are __________
- Controlled by the ____________
- Increased reductive biosynthesis causes __________
- When demand for NADPH slows, ____________
- irreversible
- NADP+ -to- NADPH ratio
- NADP+ to rise, leading to allosteric activation of Glu6PDHase & greater flux through PPP
- [NADP+] drops and PPP slows
causes cellular damage; especially harmful to RBC’s
ROSs
GSH acts as “sulfhydryl buffer” in RBCs:
- Maintains Cys residues of Hb in reduced state
- Keeps iron in reduced, Fe2+, state
- Detoxifies peroxides and hydroxyl free radicals
•Otherwise: Hb tetramers-Heinz bodies-hemolysis-hemolytic anemia
