Chapter 17 Flashcards
Gluconeogenesis & pathway overview
- Pyruvate to glucose
- Gluconeogenesis = synthesis of glucose from non-carbohydrate precursors
- Gluconeogenesis = NOT a complete reversal of glycolysis
–> some enzymes for reversible steps of glycolysis will remain same during gluconeogenesis (reversible steps will utilize same enzymes as in glycolysis)
- Major site = liver, can occur in kidney
- Important during fasting/starvation
- Gluconeogenic pathway converts pyruvate into glucose from non-carbohydrate precursors: lactate, amino acids, and glycerol
Pyruvate can be formed from muscle-derived lactate in liver by lactate dehydrogenase
Carbon skeletons of some amino acids can be converted into gluconeogenic intermediates
Glycerol (derived from hydrolysis of triacylglycerides) can be converted into dihydroxyacetone phosphate (DHAP), which can be processed by gluconeogenesis or glycolysis
Glucose = primary fuel for _____ and only fuel for _____
brain; RBC
List the 4-distinct enzymes in gluconeogenesis are needed to counter the irreversible steps of glycolysis
- Pyruvate carboxylase
- Phosphoenolpyruvate (PEP) carboxykinase (PEPCK)
- Fructose 1,6-biphosphatase
- Glucose-6-phosphatase
Enzymes for gluconeogenesis located in cytoplasm EXCEPT …
- pyruvate carboxylase (in mitochondria)
- glucose-6-phosphatase (membrane bound in ER)
Irreversible enzymes/steps of glycolysis and the enzymes/steps that bypass them in gluconceogensis
Irreversible steps and enzymes in glycolysis are hexokinase (glucose - G6P), PFK (F6P - F,16-BP), and pyruvate kinase (Phosphoenolpyruvate - pyruvate)
In gluconeogenesis, these 3 reactions are bypassed by the following steps & enzymes:
ENZYME Glucose 6-phosphatase converts glucose 6-phospahte plus H2O into glucose + Pi
ENZYME Fructose 1,6-biphosphatase converts fructose 1,6-biphosphate plus H2O into fructose 6-phosphte + Pi
ENZYME Phosphoenolpyruvate carboxykinase (PEPCK) converts oxaloacetate + GTP into phosphoenolpyruvate + GDP + CO2
ENZYME Pyruvate carboxylase converts pyruvate + CO2 + ATP + H2O into oxaloacetate + ADP + Pi
- Pyruvate to oxaloacetate via enzyme Pyruvate Carboxylase
- Conversion of pyruvate to oxaloacetate at expense of 1 ATP
- Pyruvate = 3 carbon molecule. Oxaloacetate = 4 carbon molecule. Three steps:
- Bicarbonate (HCO3) is phosphorylated, 1 ATP is used
- CO2 from biotin is transferred to biotin arm of enzyme, called carboxybiotin
–> requires vitamin B7 (biotin) as a cofactor
- CO2 is added to pyruvate, allows for generation of oxaloacetate
** This step occurs in mitochondria, but ALL OTHER ENZYMES for gluconeogenesis exist in cytoplasm
** In aqueous solution, CO2 exists as bicarbonate
Biotin as enzyme swing arm
Biotin = covalently attached prosthetic group that serves as the carrier of activated
- Pyruvate carboxylase requires vitamin B7 (biotin) as cofactor
- Biotin serves as carrier of activated CO2
- Carboxylate group of biotin is linked to lysine
- Biotin is covalently attached to biotin carboxylase carrier domain
- Biotin transports CO2 from biotin carboxylase active site to pyruvate carboxylase active site of an adjacent subunit (acting as swing arm, or tether)
–> biotin is NOT carboxylated UNLESS acetyl CoA is present (allosteric regulation)
Oxaloacetate must be transported to the cytoplasm to complete the synthesis of phosphoenolpyruvate. Explain this process.
- There is no specific transport of oxaloacetate from mitochondria into cytoplasm, thus, to complete the synthesis to phosphoenolpyruvate, oxaloacetate is reduced to malate, then transported into cytoplasm
- This involves enzyme malate dehydrogenase
- In cytoplasm, malate is re-oxidized to re-form oxalacetate, which alongside generates cytoplasmic NADH
- NADH is utilized in subsequent steps of gluconeogenesis
- Oxaloacetate to phosphoenolpyruvate via enzyme phosphoenolpyruvate carboxykinase (PEPCK)
- Phosphoenolpyruvate carboxykinase (PEPCK) generates phosphoenolpyruvate from oxaloacetate in phosphorylation and decarboxylation reactions
- This step occurs at expense of 1 GTP
- Phosphoryl donor is GTP. CO2 that was added to pyruvate by by pyruvate carboxylase gets taken off
**addition of phosphoryl group to pyruvate is very unfavorable and an endergonic reaction
**carboxylation and decarboxylation reactions are favorable and used to power phosphorylation
**CO2 that was added by pyruvate carboxylate comes which helps set up the unfavorable reactions
** decarboxylations often drive reactions that are highly endergonic
- Fructose 1,6-BP to F6P via enzyme fructose 1,6-bisphosphatase
Phosphoenolpyruvate is metabolized by the enzymes of glycolysis in the reverse direction until the next irreversible step: hydrolysis of fructose 1,6-biphosphate)
- Enzyme for this reaction = fructose 1,6-biphosphatase (highly regulated allosteric enzyme)
- This step and activity is reciprocal of enzyme of PFK in glycolysis
- G6P to glucose via enzyme glucose 6-phosphatase
- F6P is converted into G6P by phosphoglucose isomerase (not regulated or irreversible step)
- Free glucose is generated from G6P via glucose 6-phosphotase (important control point)
- Takes place in liver and reverses the activity of glucokinase in liver
- Glucose 6-phosphotase is ER membrane anchored and reactions catalyzed by this enzyme take place on inner surface of ER
** this allows the phosphate to be removed and glucose to move out of the cell via GLUT transporters
- Other tissues (muscle) lack this phosphate; thus gluconeogenesis occurs up to point of generating G6P
- G6P is then added to top up tissue glycogen rather then export free glucose
Net reaction of gluconcegenesis
- Net reaction for gluconeogenesis is energetically unfavorable unless it is coupled to favorable reactions
- Cost of gluconeogenesis = 4 ATP and 2 GTP w/ 6 high-transfer potential phosphoryl groups required in synthesis of glucose from pyruvate
2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 2(H+) + 6 H2O –> glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+
- Evidence of coupling (ATP/decarboxylation) drives unfavorable reactions
- Glycolysis: only 2 ATP net are generated
- Gluconeogenesis and glycolysis are reciprocally regulated so that within a cell, one pathway is relatively inactive while other is highly active
** The rationale for reciprocal regulation**
Glycolysis will predominate when glucose is abundant
Gluconeogenesis will be highly active when glucose is scarce
The level of fructose 2,6-bisphosphate (F-2,6-BP) is ____ in the fed state and _____ in starvation
high; low
The key regulation site in the gluconeogenesis pathway is the interconversion of
a) glucose and glucose 3-phosphate
b) fructose 3-phosphate and fructose 1,3-bisphosphate
c) fructose 6-phosphate and fructose 1,6-bisphosphate
c) fructose 6-phosphate and fructose 1,6-bisphosphate
Fill in the blanks with regards to the key regulation site of the gluconeogenesis pathway.
Consider first a situation in which energy is needed. In this case, the concentration of AMP is _____. Under this condition, AMP stimulates _______ but inhibits _________. Thus, glycolysis is turned _____ and gluconeogenesis is ______
high; PFK; fructose 1,6-bisphosphatase; on; inhibited
