Lecture 17 Flashcards
What is gluconeogenesis?
- indirect synthesis of glucose
- - not exactly the reverse of glycolysis
Which of the following is NOT a reason as to why our bodies spend energy to make glucose?
A) we can only store 190g of glucose at any time
B) stores of glucose only last for little over a day.
C) we need to make sure enough glucose is present for the brain
D) We don’t actually make glucose and its all a hoax
D
True or false.
The organs that are capable of gluconeogenesis are the liver and the kidney
true
True or false
non-carbohydrate precursors that can enter gluconeogenesis include glycerol, amino acids, and lactate
true
T or F, major tissue for gluconeogenesis is the LIVER and occurs mainly in cytosol, though some precursors come from the mitochondria
True
Why isn’t glucose synthesis simply the reverse of glucose metabolism?
– Energetics
– also there are 3 key reactions in glycolysis are energetically very favorable – i.e. deltaG is negative:
glucose –> glucose 6-P
fructose 6-P –> fructose 1,6-BP
PEP –> pyruvate
– this means the reactions are, essentially, irreversible and thus evolution needed a way around this to synthesize glucose
What are the different enzymes involved in gluconeogenesis?
- pyruvate carboxylase, – phosphophenol –pyruvate carboxykinase
- fructose 1 6 bisphosphatase and
– glucose 6 phosphatase
Where does gluconeogesis occur?
– cytosol except for the initial step that occurs in the matrix
Which enzyme is involved in the conversion of pyruvate to oxaloacetate?
– pyruvate carboxylase
– carboxylation of pyruvate occurs in mitochondria – oxaloacetate must then be sent to the cytoplasm for conversion to PEP
– occurs through a malate intermedediate and uses a malate transporter
What is the net input/output during the formation of oxaloacetate from pyruvate?
will input 1ATP, 1 HCO3 (bicarbonate). will release and ADP and Pi
Which cofactor is necessary for the first reaction of gluconeogesis?
biotin
True or false
There is a transfer of NADH from inside the matrix to outside the matrix during gluconeogenesis
True
Which enzyme is involved in the use and reformation of NADH/NAD+ during the first step of gluconeogesis
malate dehydrogenase
– because remember oxaloacetate is converted to malate intermediate
True or false
Lactate can be used as precursor to pyruvate and amino acids can be formed into oxaloacetate
True
Which enzyme is responsible for the formation of phosphophenol pyruvate (PEP) from oxaloacetate
PEP carboxykinase
– a decarboxylation and phosphorylation reaction converts oxaloacetate to PEP while burning a GTP
What is the next in/output of conversion of oxaloacetate to PEP?
– 1 GTP input and an output of one GDP and CO2
True or false
a carboxylation followed by a decarboxylation is a coupling event that occurs in gluconeogenesis in order to overcome the positive free energy of converting to PEP
True
How is gluconeogenesis different from glycolysis?
- energetics
- deltaGknot prime for reverse glycolysis = + 31 kJ/mol – Not going to happen
– gluconeogenesis uses a carboxylation/decarboxylation cycle to get PEP from pyruvate, and this is much more energetically favorable deltaG = -25 kJ/mol
– however, it has to utilize an ATP and a GTP, energetically expensive
T or F, after formation of PEP, gluconeogenesis proceeds as reverse glycolysis
True; reaction direction is determined by the concentrations of reactants, until we hit the fructose 1,6-BP step
What is the net output/input between PEP to FBP?
-1ATP -1NADH -1H20 and + 1 ADP + 1Pi
Which enzyme is responsible for the conversion of FBP to F6P
– FBPase –> allosteric enzyme under
– Fructose 1,6 - BP is converted to fructose 6-P by dephosphorylation
what are the products of the reaction of FPasE?
– one Pi
What enzyme is responsible for the formation of glucose from G6p?
– glucose 6-phosphatase
– Fructose 6-P readily converts to glucose 6-P
– this enzyme allows liver and kidney want to send glucose into the bloodstream
– this enzyme is also expressed primarily in those tissues
true or false GPase is present only in the liver and kidney
true
true or false
FPase is an allosteric enzyme
true
What is the next product of gluconeogenesis when 2 pyruvate are used?
- 1 glucose
- 4ADP
- 2GDP
- 6Pi
- 2NAD+
- 2H+
How many NTPS are burned per glucose regenerated from pyruvate?
- 6
- 4ATP
- 2GTP
T or F, lactate can serve as a precursor for gluconeogenesis via the Cori Cycle
True
T or F, under heavy exertion, lactate produced by muscles isn’t wasted, as it can be regenerated into glucose by the liver
True
Which two amino acids are mainly involved in the gluconeogenic pathway?
– Alanine and Glutamine
When would amino acid metabolism occur?
– when fasting or starving, muscle protein will breakdown
– main source of pyruvate
What are major control mechanisms?
– substrate-level control, concentration of reactants or products of the reaction activating/inhibiting the reaction
– feedback control, products of Citric Acid Cycle influencing enzymatic reactions.
At high ATP concentrations, flux through Citric Acid cycle slows and intermediates accumulate
Describe phosphofructokinase regulation
– PFK an allosteric enzyme that is activated (R state) by low energy charge (low ATP) and inhibited by high energy
– ATP is also a substrate for the reaction, so binds in the active site
– it binds at the allosteric site with lower affinity so only inhibits at HIGH ATP concentrations
True or false
PFK is a homotetramer
true
What are the pathways that F6P can take in the context of glucose breakdown?
it can turn into F16BP or turn into F26BP
True or false
The non phosphorylated PFK2 enzyme stimulates glycolysis
True
– regulation by phosphorylation, activating one activity versus the other
– kinase activity versus phosphatase activity is controlled by hormone regulation in liver
How does the conformation of PFK change when phosphorylated vs not?
– when phosphorylated the FBpase site is connected and the kinase site is separated, resulting in breakdown of fructose 26 bisphosphate.
– When not, the kinase site is together and the phosphatase site is not, resulting in increase kinase activity and formation of F26BP
True or false
insulin stimulates FBP2 by increasing protein phosphatase activity
True
True or false
glucagon exerts its influence on F2BP levels by increasing activity of cAMP-dependent protein kinase
True
True or false
pyruvate kinase is allosterically activated by F16BP
true
Which of the following is pyruvate kinase not inhibited by?
A) acetyl coa
B) ATP/alanine
C) F16BP
D) cAmp dependent phosphorylation in the liver
C
true or false
PC and PEPCK are activated by acetyl goa
true
True or false
glucagon stimulates PEPCK and insulin inhibits it
true
true or false
hexokinase is inhibited by G6P in non-liver cells
true; however liver isozyme (hexokinase IV/glucokinase) is NOT subject to feedback inhibition
Which of the following stimulate PFK activity? A) Citrate B) ATP C) AMP/ADP D) G6P
C; AMP, ADP and also F2, 6 BP
True or false
citrate and ATP stimulate FBPASE
false. inhibit pfk
true or false
FBpase is inhibited by F26P and amp
true
true or false
the reactions between G69 and glucose and vice versa are regulated by substrate level control except in the liver
true
Describe the regulation of Hexokinase
– inhibited by product G-6-P; however, the liver isozyme (hexokinase IV/glucokinase) is NOT subject to feedback inhibition
Explain how insulin and glucagon activate glycolysis and gluconeogenesis.
– insulin activates kinase activity (high glucose, F2, 6B activates Glycolysis inhibits gluconeogenesis)
– glucagon activates phosphatase activity (low glucose, no activation of Glycoloysis)
T or F, pyruvate kinase is allosterically inhibited by: Acetyl- CoA
True, also ATP, amino acids and hormonal regulation in liver isoforms
T or F, PC and PEPCK are activated by Acetyl-CoA
True
T or F, glucagon tells us about low blood sugar
True, this activates the synthesis of phosphoenol-pyruvate carboxykinase
