Gluconeogenesis/Ketogenesis Flashcards
What is gluconeogenesis? precursors? where does it happen?
- synthesize glucose from non-carbohydrate precursors when glucose levels are low
- precursors:
pyruvate, lactate, propionate, glycerol, and amino acids
-tissues:
liver (major), kidneys and small intestine (minor)
- early stage of fasting, kidney provides 10% of glucose and during extended fasting, kidney provides 40%
- supports plasma glucose levels in brain, testes, RBCs, and kidney medulla
- insulin/glucagon low
3 irreversible steps of glycolysis?
1- Glucokinase/hexokinase
3- PFK1
10-Pyruvate kinase -these barriers must be overcome in GNG
What is the first barrier to overcome in GNG? (7)
barrier: pyruvate to PEP
1. Pyruvate to oxaloacetate by pyruvate carboxylase
- enzyme to overcome is pyruvate kinase
- uses 1 ATP
- Biotin as cofactor, covalently bound
- allosterically activated by acetyl CoA
2. OAA to PEP by PEP carboxykinase
- either by OAA to PEP directly, using 1 GTP in mitochondria
- or by OAA to Malate (in mitochondria and then transported to cytosol) to OAA to PEP by cytosolic PEP carboxykinase using 1 GTP
- Notes:
- pyruvate must go into the mitochondria and through several steps to bypass this step
- shuttle is used, bidirectional, malate or aspartate must move from mitochondria to cytosol
- H from NADH is moved from mitochondria to cytosol to be used for glycolysis later
Glycolysis vs GNG in 1st barrier? (8)
Glycolysis last step:
-one enzyme (pyruvate kinase) generates 2 ATP
GNG 1st step:
- two enzymes (pyruvate carboxylase and PEP corboxykinase) use 2 ATP
- PEP is in cytosol and OAA is in mitochondria so transportation across mtmemebrane must take place
What is the 2nd barrier to GNG? (9)
- F1,6bisP to F6P
- enzyme used is F1,6bisPase (overcomes PFK-1 in step 3 of glycolysis)
What is the 3rd barrier to GNG? (9)
- G6P to glucose
- enzyme used is G-6-phosphatase (overcomes hexokinase in step 1 of glycolysis)
Glycolysis vs GNG? enzymes? location? energy used?(10)
- enzymes are the same except:
- pyruvate to PEP (step 10)
- F1,6bisP to F6P (step 3)
- G6P to glucose (step 1)
- location:
- glycolysis in cytosol
- GNG in cytosol and mitochondria -glycolysis generates 2 ATP
- GNG consumes 6 ATP
Precursors to GNG? (11)
- pyruvate (reverse glycolysis)
- lactate (turns into pyruvate)
- glycerol (glycerol 3 phosphate and DHAP)
- propionate
- amino acids (alanine can turn into pyruvate, 18 can go into TCA)
How are lactate and alanine used for GNG? (12)
- lactate to pyruvate catalyzed by lactate dehydrogenase, using NAD to transfer H back to NADH, reversible
- alanine to pyruvate using aminotransferase (transaminase), reversible
What is the Cori cycle? (13)
- glucose lactate cycle
1. in RBC, no mitochondria, and high concentration of pyruvate dehydrogenase, so lactate is formed
2. blood carries lactate to liver
3. lactate turns into glucose by GNG - in each cycle, 6 ATP are spent in the liver and 2 ATP are generated in RBC
Glucose alanine cycle? (14)
- anaerobic exercise
1. glycolysis in muscle generates pyruvate which turns into alanine
2. NH2 (ammonia- toxic) is carried on alanine from muscle to liver where it is used to make urea
3. alanine converts back to pyruvate to be used for GNG in liver - input 6 ATP in liver to generate 2 ATP in muscle
Glycerol in GNG? process of turning to glucose?(15)
- glycerol is 3C molecule
- backbone of triacylglycerol (fat) with 3 fatty acids
1. glycerol leaves fat and enters blood to be carried to liver
2. in liver, glycerol kinase turns glycerol to G3P using one ATP - adipose tissue does not have glycerol kinase to it cannot use glycerol
3. G3P dehydrogenase catalyzes G3P into DHAP transferring H from NAD to NADH
4. DHAP is an intermediate in glycolysis so it can go in reverse to generate glucose
Propionate in GNG? process of getting glucose? (16)
- 3C acid molecule comes through beta oxidation(cut 2C off each time), sometimes 3C are left
1. propionate to propionyl CoA bio thiokinase, energy used, high energy
2. carboxylation using biotin to methyl malonyl CoA
3. mutase catalyzes the formation of succinyl CoA using Vit B12
4. succinyl CoA goes through TCA cycle to reach OAA, go in reverse glycolysis to get glucose
What amino acids are used for GNG? ketogensis? (17)
- GNG:
- 18 of 20 are used except lysine and leucine
- carbon skeleton of most amino acids can used to make glucose
- different entry points because of different structure
KG:
- tryptophan
- isoleucine
- phenylalanine
- tyrosine
When do you need to conduct GNG?
plasma glucose levels are low
where is the carbon backbone from in GNG?
proteins (amino acids) or glycerol
where is the energy driving GNG?
fatty acid catabolism (acetyl CoA)
-regulation comes from fatty acid beta oxidation
How to coordinate cell to cell communication in GNG?
hormones (insulin, glucagon)
Glucose usage vs storage per day?
- use 160g/day (120g in brain, 40g in RBC)
- store 190g glycogen
- 20g free floating in fluid
- enough storage for about one day
- GNG starts before depletion of glycogen
Regulation of GNG overall picture? (20)
- decreased insulin/glucagon ratio drives the breakdown of TG into fatty acids
- free fatty acids enter the mitochondria where they are converted to acetyl CoA by beta oxidation, forming NADH
- NADH and acetyl CoA inhibit pyruvate dehdyrogrenase so that pyruvate is converted into OAA rather than more acetyl CoA
- pyruvate carboxylase is activated by acetyl CoA
- pyruvate is converted into OAA
- OAA is converted directly into PEP by PEP carboxykinase (induced by PKA), transported out of mitochondria
- this is dominant at beginning because there is plenty of NADH in cytosol to complete GNG - OAA is indirectly converted to malate, transported out of mitochondria with NAD and converted back to OAA, forming NADH
- this is dominant when there is not enough NADH in cytosol to carry out GNG, NADH is transported to cytosol with Malate - OAA can also be converted to aspartate by aminotransferase and then transported out to cytosol by anti porter glutamate aspartic transporter, turns back to OAA
- ammonia carried will be made into urea - pyruvate carboxykinase is induced by high glucagon to convert OAA into PEP
- pyruvate kinase (PK) is inhibited by increased glucagon which activates cAMP to phosphorylate PK so that it does not form more pyruvate, but completes GNG
- glucose is formed by GNG
Explain the hormonal regulation of F2,6bisP? (22, 23, 24)
- increased glucagon (PKA) phosphorylates F2,6Pase to convert more F2,6bisP to F6P
- lack of F2,6bisP inhibits PFK-1 and does not inhibit F1,6Pase, so increased gluconeogenesis - increased insulin (phosphatase) dephosphorylates PFK-2 to convert more F6P to F2,6bisP
- F2,6bisP stimulates PFK-1 to increase glycolysis and inhibits F1,6Pase to decrease gluconeogenesis
What are F1,6Pase and G-6-phosphatase induced by? regulation of 2nd and 3rd barriers? (25)
- glucagon
- G-6-phosphatase is located in ER so G6P needs to translocated to ER by G6P translocase
- deficiency of either proteins (translocase or G6Pase) leads to glycogen storage disease (can synthesis but not use)
- glucokinase nuclear translocation prevents glucose phosphorylation
What is going on during short term fasting? (27)
- lower blood glucose
- lower insulin levels
- higher glucagon - phosphorylation of enzymes to turn glycogen into G6P which turns into glucose to be transported out into blood (brain, RBC)
- brain uses glucose for energy
- RBC convert glucose to lactate in Cori cycle and transport it to liver (11)
- adipose breaks down triacylglycerol into glycerol and free fatty acids
- glycerol goes to liver for GNG (12) - fatty acids go to liver or muscle to be turned into acetyl CoA
- acetyl CoA goes to make ketone bodies
- ketone bodies go to muscle where they are converted back to acetyl CoA, and ATP
- amino acids broken down in muscle, sent to liver to be used in GNG (alanine)
- ammonia from amino acids will be used to make urea
Long term starvation? (28)
- low glucose
- low insulin
- high glucagon
- glycogen depleted - glucose from liver mainly goes to RBC to support cori cycle because RBC can only use glucose
- amino acids are used sparingly
- adipose tissue is broken down into glycerol for GNG and free fatty acids
- fatty acids are converted to acetyl CoA in muscle and liver to make ATP and ketone bodies - ketone bodies go straight to brain to make ATP
- brain prefers to use ketone bodies but usually use glucose because KBs are not available
- in this case, brain uses KBs to save glucose for RBC which cannot use anything else - urea and urine is reduced
summary graph of short term and long term fasting? (29)
graph
- ketone bodies increase exponentially
- fatty acids increase linearly
- how long you can last without food, depends on how much fat you have
How are triacylglycerols mobilized in fat to liver during fasting state? hormones and enzymes?(30)
- decreased insulin
- increased glucagon, epinephrine, norepinephrine, AMPK (sensing levels of AMP, low energy means more AMP)
- hormone sensitive lipase (HSL) is phosphorylated and is activated
- lipase breaks down diester bond from triacylglycerol to form glycerol and 3 fatty acids
- glycerol goes to liver which has glycerol kinase to produce DHAP for GNG
- adipose lacks enzyme to re-use glycerol - fatty acid is carried by albumin in blood to muscle and liver for beta oxidation for energy (acetyl CoA in liver as well for ketone bodies)
Process of synthesis of ketone bodies? (32)
- 2 acetyl CoAs are combined by beta-ketothiolase to form acetoacetyl CoA
- another acetyl CoA is added by HMG-CoA synthase to form HMG-CoA (rate limiting-irreversible)
- HMG-CoA lyase removes one of the original acetyl CoAs to form acetoacetate
- spontaneously loses CO2 to form acetone
- the rest is reduced by beta hydroxybutarate dehydrogenase to form beta hydroxybutarate
- these 3 molecules (beta-hydroxybutarate, acetone, and acetoacetate) are considered the ketone bodies
- only can use two of them for energy (acetone is not usable form of energy, it is evaporated)
Summary of ketone bodies synthesis?
- increased acetyl CoA due to increased beta oxidation
- idling of TCA cycle - a lot of NADH which inhibits TCA
- HMG-CoA synthase is rate limiting (irreversible) is induced by glucagon
- liver cannot use KBs because they lack beta-ketoacyl -CoA transferase
- KBs used by muscle, kidney, brain
How do certain tissues use Ketone bodies? (35)
- heart, muscle, brain use them
1. beta-hydroxybutarate (more energy rich) carries two extra H’s compared to acetoacetate, which are transferred to NADH for energy
2. the enzyme, beta-ketoacyl-CoA transferase is necessary for ketone bodies to be used
3. this enzyme takes the CoA from succinyl CoA and adds it to acetoacetate to make acetoacetyl-CoA
4. acetoacetyl CoA is turned into two acetyl CoAs by beta-ketothiolase
5. acetyl CoA goes through TCA to form ATP
Summary of everything? (37)
explain pic
