• After long fasts, between meals or vigorous exercise, glycogen reserves are depleted
• at these times the body needs to synthesis glucose from noncarbohydrate precursors
• the body converts pyruvate and related 3/4 carbon compounds into glucose

• all micoorganisms, plants, fungi and animals do it and in all tissues
• precursors include lactate, pyruvate, glycerol and certain amino acids
•  focus on the pathway in the mammalian liver

How and why does Gluconeogenesis Differ from Glycolysis?

• shares 7 of the 10 reverse steps of glycolysis but 3 of the glycolysis steps are irreversible in vivo
• Steps with a very negative delta G in glycolysis are bypassed by a separate set of enzymes
• these three steps are conversion of glucose to G6P by hexokinase, phosphorylation of F6P to F1,6 BP by PFK, and conversion of PEP to pyruvate by PK

• components in cytosol and mitochondria
• pyruvate transported from cytosol-->mitochindria or generated from alanine in mito. via transamination
• then pyruvate carboxylase, which requires coenzyme biotin, converts pyruvate to oxaloacetate
  
• [image]
  
• biotin carries activated bicarbonate and HCO3- is phosphorylated by ATP to form a carboxyphosphate
• biotin displaces the phosphate in formation of carboxybiotin
• [image]
• This reaction is posiively regulated by Acetyl CoA (created by fatty acid oxidation, signaling an increase in fatty acids
  

• Mitochondria membrane has no oxaloacetate transporter and the oxaloacetate formed from pyruvate must then be reduced to malate by malate dehydrogenase
• oxaloacetate+NADH + H(+) ->Lmalate+ NAD(+)
• malate leaves mitochondria thru transporter and oxidized back to oxaloacetate
• Malate + NAD(+)-> Oxaloacetate + NADH + H(+)

• Oxaloacetate -> PEP by phosphoenolpyruvate carboxykinase
• Oxaloacetate + GTP-> PEP + CO2 + GDP
• reversible
• [image]
• Summary of 1st bypass: pyruvate + ATP + GTP + HCO3(-) -->PEP + ADP + GDP + Pi + CO2, delta G cell = -25 kj/mol (- because [PEP] is low; used quickly)

• [NADH]/[NAD+] in cytosol is 10^5times lower than in mitochondria
• b/c NADH is consumed in gluconeogenesis(1,3 BPG ->G3P), NADH needs to be available to make glucose
• transport of malate from mitochondria to the cytosol and its conversion there to oxaloacetate moves reducing equivalents to the cytosol (where [NADH] is low...)
• this balances NADH made/used in cytosol during gluconeogenesis

• lactate produced by erythrocytes or anaerobic muscle
• isozyme for PEP carboxykinase (encoded by different genes for mito/cyto localization)
• [image]

• FBPase-1 promotes irreversible hydrolysis of C-1 Phosphate
• F1,6BP + H2O -> F6P + P1, standard delta G = -16.3

• final reaction of gluconeogenesis
• to reverse hexokinase a phosphoryl group transfer to ADP would have to occur, which is energetically unfavorable
• Glucose 6-phosphatase  hydrolyzes the phosphate ester:G6P + H2O-> glucose + Pi      std delta G = - 13.8 kj/mol
• occurs in ER of hepatocytes, which makes sense because if other tissues had this enzyme, G6P used for gycolysis would be degraded

• 2 Pyruvate + 4ATP + 2GTP + 2NADH + 2H(+) +4H2O->glucose + 4 ADP + 6Pi + 2NAD(+)
• the expensive process essentially ensures that gluconeogenesis is irreversible!

• Intermediates in TCA like citrate, isocitrate, a-ketoglutarate, succinyl-CoA, succinate and fumarate can be oxidized to oxaloacetate
• alanine and glutamine, which transport amino groups from outside to inside the liver, after having the amino groups taken off in the mitochindria, the remaining C skeleton (pyruvate and a-ketoglutarate) are used for gluconeogenesis

• mammals can't convert fatty acids to glucose
• catabolism of fatty acids yeilds only acteyl-CoA, tho plants and fungi can convert it to oxaloacetate
• mammals can use triaglycerols(fats)  to make glucose