Glycolysis/TCA Flashcards
What is the difference between hexokinase and glucokinase
They both convert glucose into glucose-6-phosphate.
- Blood glucose at ALL levels
Hexokinase: provides glucose 6 phosphate.
-in all tissues
-low Km for glucose (Km is amount of substrate concentration to reach (1/2 the maximum velocity). So if Km is LOW, that means less glucose is needed to become saturated. Therefore it works high at all concentrations.
-Has feedback inhibition*** - Blood glucose is high (insulin action)
- found in liver
- has a high Km so needs high levels of glucose
- it is induced by insulin**
- no feedback inhibition*
How does glucokinase have regulation?
In the presence of fructose 6 phosphate, GKRP forms a ternary complex that exhibits competitive inhibition of glucokinase
In welled state, glucose accumulation leads to displacement of fructose 6 phosphate/GKRP complex and GK activity increases.
Regardless of glucose concentration, fructose 1-phosphate (from plasma fructose) competitively binds to GKRP and causes the ternary complex to dissociated, making GK activity uninhibited.
What is phosphofructokinase and what is its regulation.
PFK2
Phosphofructokinase is the first committed step of glycolysis. It will phosphorylate glucose -6 -phosphate to make fructose-1,6-biphosphonate.
To make fructose - 2,6- bis-phosphate.
In the liver it functions rapidly when blood glucose is HIGH
in muscle it functions rapidly when ATP is LOW
Regulation ———–
-abundance of high energy compounds inhibit PFK-1 (**ATP and CITRATE)
-it is activated by low energy molecules like AMP.
——————————
-it has allosteric regulators (fructose 2,6-bisphosphate)
PFK2
(like HMGcoA reductase, insulin ACTIVATES by dephosphorylation
-glucagon inhibits PFK-2 by phosphorylation
PFK2 then makes fructose 2,6 biphosphate which activates PFK1**
this insulin, glucagon, PFK2 stuff is ONLY IN LIVER***
citrate is the inhibitor in muscle and AMP is activator.
What is pyruvate kinase and what is its regulation?
Pyruvate kinase converts phosphoenolpyruvate to pyruvate.
Like all the others, it is active in fed state because of presence of substrate
Allosteric regulation: it has feed forward activation by fructose 1,6 biphosphate.
Insulin promotes
Glucagon inhibits
What occurs in glucokinase disorder?
You will have elevated blood glucose. As a result you need higher blood glucose levels than normal to trigger insulin secretion. it is a form of diabetes and it is mild, just higher than normal blood glucose and can be managed by diet for HETEROZYGOTES
Both allelic mutations will lead to neonatal diabetes
ACTIVATING: disorder will cause hyperinsulinemia hypoglycemia that can cause life threatening seizures
Describe pyruvate kinase
deficiency:
there is a disruption of glycolysis and build up of intermediates.
- you have decreased ATP from glycolysis to drive membrane pumps
- there is an Na/K ATPase pump that exports sodium and imports potassium
- increased intracellular sodium leads to swelling and lysis - hemolytic anemia
- also more spherical so that it gets destroyed in spleen.
- they can’t maintain membrane integrity leading to spiculated cells **Burr cells)
- there is increased glycolytic intermediates like 2,3 BPG because 1,3 BPG is an intermediate of glycolysis and with accumulating substrate, there is mutase activity.
How do you stabilize blood glucose levels in patient samples?
You give fluoride which inhibits enolase, an enzyme that is part of glycolysis which will prevent the loss of plasma glucose
What happens to pyruvate?
Pyruvate can be converted to oxaloacetate which is a part of gluconeogenesis
converted to acetyl coA which enters TCA cycle
-can be converted to lactate which itself can be converted back into pyruvate.
What is the role of PDH
PDH conducts the irreversible reaction between (3C) pyruvate to acetyl coA (2C). That is the starting point of TCA.
Pyruvate is brought into mitochondrion and then converted there
in the process, Co2 is released (2CO2 from 6 carbon) and NADH is produced.
It is a complex of three enzymes ,E1, E2, E3.
with five cofactors (TLCFN) tender love and care for nancy
thiamine pyrophosphate, FAD, NAD< CoA and lipoic acid
Describe PDH and its mutations.
E1: is affected by thiamine deficiency which can cause wernicke-korsakoff syndrome and beriberi
E2: is affected by arsenic, mercury
E3:
Describe regulation of PDH complex
Allosteric control (like glycolysis), high energy charge inhibits it
- ATP
- Acetyl coA
- NADH
Low energy charge activates PDH
- ADP
- CoA
- NAD+
- pyruvate
- dicholoroacetate
Covalent modification
- dephosphorylation activates PDH (like insulin activating PFK2)
- phosphorylation inhibits PDH
Describe PDH deficiency
The brain uses glucose as a fuel so without it you degenerate the gray matter in the brain
Pyruvate will accumulate leading to alanine and lactate accumulation leading to lactic acidosis:
acidosis will cause hyperventilation
- muscle pain and weakness
- abdominal pain and nausea
treatment is a high fat, low carb diet (ketogenic diet)
-or dichloroacetate which stimulates PDH.
What are the 3 irreversible enzymes in the TCA cycle
- Citrate synthase - converts acetyl coA to citrate
- inhibited by citrate - Isocitrate dehydrogenase **committed step
- it converts isocitrate to alpha-ketoglutarate
- inhibited by NADH, ATP
- stimulated by ADF, Calcium - *alpha-ketoglutarate dehydrogenase - converts alpha-ketoglutarate to succinyl coA
- inhibited by NADH, succinyl coA
- stimulated by calcium
basically when energy level is high, TCA IS INHIBITED and when energy level is LOW, TCA is stimulated.
Describe the products of TCA
- net carbon yield is zero as two carbons are introduced as acetyl coA and two carbon are released as carbon dioxide.
- Oxaloacetate is converted to citrate with combination to acetyl coA. There must be a supply of carbons which is provided by conversion of pyruvate to oxaloacetate. Example of the anaplerotic reaction.
With one acetyl CoA
- 3 NADH / 3
- 1 FADH2 /2
- 1 GTP /1
12 ATP per cycle
24 ATP total from TCA, 2 ATP from glycolysis, 2 NADH2, and 6 ATP from PDH.
What regulates the three regulated enzymes of TCA?
Citrate synthase: inhibited by citrate
(committed step)Isocitrate > alpha ketoglutarate
Enzyme: isocitrate dehydrogenase is inhibited by NADH, ATP
stimulated by ADP and calcium (signs of ATP need)
alpha ketoglutarate > succinyl coA.
Inhibited by NADH and succinyl coA (product feedback)
Stimulated by calcium..hmmm
