Important enzymes for glycolysis Flashcards
Hexokinase
1) Converts glucose to glucose 6-phosphate
2) Inhibited by its product
3) Present in most tissues
4) Low Km
Glucokinase
1) Present in liver and pancreatic cells
2) High Km
3) Induced by insulin in hepatocytes
Phosphofructokinase-1 (PFK-1)
1) Inhibited by ATP and citrate. Activated by AMP.
2) Converts fructose 6-phosphate to Fructose 1,6 bisphosphate using ATP
3) Insulin stimulates, glucagon inhibits
What is the method by which insulin/glucagon stimulate and inhibit?
1) Insulin activates PFK-2 to convert a small amount of fructose 6-phsopate to fructose 2,6 bisphosphate.
2) Fructose 2,6 bisphosphate activates PFK-1
Glyceraldehyde 3-phosphate dehydrogenase
1) catalyzes an oxidation and addition of inorganic phosphate to its substrate glyceraldehyde 3-phosphate
2) This produces high energy 1,3-biphosphoglycerate and NAD+->NADH reduction.
3-Phosphoglycerate kinase
transfers the high energy phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate (Substrate level phosphorylation)
Pyruvate kinase
1) Last enzyme in glycolysis, catalyzes a substrate level phosphorylation of ADP using high energy substrate phosphoenolpyruvate (PEP).
2) Activated by fructose 1,6-bisphosphate from the PFK-1 reaction
Lactate dehydrogenase
key fermentation enzyme that oxidizes NADH->NAD+
Dihydroxyacetone phosphate (DHAP)
1) used in hepatic and adipose tissue for triacylglycerol synthesis.
2) Can be isomerized to glycerol 3-phosphate which can then be converted to glycerol
What are the high energy intermediates used to generate ATP via substrate level phosphorylation?
1) 1,3-Bisphosphoglycerate (1,3-BPG)
2) Phosphoenolpyruvate (PEP)
what are the irreversible enzymes of glycolysis?
1) hexokinase
2) glucokinase
3) PFK-1
4) Pyruvate kinase
Glycogenesis
synthesis of glycogen granules
What is the rate limiting enzyme of glycogen synthesis and what does it do?
Glycogen synthase. Forms the alpha-1,4 glycosidic bond found in the linear glucose chains of the granule.
Branching enzyme
responsible for introducing alpha1,6 linked braces into the granule as it grows
Glycogenolysis
the process of breaking down glycogen
glycogen phosphorylase
Breaks alpha1,4 glycosidic bonds releasing glucose 1-phosphate from the periphery of the cell
Debranching enzyme
Step 1) Breaks alpha 1,4 bond adjacent to branch point
Step 2) Forms new alpha 1,4 bond with small oligoglucose chain with the end of the old chain
Where does gluconeogenesis occur?
cytoplasm, mitochondria mostly in liver and some in kidneys
How does gluconeogenesis bypass pyruvate kinase?
1) Pyruvate carboxylase converts pyruvate into oxaloacetate
2) Phosphoenolpyruvate carboxykinase (PEPCK) converts oxaloacetate to phosphoenolpyruvate
3) Pyruvate carboxylase is activated by acetyl-CoA from Beta-oxidation
4) PEPCK is activated by glucagon and cortisol
How does gluconeogenesis bypass phosphofructokinase-1?
1) Fructose-1,6-bisphosphatase converts fructose 1,6-bisphosphate to fructose 6-phosphate
2) Activated by ATP directly and glucagon indirectly.
3) Inhibited by AMP directly and insulin indirectly
How does gluconeogenesis bypass glucokinase?
1) Converts glucose 6-phosphate to free glucose
2) Found only in endoplasmic reticulum of the liver
What is the purpose of the pentose phosphate pathway (PPP)?
1) Also known as the hexose monophosphate HMP shunt occurs in the cytoplasm of most cells generating NADPH and sugars for biosynthesis (from ribulose 5-phosphate)
What is the rate limiting enzyme of the pentose phosphate pathway?
1) Glucose 6-phosphate dehydrogenase
2) Activated by NADP+ and inhibited by NADPH and insulin
What are the two glucose transport channels?
1) GLUT 2: found in the liver for glucose storage and pancreatic beta-islet cells (glucose sensor). It has a high Km.
2) GLUT 4: found in adipose tissue and muscle and is stimulated by insulin. It has a low Km.
