Glycolysis, Glycogen, Gluconeogenesis, etc Flashcards
What are the glucose transporters?
GLUT 1- 4
What is GLUT 2?
low-affinity transporter in hepatocytes and pancreatic cells; capture excess glucose primarily for storage (picks up excess glucose and stores it preferentially after a meal)
What is GLUT 4?
in adipose tissue and muscle and responds to glucose concentration in peripheral blood
What is glycolysis?
a cytoplasmic pathway that converts glucose into two pyruvate molecules, releasing a modest amount of energy captured in two substrate-level phosphorylations and one oxidation reaction
What is the Km of GLUT 2?
~15 mM (high)
What is the Km of GLUT 4?
~5 mM (low)
Can GLUT 2 be saturated under normal glucose levels?
No; not at normal physiological conditions
Can GLUT 4 be saturated under normal glucose levels?
Yes; saturated when glucose levels are only slightly above 5 mM
How does glucose enter the cell?
facilitated diffusion or active transport
What do kinases do in general?
attach a phosphate group from ATP to their substrates
What are two kinases involved in glycolysis?
hexokinase and glucokinase
What do kinases in glycolysis do?
convert glucose to glucose 6-phosphate
What prevents glucose from leaving the cell?
GLUT transporters that move glucose in and out of the cell are specific to glucose and not Phosphorylated glucose so it cannot leave the cell
Describe hexokinase
widely distributed in tissues and is inhibited by its product, glucose 6-phosphate
Describe glucokinase
found only in liver cells and pancreatic beta-islet cells; induced by insulin in liver
What are liver cells called?
hepatocytes
What is the Km of hexokinase?
low; reaches maximum velocity at low glucose
What is the Km of glucokinase?
high; acts on glucose proportionally to its concentrations
What is phosphofructosekinase-1 (PFK-1)?
rate-limiting enzyme and main control point of glycolysis; phosphorylates fructose 6-phosphate to fructose 1,6-biphosphate
PFK-1 is inhibited by _____ and _____ and turned on by _____
ATP and citrate; AMP
How does insulin stimulate PFK-1?
Insulin activates phosphofructokinase-2 (PFK-2) which converts a tiny amount of fructose 6-phosphate to fructose 2,6- biphosphate (F2,6-BP) which activates PFK-1
How does glucagon inhibit PFK-1?
it inhibits PFK-2 which lowers F2,6-BP and inhibits PFK-1
Where is PFK-2 found?
mainly in the liver
Why is PFK-2 useful?
it activates PFK-1 and overrides PFK-1’s need for ATP, allowing the cell to continue glycolysis even when energetically satisfied and can be put into storage molecules
What does glyceraldehyde-3-phosphate dehydrogenase do?
catalyzes an oxidation and addition of inorganic phosphate (Pi) to its substrate, glyceraldehyde 3-phosphate; results in the production of a high-energy intermediate 1,3-biphosphoglycerate and the reduction of NAD+ to NADH
What does 3-phosphoglycerate kinase do?
transfers the high-energy phosphate from 1,3-biphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate
What is substrate-level phosphorylation?
when ADP is directly phosphorylated to ATP using a high-energy intermediate
What does pyruvate kinase do?
catalyzes a substrate-level phosphorylation of ADP using the high-energy substrate phosphoenolpyruvate (PEP); activated by fructose 1,6-biphosphate from the PFK-1 reaction
What is feed-forward activation?
the product of an earlier reaction of glycolysis stimulates a later reaction
What is lactase dehydrogenase?
oxidizes NADH to NAD+; reduces pyruvate to lactase
What is dihydroxyacetone phosphate (DHAP)?
used in hepatic and adipose tissue for triacylglycerol synthesis; formed from fructose 1,6-biphosphate
What is 1,3-biphosophoglycerate (1,3-BPG)?
a high energy intermediate used to generate ATP by substrate-level phosphorylation
What is phosphoenolpyruvate (PEP)?
a high energy intermediate used to generate ATP by substrate-level phosphorylation
What are three enzymes used for irreversible reactions in glycolysis?
glucokinase/hexokinase
PFK-1
Pyruvate kinase
What is biphosphoglycerate mutase?
produces 2,3-biphosphoglycerate (2,3-BPG) from 1,3-BPG in glycolysis
What are mutases?
enzymes that move a functional group from one place in a molecule to another
What does 2,3-biphosphoglycerate do?
binds allosterically to the beta-chains of hemoglobin A (HbA) and decreases its affinity for oxygen
What does galactokinase do?
phosphorylates galatose
What does galactose-1-phosphate uridyltransferase do?
converts galactose 1-phosphate to glucose 1-phosphate
What does fructokinase do?
phosphorylates fructose
What does the pyruvate dehydrogensase complex reaction do?
converts pyruvate to acetyl-CoA
What is glycogen?
a branched polymer of glucose that acts as a storage form
What is starch?
long alpha-linked chains of glucose
What is glycogenesis?
synthesis of glycogen granules
What does glycogen synthase do?
forms the alpha-1,4 glycosidic bond found in the linear glucose chains of the granule; rate limiting enzyme of glycogen synthesis
What is glycogen synthase inhibited and stimulated by?
stimulated by glucose 6-phosphate and insulin; inhibited by epinephrine and glucagon
What does branching enzyme/glycosyl alpha 1,4:alpha-1,6 transferase do?
responsible for introducing alpha-1,6-linked branched into the granule as it grows
What does glycogen phosphorylase do?
breaks alpha-1,4 glycosidic bonds, releasing glucose 1-phosphate from the periphery of the granule; the rate-limiting enzyme of glycogenesis
What does debranching enzyme do?
two-enzyme complex that deconstructs the branches in glycogen that have been exposed by glycogen phosphorylase
What are isoforms?
slightly different versions of the same protein
What are glycogen storage diseases?
diseases that are caused by the accumulation or lack of glycogen in one or more tissues
Which organ maintains gluconeogenesis?
liver; kidney to a lesser degree
what is pyruvate carboxylase?
a mitochondrial enzyme that is activated by acetyl-CoA (from beta-oxidation)
What is phosphoenolpyruvate carboxykinase (PEPCK)?
induced by glucagon and cortisol, which generally act to raise blood sugar levels`(used in glucogenesis)
