T03 - Carbohydrate Metabolism II Flashcards
The allosteric mode of regulation acts on what order of time?
milliseconds
The protein localization mode of regulation acts on what order of time?
seconds-minutes
The covalent modification mode of regulation acts on what order of time?
minutes
The gene expression mode of regulation acts on what order of time?
hours
Describe how the liver responds to high blood [glucose] levels.
liver uses GLUT2 transporter + glucokinase to detect and then stores excess glucose as glycogen
Describe how pancreatic beta cells respond to high blood [glucose] levels.
pancreatic beta cells use GLUT2 transporter to detect and then release insulin to lower blood [glucose]
Describe the interplay between insulin and glucagon in type 1 diabetes and the resulting effect.
insulin lacking and therefore glucagon chronically high, causing hyperglycemia
Describe the interplay between insulin and glucagon in type 2 diabetes and the resulting effect.
insulin signaling lacking (i.e. defect in receptor) and glucagon action is chronically high
What is the effect of glucagon?
raises blood [glucose]
(T/F) Glucose levels fluctuate significantly over the course of a day.
False. Glucose levels actually remain relatively constant, with fluctuations maxing at 5 mM.
(T/F) GLUT3 is only expressed in the brain.
False. It is expressed in most tissues, but is the predominant transporter in neurons, which is why it is designated as the “brain” glucose transporter.
Which of the four GLUT transporters are NOT expressed in all tissues?
GLUT2 (expressed in liver and pancreatic beta cells); GLUT4 (expressed in muscle and fat)
Which of the four GLUT transporters are insulin-dependent for activation?
GLUT4 only
Does glucose-6-phosphate inhibit hexokinase IV/glucokinase?
No.
Glucokinase is highly expressed in which tissues? (2)
liver; pancreatic beta cells
What is the significance of hexokinases I-III lacking a GK-RP form of regulation?
lack of GKRP-type regulation means that organs such as brain and muscle can use glucose even at low blood [glucose]
What are the key three reactions within glycolysis that are highly regulated?
glucose → G6P [hexokinase]
F6P → F-1,6-bP [PFK]
PEP → pyruvate [pyruvate kinase]
What is the major site of expression for hexokinase I?
brain (and many other tissues)
What is the major site of expression of hexokinase II? (2)
fat
muscle
What is the major site of expression of hexokinase III?
lung
What is the major site of expression of hexokinase IV? (2)
liver
islet beta cells of pancreas
Draw a graph that shows relative enzyme activity for hexokinase and glucokinase.
Which enzyme is thought to be the most important regulatory point for glycolysis?
PFK
Why does H+ inhibit PFK?
H+ is associated with lactate production, which is a signal that there is already enough energy
Which molecule is the most important regulator of PFK?
F-2,6-bP
What are the two mechanisms by which F-2,6-bP upregulates PFK activity?
(1) F-2,6-bP antagonizes ATP inhibition of PFK
(2) lowers Km of enzyme for F6P, the PFK substrate (i.e. increases affinity)
What is a bifunctional protein/enzyme? In what context is it used?
contains both kinase and phosphatase domains
used to interconvert F6P and F-2,6-bP
How does the fasting state affect the bifunctional protein?
activates phosphatase function, which reduces F-2,6-bP and slows down glycolytic flux
How does the feeding state affect the bifunctional protein?
activates kinase domain, which increases F-2,6-bP and increases glycolytic flux
What are the activators and inhibitors of pyruvate kinase?
activated by F-1,6-bP
inhibited by ATP and alanine
Describe the activity of HIF in normoxic conditions.
HIF is turned over as quickly as it’s made, mediated by binding of VHL (and is ultimately degraded by proteosome)
Describe the activity of HIF in hypoxic conditions and the resulting effect on glycolysis.
HIF forms active heterodimer with ARNT and avoids degradation → HIF target genes are glycolytic enzymes (since glycosis doesn’t require O2)
in short, hypoxia means activation of glycolysis
Describe the activity of HIF in cancer cells.
HIF is always active in many cancer cells (Warburg effect — cancer cells are highly glycolytic)
What are the inhibitors of fructose-1,6-bisphosphatase?
F-2,6-bP
AMP
What are the activators of fructose-1,6-bisphosphatase?
citrate
What are the activators of the pyruvate kinase reaction?
F-1,6-bP
What are the inhibitors of the pyruvate kinase reaction?
ATP
alanine
What are the inhibitors of the PEP carboxykinase reaction?
ADP
What are the activators of the pyruvate carboxylase reaction?
acetyl CoA
What are the inhibitors of the pyruvate carboxylase reaction?
ADP
Draw a diagram illustrating the reciprocal regulation of glycolysis and gluconeogenesis.
What is the relationship between glucagon and PEP carboxykinase?
glucagon promotes transcription of PEP carboxykinase
Draw a diagram showing how the phosphorylation/dephosphorylation of PFK works in normoglycemic and hypoglycemic conditions.
What specific component of the HIF system is mutated in cancer cells that exhibit the Warburg effect?
prolyl hydroxylase is mutated → it can’t bind HIF → HIF accumulates → cell thinks it’s in hypoxia and therefore drives glycolysis
HIF does not regulate the normal bifunctional enzyme. Describe how one of the isoforms of the bifunctional enzyme is regulated by HIF.
HIF regulates an isoform (iPFK-2) that has a mutation in the phosphatase domain → kinase domain always on → constitutive production of F-2,6-bP → high glycolytic rate as F-2,6-bP is an activator of PFK
