17. regulation of glycolysis and GNG Flashcards
how can flux through enzyme catalyzed pathways be modulated
changes in the number of enzyme molecules or changes in the catalytic activity of the enzyme molecules
T or F: a protein has an infinite lifetime
false; it has a finite lifetime
is rapid turnover energetically favorable?
no; it is energetically expensive, but it allows more rapid changes in steady state levels for quicker responses to new cellular conditions
how many glucose transporters do humans have
12
what are glucose transporters used for
regulate the passive intake of glucose from the blood into the cell at different rates and in different tissues
what is the average glucose concentration in blood plasma
4-8mM
where does GLUT1 have high expression
erythrocytes
where does GLUT1 have low expression
most tissues
where is GLUT3
brain
what does GLUT3 represent
basal glucose uptake
where is GLUT2 present
liver and pancreatic beta cells
what does low Kt represent
low Kt= high affinity to glucose
which GLUT (1-3) has the lowest Kt, and which has the highest
GLUT3 has low Kt (since it’s the basal glucose uptake). GLUT2 has a higher Kt
why is it significant that GLUT have different Kt values
- muscle glucose is for making ATP for contraction
- liver glucose will be stored as glycogen
- at basal levels, lots of gluc will be moving through transporters, and GLUT1 is more effective than GLUT2, but because of the Kt GLUT2 has the ability to respond to glucose changes better than GLUT1
which transporter is involved with insulin
GLUT4
where is insulin released from
the pancreas
what happens to GLUT4 when insulin is released
GLUT4 transporters move from intracellular vesicles to the PM to passively move glucose into adipocytes and myocytes
T or F; we need to make sure the distinct enzymes in a paired pathway aren’t working at the same time
true
are paired pathways reversible or irreversible? why
irreversible; an enzyme converts one thing to another, and then a different enzyme converts that back into the starting product
which enzymes in glycolysis regulate flux: the ones the two share, or the ones they don’t share
only the ones they don’t share are able to regulate flux
how many hexokinase isozymes do humans have
4
which hexokinase isozyme is the main myocyte isozyme
hexokinase II
describe the Km for hexokinase II
very low Km (0.1mM)
what inhibits hexokinase
glucose 6-phosphate
why does glucose 6-phosphate inhibit hexokinase
hexokinase is the enzyme that converts glucose to glucose 6-phosphate in glycolysis. So having more product means less product needs to be made, which means hexokinase is inhibited
where is hexokinase IV present
the liver
what is the name of hexokinase IV
glucokinase
describe the Km of hexokinase IV
high Km (10mM)
is hexokinase IV inhibited by G6P
no
why isn’t hexokinase IV inhibited by G6P
in the liver, we aren’t consuming glucose, but we’re maintaining glucose homeostasis by building up or breaking down glycogen stores. The high Km (low glucose affinity) allows the liver to regulate blood glucose levels after a meal or a fast
describe what happens when blood glucose levels increase (in regards to GLUT and hexokinase)
influx of glucose to the liver due to GLUT2, hexokinase IV works faster as glucose concentration rises, glucose is converted to G6P which cannot leave the cell and will be funneled into glycogen synthesis
describe what happens when blood glucose levels decrease (in regards to GLUT and hexokinase)
glucose is low relative to the hexokinase IV Km. Low enzyme velocity. Any glucose that is released from glycogen is not going to be phosphorylated, so it leaves the cell to raise blood glucose levels
describe hexokinase IV inhibition during a fast
F6P promotes hexokinase movement into the nucleus to it can’t do it’s job
describe hexokinase activity after a meal
glucose promotes the removal of the regulatory protein so the hexokinase can return to the cytosol and produce G6P
describe the effect of ATP concentration on hexokinase
high ATP = reduced hexokinase transcription
what is the commitment step of glycolysis (and what enzyme is used)
F6P –> F1,6P
enzyme used: phosphofructokinase-1
describe the structure of PFK1
a homotetramer, where each subunit has a catalytic site and a binding site for allosteric regulators
what inhibits PFK1
ATP
how does ATP inhibit PFK1
it binds to the allosteric site and lowers its affinity for F6P
describe the effect of citrate of PFK1 activity
citrate inhibits PFK1: it’s in the CAC and it means that ATP will later be produced, and ATP is an inhibitor (which makes both of these inhibitors)
since PFK1 was step 3 of glycolysis, which is the corresponding enzyme for GNG (step 8)
fructose 1,6-bisphosphatase (FBPase-1)
what does FBPase-1 do
converts F1,6BP to F6P
describe how FBPase-1 is regulated
inhibited by AMP to prevent gluconeogenesis when ATP stores are low
describe how hormone levels affect PFK1 and FBPase1 at low blood glucose levels
glucagon is released by the pancreas which signals the liver to stop consuming glucose and begin producing and releasing it. Glycogen stores broken down and GNG is upregulated. Glucagon ramps up FBP1=F6P=GNG
describe how hormone levels affect PFK1 and FBPase1 at high blood glucose levels
insulin is released by the pancreas which signals the liver to consume and store glucose. Glycogen stores build up, glycolysis is upregulated. Insulin ramps up PFK1=F1,6BP=glycolysis
describe the impact F26BP has on PFK1
PFK1 has an increased affinity for its substrate in the presence of F26BP, meaning it has a reduced affinity for its inhibitors (ATP and citrate)
describe the impact of F26BP on FBPase-1
FBPase-1 has a reduced affinity for its substrate in the presence of F26BP, and it has an increased affinity for its inhibitors (AMP)
where does F26BP come from? what synthesizes and degrades it
formed from F6P by PFK-2
degraded to F6P by FBPase-2
what controls PFK-2 and FBPase-2 levels
concentration of glucagon and insulin in the liver
describe the structure of PFK-2 and FBPase-2
kinda like a conjoined enzyme
when the joint enzyme is phosphorylated, which half is active
FBPase-2 is active
when the joint enzyme is not phosphorylated, which half is active
PFK-2 is active
how many isozymes does pyruvate kinase have in humans
3
which two pyruvate kinase isozymes are we focusing on
liver (L) and muscle (M)
what inhibits pyruvate kinase isozymes
ATP, Acetyl-CoA, and fatty acids (all signaling an abundance of energy)
what activates the pyruvate kinase isozymes
F1,6BP
is pyruvate kinase active or inactive when phosphorylated
inactive
what phosphorylates pyruvate kinase to make it inactive
Protein Kinase A
what activates Protein Kinase A so it can phosphorylate and inactivate pyruvate kinase
glucagon
in regards of pyruvate, how is GNG initiated
pyruvate is carboxylized to initiate GNG
what activates carboxylation of pyruvate to initiate GNG
Acetyl-CoA
other than activating carboxylation of pyruvate, what does Acetyl-CoA do
it inhibits pyruvate oxidation
