metabolic pathways

Question 1

how is homeostasis maintained?

Answer 1

by keeping the concentrations of most metabolites at steady state. steady state = rate of synthesis of metabolite is equal to rate of breakdown of metabolite
pathways are at steady state unless perturbed (eg a tiger attack) and return to new steady state after perturbation

Question 2

what is feedback inhibition?

Answer 2

in many cases, ultimate products of metabolic pathways inhibit their own biosynthetic pathways. eg ATP inhibits the commitment step in glycolysis

Question 3

what determines rates of biochemical reactions?

Answer 3

[R] vs [P]
activity of catalyst
[effectors]
temp

Question 4

when is rate most sensitive to [substrate]

Answer 4

rate is more sensitive to [substrate] at low concentrations when frequency of substrate meeting the enzyme matters. at high [substrate] the enzyme is nearly saturated and rate is insensitive to changes in [substrate]. remember the Michaelis-menten curve drawings and that physiological concentration is usually near Km

Question 5

most reactions in metabolic pathways operate near equilibrium. what reactions operate far from equilibrium?

Answer 5

key enzymes at points of regulation operate far from equilibrium. they control the flow through the pathway. all enzymes operate at the same rate so changing one enzyme rate will change entire pathway rate

Question 6

why is AMP a key regulator?

Answer 6

small changes in [ATP] have dramatic effects on [AMP] so AMP is a good indicator of ATP levels and need for increased/decreased activity of ATP metabolic pathways

Question 7

how does AMP affect pathways?

Answer 7

through AMPK, AMP activated protein kinase. AMPK has different effects on different tissues

Question 8

what enzyme effects glycolysis flux the most?

Answer 8

Hexokinase. since hexokinase provides the starting material Glc, PFK-1 activity relies on hexokinase activity and so does not determine flux as much as hexokinase.

Question 9

what is an elasticity coefficient?

Answer 9

measures the responsiveness of an enzyme to substrate, on Michaelis-menten graph a steeper slope is a higher elasticity coefficient

Question 10

what are the two important isoenzymes of hexokinase?

Answer 10

hexokinase I: expressed in all tissues to different levels
hexokinase IV: only expressed in liver. has higher Km and higher responsiveness to [glucose]. is not inhibited by Glc-6-phosphate so functions at higher [glucose]. functions to clear blood glucose at high [glucose] for storage as glycogen
draw the curves for both types. slide 20

Question 11

what is the effect of ATP on PFK-1?

Answer 11

ATP inhibits PFK-1 activity basically down to nothing. at high [ATP] the Km is much higher. slide 22

Question 12

how are PFK-1 and fructose 1,6 bisphosphatase regulated separately?

Answer 12

PFK-1 is inhibited by ATP and citrate and is stimulated by ADP and AMP, meaning glycolysis activates if AMP is high and ATP is low
fructose 1,6 bisphosphatase is inhibited by AMP, meaning that gluconeogenesis activates if AMP is low

Question 13

what is fructose 2,6 bisphosphate?

Answer 13

not a glycolytic intermediate, only a regulator. produced specifically to regulate glycolysis and gluconeogenesi by activating PFK (glycolysis) and inhibiting fructose 1,6 bisphosphatase (gluconeogenesis)

Question 14

how is fructose 2,6 bisphosphate regulated?

Answer 14

it is regulated by PFK-2 (causes formation) and fructose 2,6-bisphosphatase (causes breakdown).
insulin activated phosphoprotein phosphatase which removes a phosphate group and causes PFK-2 to be active, forming F-2,6-bP and stimulating glycolysis.
glucagon activates cAMP-dependent protein kinase which adds a phosphate group and causes FBPase-2 to be active, breaking down F-2,6-bP and inhibiting glycolysis
slide 27

Question 15

how is pyruvate kinase regulated?

Answer 15

activated by fructose 1,6 bisphosphate
inhibited by abundant energy, ATP, acetyl-CoA, alanine
inactivated by phosphorylation in response to signs of glucose depletion (glucagon) in liver only

Question 16

what does FOXO1 do?

Answer 16

activates transcription of PEP carboxykinase and glucose 6-phosphatase (used in gluconeogenesis). in response to insulin (high blood glucose), FOXO1 is phosphorylated and does not activate transcription of gluconeogensis enzymes, so glycolysis is supported

Question 17

how are glucose residues removed from glycogen?

Answer 17

by glycogen phosphorylase removes residues on the nonreducing end, releasing glucose 1-phosphate. works until it reaches four residues from an a1-6 branch point

Question 18

how are branch points dealt with when removing glucose from glycogen?

Answer 18

a debranching enzyme transfers a block of three residues to the non-reducing end of the chain and cleaves the single remaining a1-6 linked glucose. slide 35

Question 19

what must be done to the glucose released from glycogen before metabolism?

Answer 19

the glucose-1-phosphate must be isomerize to glucose-6-phosphate by phosphoglucomutase. glucose-6-phosphate is dephosphorylated in the liver for transport out of liver, or it can enter glycolysis directly in the liver

Question 20

how is glycogen synthesized?

Answer 20

by glycogen synthase. it uses UDP-glucose and makes a1-4 attachments only
branching requires a glycogen branching enzyme which takes a linear section and moves it backwards so it is attached to another residue at a1-6 branch point. slide 39

Question 21

what signals glycogen breakdown?

Answer 21

glucagon (present when Glc is low) and Epinephrine (present during fight or flight when energy is needed immediately) start a phosphorylation cascade via cAMP that activates glycogen phosphorylase, cleaving off glucose-1-phosphate residues

Question 22

what signals glycogen synthesis?

Answer 22

insulin increases glucose import into muscle, stimulates activity of muscle hexokinase, and activates glycogen synthase