Regulatory enzymes in glycolysis How are they regulated?
1. Hexokinase (glucokinase in liver) 2. PFK-1 (committing step) 3. Pyruvate Kinase **remember this are irreversible and can't be used for gluconeogensis for glucose production*** By small-molecular weight regulators (allosteric regulation) and by hormones on the genetic level (enzyme synthesis)
What is special about Glut 1 and Glut 3
They are on the surface of many cells are general transporters of glucose and they are ONLY regulated by glucose concentrations
Where can you find Glut 2? Why is it special?
Plasma membrane of liver, intestines, pancreas and kidneys Liver and and kidney need bidirectional because glucose is put back into the circulation when glucose level is dropping, this glucose is transported to keep blood glucose level mainly. Pancreas- in flowing glucose trigger insulin formation, but excesis sent back to circulation because that's a sensor of insulin production These are bidirectional
Where can you find glut 5
Apical surface of intestine Liver (distributes every sugar so has to have one for fructose) Muscle (use fructose) Testes (fluctuating glucose would never affect spermatozoa)
What's speicla about Glut-4? Where can you find it?
Glut-4 is insulin sensitive and found in the muscle, adipose, heart and blastocytes.It's an extra glucose transporter when glucose is high in addition to glucose 1 to clear excess glucose from circulation. Insulin up regulates Glut-4. More insulin means more Glut 4(especially in skeletal and muscle cells) and they take more glucose but when it's not available they switch to LIPIDS
Explain how there is moe Glut 4 receptors when insulin is high
1. Insulin binds to receptor in the membrane 2. This now activated receptors promotes recruiting of glucose transporters 3. Glucose transporter increase the insulin-mediated glucose uptake (making Glut 4 available in addition to the Glut 1 that was already there) 4. When insulin decrease, glucose transporter move from membrane to intracellular pool where they are recycled 5. Vesciesl fuse to form endosomes
How is glucose kept in the cell
Through phosphorylation Glucose becomes Glucose 6-P with the enzyme hexokinase. It's now negatively charged and not a substrate of transporters
Explain how each of the small molecular weight effectors regulate glycolysis
1. Citrate is from the TCA cycle-- when it is high it says spare the glucose and don't run glycolysis because the TCA (kerbs) cycle is full (inhibits glycolysis) 2. Alanine- comes in when there is a need for glucose so we want to up regulate gluconeogenesis and shut down glycolysis. (inhibits glycolysis) 3. AMP/ATP- are energy sources. When ATP is high no need for glycolysis (inhibits glycolysis) AMP wants to be converted to ATP so when its high it activates glycolysis *** note that regulation of gluconeogensis is just the opposite***
Explain hormonal regulation of the 3 regulatory enzymes What organs are regulated by these hormones
They are all inhibited by glucagon and stimulated by insulin liver, muscle and adipose. Most other cells ALWAYS need glucose.
Explain the opposite affects of glucagon and insulin
Insulin up regulates phophatases (makes sense, if there's a lot of glucose then why not make energy out of it by glycolysis) so dephosphorylates and increases activity Glucagon up regulates kinases so decrease in activity
What regulates Pyruvate Kinase
Up regulates--fructose 1-6 Bi-phos Down regulated by alanine and ATP
What regulates PFK-1
Down regulated by ATP, citrate and H+ Up regulated by AMP, Fructose 2, 6-P2
What regulates Hexokinase
Down regulated by glucose 6-P
What is Km
Substrate concentraion which the enzyme is working at half maximal velocity
Explain the difference in hexokinase and glucokinase. When are each important/good? What inhibits them?
Hexokinase has a low Km (.1mM) and is saturated at normal blood glucose levels-- good for the brain because it needs glylsis to run even when glucose is low **inhibited by Glucose 6-P, its product**
Glucokinase (liver) has a higher Km (7mM) so it turns on when glucose is high ( when glucose is high (sensed by pancreas and secrete insulin). Inhibited by fructose 6-P
Explain the specific regulation of glucokinase
Gluckinase is a helper (only works when glucose is high) and it is inhibited by Fructose 6-P downstream.
Glucokinase is stored in the nucleus with a regulatory protein
Positive reg- when glucose is high that sends a signal to release the enzyme; Negative reg- when fructose 6-P is high is sends glucokinase back to the nuclues
Affects of too much fructose
Fructose is phosphorylated by fructokinase to Fructose 1-P (very fast process) F-1-P next step is aldolase very slow so we have a build of F-1-P we've used ATP to phosphorylate so too much ATP expended can cause cell death because of lack of energy.
F-1-P also activates glucokinase. So more glucokinase enters the cytosol so more glycolysis and can cause cell death as well due to lack of energy.
What's important about it?
postitive regulatory effectors?
It's the most important regulatory enzyme (last steop of priming) and the rate limiting step
Postive by 1. AMP (telling us the cell has low energy levels so run glycolysis) 2. Fructose 2-6 bi phos
Negative allosteric effectors- 1. ATP(telling us we have enough energy), 2. citrate (telling us Kerbs is getting energy from fatty acid/ketones and save glucose for cells that need it), 3. H+ (NOT LACTATE- telling us the glycolysis has made enough H+ and lactic acid and we need to avoid lactic acidosis)
How does PFK-1 regulation potentially protect from lactic acidosis
One of the negative regulators is H+ which is an end product of glycosis along with lactic acid. When there is build up, H+ triggers the stop of glycolysis allowing the symport of H+ and lactic acid to clear it from the cell
If there isn't alot of blood flow this clearing can be compromised causing pain.
Hormonal Control of PFK2
Hormonal activator- Fructose 2,6- Bipho -- this is NOT part of glycolysis but made by a glycolysis intermediate F6P and based on phos or dephos of a serine residue.
PFK-2 is regulatory and it is bifunctional and has a kinase and phosphotase ability. When the serine residue it phos. it inactivates the kinase; when it's dephos it activates the is activates the kinase
the kinase is activate is phos Fructose 6-P and it goes to Frcuotse 2, 6 Biphos upregulating PFK-1
When the phosphotase is activate is take off the phosphate group so Frctose 2-6 biphosphate becomes fructose 6-p and goes back to glycolysis
Hormonal control of PFK 1
Control of PFK-1 is through Fructose2-6 biphosphate positively
Negatively through fructose 1-3 biphos
PFK-2 in the heart
The phosphatase unit is in the heart
Explain how Fructose 2-6 biphosphate works
Special in the liver because it influences glyoclysis anf gluconeogensis
Its one polypeptide with two activate certains with a kinase and phosphatase so depending on which activate center is active is will either phos or depho Fructose 6 phophotase.
Glucagon-- phos--- inactivate kinase (so towards gluconeogensis)
Insulin--- dephos-- activate kinase (so high glycolysis)
Explain hemolytic anemia
Anemia due to excessive RBC destruction that can be due to the breakdown in pyruvate kinase deficienty.