Lecture 2: Carbs 2 Flashcards
Key regulation mechanisms: 4 of them
- substrate/ product (stimulation or inhibition)
- Km (concentration if substrate that gets you to half max) or Vmax. Good example are GLUT transporters, 1 and 3 had low infinity.
- allosteric effectors: is any substance that will effect the rate of activity of an enzyme by binding to it other than its active site. It changes its affinity, ie increase or decrease.
- covalent modulation: an enzyme that you can regulate the activity of an enzyme by phosphorylating it, you change its activity so it can work better or slower on a substrate.
Definitions: life will be a lot easier if you remember these Enzyme names: Kinase Phosphorylase Phosphatase
Definitions:
Enzyme names:
Kinase eg protein kinase: so it adds a phosphate to a protein
-any enzyme that adds phosphate using ATP as the phosphate donor. Another ex, pyruvate kinase➡ adds phosphate group to pyruvate.
Phosphorylase eg glycogen phosphorylase
-any enzyme that’s adds phosphate using inorganic phosphate (Pi) as the phosphate donor. Doesn’t need ATP, just grabs it from free floating phosphates.
Phosphatase eg protein phosphatase
-any enzyme that removes phosphate as inorganic phosphate (Pi)
Glycolysis
Major pathways for cabo metabolism.
Function: catabolism of carbs as glucose to produce energy in form of ATP.
Unique features:
-reactions proceed through phosphorylated intermediates.
-substrate level phosphorylation
Location: it occurs in soluble cytoplasm.
When is glycolysis important for ATP production?
High glucose intake- very important
Stress
Exercise
Availability of glucose is a major driver for glycolysis.
When availability is high it is very important.
Tissue dependant on glycolysis
- Brain- only uses glucose
- Erythrocytes, rely on anaerobic glycolysis only
- Type 2 muscles
- Foetus
- Renal medulla
- Retina
- Skin
The glycolytic pathway
-6 carbon substrates going in. This 6 carb goes to two 3 carb substrate.
Total energy generated:
Investment phase- 2 ATP
Pay off phase: 4 ATP
Nett: 2 ATP
Hexo(means 6) kinase (phosphorylates using ATP)
-Hexokinase phosphorylated glucose to 2 glucose-6-phosphate using ATP (phosphate added)
-phosphofructokinase make fructose 6-phosphate to fructose 1,6-biophosphatase using ATP
Etc
Regulation of glycolysis
Regulatory steps
- 3 parts can’t go backwards
- they are allosterically regulated.
Hexokinase- anchors glucose
He phosphorylates (adds phosphate) glucose. This phosphate group makes it stay in the cell, the glucose would run away.
- hexokinase has very low Km (high affinity enzyme for hexoses) low vmax. Slow Vmax can be a problem when you have bulk glucose! Doesn’t go fast enough to anchor the glucose.
- found in all cells
- controlled allosterically by product glucose 6-P
- not specific for glucose
Glucokinase (liver in humans)
Km= 10 mM, higher Vmax.
It phosphorylates glucose
-specific for glucose
-not in liver of ruminants coz they have low glucose absorption.
-the additional activity of glucokinase enables liver to phosphorylate heaps of glucose to decrease glucose concentration.
Phosphofructokinase
Th most important rate regulator!
It works on fructose-6-phosphate. Hence the fructo part of the name, and its a kinase so its going to use ATP and add a phosphate group onto it, so you end up with a fructose 1-6 biphosphate.
It’s controlled by a few things
Inhibition:
-ATP inhibits it, when glycolysis has been moving too fast. Thus ATP feeds back and slows glycolysis down by inhibiting this enzyme.
-citrate
-H+: when? After excercise, it becomes acidic and inhibits this enzyme.
Stimulators:
Fructose 2, 6-bisphosphate (single most potent effector of this enzyme) it goes in and allosterically regulates PFK1-phosphofructokinase. It is made by fructose 2,6-bisPase.
Pyruvate kinase:
Inhibitors
Stimulators
Inhibitors: Allosteric- ATP, alanine Covalent- glucagon, switches on an enzyme which adds a phosphate to pyruvate kinase, by doing that it will slow it down and inhibit it. -around during (low sugar) starvation -slows down glycolysis
Stimulation:
Allosteric: fructose 1,6-bis phosphatase (feed-foreword control)
Why do you get lactic acid?
So you can recycle NADH so glycolysis can proceed.
There is only
Anaerobic glycolysis
Lactate dehydrogenase
Can make lactic acid from pyruvate and pyruvate from lactic acid. Slide ?
Essential additional enzyme for this anaerobic scenario
Aerobic glycolysis.
Oxygen as the final electron acceptor
What enzyme enables pyruvate to be converted to acetyl coA?
Pyruvate can be fed into aerobic glycolysis called TCA cycle.
Pyruvate dehydrogenase catalysed this reaction. It converts pyruvate to acetyleCoA. You can reverse this step.
You can’t convert acetyleCoA back to glucose.
Pyruvate dehydrogenase is regulated by?
- End product inhibition
- Covalent modulation
If you have high levels of pyruvate it will slow this enzyme down
If you have high levels of acetyleCoA you will speed this enzyme up. So will
-NADH (like when your metabolising fat) which generates heaps of NADH, acytlecoa which switches off kinase which switches off dehydrogenase.
-the enzyme reaction if controlled by the phosphorylation status of pyruvate dehydrogenase
-inhibited when phosphorylated this the oxidation of acetyleCoA can’t proceed
Look at pg 9
