It's about 3 a.m. and I've been studying for the last two hours or so. When I heard my alarm go off, I was scared to death that it was morning and I'd missed my study time.
pyruvate-citrate cycle
I was thinking about that pyruvate-citrate cycle and basically, it's a means for synthesizing or creating fatty acids. To avoid excess glucose (carbs) from being synthisized into fatty acids there needs to be a demand for ATP. We're going to end up with Citrate inside the mitochondria which are found inside our cells. The mitochondria is the place where the TCA Cycle takes place. The TCA Cycle is a MAJOR source for producing ATP molecules, ATP means energy and every crank of the TAC Cycle produces 12 ATPs and one molecule of glucose provides enough carbon atoms (six of them) to result in two cranks of the TCA Cycle which results in 24 ATPs (12 ATPs per cycle and two cycles per glucose or 12 times 2)
So, if you have an ATP demand then the flagship metabolite, Citrate, will go through the TCA cycle and no fat will be formed. How do you create an ATP demand? It might be easier to answer how NOT to create an ATP demand. To NOT have an ATP demand (or to minimize the demand for ATP) then one should not move very much and stay very still. To create a demand for ATP, one should move around. Exercise creates a high demand for ATP.
IF Citrate cannot be used in the mitochondrian to run through the TCA cycle, then ATP is out of a job! It's kindly asked to leave the mitocondrian and goes back out into the cytoplasm of the cell. It's an outcast and then degenerates into the molecules from which it came. One of these molecules is Acetyl Co-enzyme A (Acetyle CoA) which is the starting material used to make fat.
Citrate is out of a job when we eat too many carbohydrates, then Citrate is decomposed and becomes the starting material to make fat.
Pyruvate-Alanine Cycle
What if we don't eat enough? After about 4 hours after a meal, we start to get more fuel from stored sources such as glycogen and if we continue without eating, such as we might experience in the time span between dinner, sleeping and breakfast in the morning (maybe 12 hours or so) then we can start to use amino acids (constituites of proteins) from our muscles to be used for fuel.
Skeletal muscle gets converted to alpha-Amino Acids to alpha-ketoacid to glucose which ends up as pyruvate, then the pyruvate gets converted to alanine and leaves the muscle cells to head over to the Liver.
The liver is made up of proteins - why doesn't the liver just use its own protein!?!??
The liver may be dumb but, it's not stupid - when an organism is using up protein from it's own tissue, it's literally eating itself - catabolism! just like cannibals which is where the word comes from. If the liver starts to eat itself then it's not going to be good for anything. So, the muscles are more expendable and we take our proteins from muscles to send to the liver to be metabolized into glucose then injected into our blood plasma and raise up our blood sugar levels.
Besides, skeletal muscle cannot inject glucose into our blood plasma, it lacks the necessary enzymes - so, skeletal muscle sends out what it can - ie, alanine and Liver takes it from there.
When we go through this process, we stay alive and keep glucose in our blood but not only are we catabolizing our own muscle, we're entering metabolic acidosis and the kidneys are going to need to excrete ammonia ions to help stabilize pH levels.
Better to eat a meal - the liver has better things to do than to be preoccupied with making glucose out of alanine.
Fatty Acid Mobilization
What else can our body do when our blood sugar levels are low? The pancreas releases the hormone glucagon. Glucagon travels around our system until it meets up with adenyl cyclase which is found in the cell wall of adipocytes (fat cells). Outside the cell, we have glucagon. Inside the cell, the glucagon interaction with adenyl cyclase gives rise to cAMP which is the intracellular version of glocagon. cAMP sets off a series of reactions which breaks up triacylglycerides into fatty acids and glycerol which can then leave the cell and become Free Form Fatty Acids (FFAs)
Fatty acids are not carbohydrate precursors. But, they will provide ATP (via oxidation) for gluconeogenesis. (gluconeogenesis is essentially, glycolysis backwards)
So, what can we do with fatty acids? Through a cute little four step process, called beta-oxydation, we use two of the carbons from the fatty acids to produce Acetyl-CoA. What's so special about Acetyl-CoA? It's one of the things we need to make Citrate! And - citrate is a nice starting point for the TCA cycle. The production of acetyle-CoA inhibits glycolysis which preserves glucose.
Still ...We need that energy demand in order to elicit the generous TCA cycle.
OK - I think I've figured something out ...maybe not... let's see ...
The liver makes ketone bodies (ketogenesis) from FFA, those ketone bodies inhibits synthesis of FFA from Fat Cells. Also, Insulin, which is released in response to rising blood sugars, inhibits the production of FFA.
It's about 5 a.m. now - I have to leave in an hour - I'll need to review lipoproteins before I leave for school at 6 a.m. From 7 to 8 a.m. I'll be studying for my philosophy test which is immediately before my biochemistry test. Half of our class will also have to have their Diversified practical, then philosophy, then biochem, then professional development.
Highest priority is biochem, next is philosophy.
oh yeah - picture of the day ...it's gotta be the Mitochondria!
This is a pretty nice picture illustrating the mitocondria. In the upper left corner we see the entire cell then the small mitochondria floating around in the cell is taken out with that grey arrow and shown by itself. I've heard talk that at one time the mitochondria may have been an organism all to itself. The number of mitochondria in our cells can be determined by how much we use them - and what are they for? It's where the TCA cycle takes place! The TCA cycle is used in response to a demand for ATP - We demand ATP by our daily energy requirements. If we're lazy and don't demand much energy we probably won't have a lot of mitochondria in our cells - be active & exercise then we have a demand for ATP, more mitochondria and then Citrate can keep it's job and crank through the TCA cycle and that means it Citrate won't end up becoming fat beneath our chin and around our waist. :)
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