Creatine, creatine phosphate, and energy storage

Our muscles don’t wait until a crisis to stock up on essentials - they “stock up on” some energy - but not directly in the “universal energy currency” of ATP. Instead, if your muscles need energy ASAP, they use creatine phosphate to quickly make ATP. Creatine? What does that mean? An amino acid that’s not in protein?  

blog form: http://bit.ly/creatineatp ; YouTube:    • Creatine, creatine phosphate, and ene...  
 
I’ve been talking a LOT about amino acids as protein letters. With their generic backbones that allow for linking through peptide bonds into peptides that can fold up into functional proteins - and their unique “side chains” that stick off that generic backbone and impart special “superpowers” to the proteins they’re in, amino acids are great for making molecular machines that can do everything from serving structural roles (like keeping cells from collapsing and acting as scaffolds to keep related things near each other) to acting as catalysts, speeding up reactions by holding the reacting partners in the right orientation in the right environment, etc. We call such biochemical catalysts enzymes and they make possible everything from linking together amino acids to form proteins to breaking down sugars to get energy.

Speaking of energy - a lot of enzymes require energy to function. And usually that energy comes from ATP. ATP (Adenosine TriPhosphate) is the RNA letter A, but it moonlights as a sort of energy currency. It’s able to do this because those three negatively-charged phosphate groups (phosphorus surrounded by oxygens) are held together by “high energy” phosphate bonds. We call them “high energy” because energy is used to clamp them together (this energy is needed because, thanks to their like charges, they don’t want to be next to each other). Similar to unclamping a spring, this energy is released if you allow them to “let go” More here: https://bit.ly/atpenergymoney 

As a result, ATP is able to act as a sort of “energy currency” akin to an arcade token. Similarly to how you go to an arcade and can stick in different forms of money and the machine will spit out tokens that you can use on any game, your cells can take things like sugars, fats, and proteins, and convert the energy they contain into ATP. Once you have the ATP there’s no “money trail” – the ATP all looks the same & can be “spent” anywhere. A lot of different enzymes can use them - to do a lot of different things. This is great and all, but it also means that you don’t want to keep a bunch of free ATP floating around - random enzymes will start using it and doing their own things and you’ll have cellular anarchy! So you need to keep it under control.

But you don’t want to waste energy. So, for long-term energy storage, instead of ATP, energy is stored in the form of things like fats and glycogen (a storage form of the blood sugar, glucose, that’s made up of lots of glucoses chained together). Your body can spend ATP money to carry out that linking, thus sticking that energy away for safe keeping until you’re ready to break the storage molecule down and make ATP from it for energy.

In the case of glycogen, getting energy out involves chopping it into individual glucose sugars and then sending those glucoses through breakdown processes called glycolysis and the tricarboxylic acid cycle (TCA cycle)(aka citric acid cycle or Krebs cycle). http://bit.ly/metabolismglycolysis

One way your cells prevent ATP buildup is by keeping ATP from being made when there’s already a lot of it. ATP inhibits glycolysis and the TCA cycle by allosterically inhibiting some of their enzymes. “Allosteric” just refers to the fact that ATP isn’t binding to the active site of those enzymes and keeping the real reactants from getting in - instead it’s binding somewhere else on the enzymes leading them to shape-shift a little so that the enzyme gets inactivated.

So, if you have a lot of ATP, glycolysis & the TCA cycle are inhibited, so you stop making more of it… until you use up that ATP. Now you no longer have inhibition of ATP-making and, additionally, you have a lot of spent ATP, converting it into Adenosine DiPhosphate (ADP), which can act as an allosteric activator of some of those enzymes needed for ATP generation. So you start making ATP again.  

That “break down and get energy” process takes time - glycolysis and TCA give you some ATP, but most of the energy at that point is held by electron carriers like NADH which then have to go through the oxidative phosphorylation (oxphos) process to give you the major ATP payout. Long story somewhat short - there are a lot of steps and stepping takes time.⠀https://bit.ly/cellularrespiration2  

But what if you need ATP ASAP? continued in comments