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?
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 serve structural roles (like keeping cells from collapsing and acting as scaffolds to keep related things near each other) to act 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) has “high energy” phosphate bonds in which energy is used to clamp together negatively charged phosphate groups (phosphorus surrounded by oxygens) that, thanks to their like charges, don’t want to be next to each other. Similar to unclamping a spring, this energy is released if you let them “let go” More in this past IUBMB Bri*fing http://bit.ly/2WiPOpg
So ATP acts as a sort of “energy currency” that a lot of different enzymes can use – to do a lot of different things. So you can’t just 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).
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, so that you no longer have inhibition of ATP-making and, additionally, you have a lot of spent ATP – Adenosine DiPhosphate (ADP), which can act as an allosteric *activator* of some of those enzymes needed for ATP generation.
That “break down and get energy” process takes time – glycolysis and TCA givesyou 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.
But what if you need ATP ASAP?
This is a problem that your muscles sometimes face. You don’t really give them a warning before you start running a marathon or running around the lab. So their ATP stocks can quickly get depleted. If they just ran out and gave up, your muscles would freeze because, as we looked at in a previous post, muscle contraction relies on the protein myosin using ATP to slide protein fibers along one another http://bit.ly/33Xi1ob
So – to keep that freezing from happening – your muscles keep a stock of energy in an easier to use form – creatine phosphate, which has a “high energy phosphate bond” like ATP does, but stuck to something else – creatine.
An enzyme called creatine kinase reversibly transfers a phosphate from ATP to an amino group of creatine to give you creatine phosphate when there’s a lot of free ATP. And then, when needed, that same enzyme can transfer the phosphoryl group from creatine phosphate to ADP to regenerate ATP that’s already been spent.
creatine + ATP ⇌ creatine phosphate (aka phosphocreatine or PCr) + ADP
In this way, creatine is able to serve as an ATP buffer – it keeps ATP production going by preventing allosteric inhibition of glycolysis & TCA while still making sure you don’t run out of ATP.
So what is creatine? It’s technically an amino acid – which is how I got to writing this post – a suggestion from a reader on my amino acid review post. Last month I took you through the 20 common amino acid protein letters with #20DaysOfAminoAcids. And I even showed you a couple “uncommon” ones – selenocysteine and pyrrolysine – which are added as weirdos because they’re able to trick the protein-making machinery to add them instead of stopping at a stop codon; as well as modified versions of the common ones that are “weirdified after adding” (post-translationally modified) – like phosphoserine (serine that’s had a phosphoryl group added on and hydroxyproline (proline with a hydroxyl (-OH) group stuck to it)
All of those (even the weirdos) were “proteinogenic” amino acids – meaning that they can be used to make and/or can be found in proteins. But “amino acids” don’t *have* to be used for protein-making. Some of the proteinogenic amino acids “moonlight” in other roles – like glutamate working as a neurotransmitter (brain signaling molecule).
But, unlike glutamate, which just does non-protein work as a side hustle, some amino acids are *never* found in proteins. An example of this is creatine – but it’s not like the amino acids we’ve looked at…
When biochemists talk about amino acids, we’re usually referring to the type of amino acids used in proteins, which are chemically classified as “alpha amino acids (α-amino acids).” The a refers to the fact that the amino group (-NH₂ or -NH₃⁺ depending on the pH) and the carboxyl group (-(C=O)-OH in the carboxylic acid form and -(C=O)-O⁻ in the carboxylate form) are attached to the same central “alpha” carbon. This is also the same carbon that is attached to unique side chains (aka R groups) that make different protein letters different.
But in order to be an amino acid definition-wise, something just needs to have an amino group and a carboxyl group – they don’t have to be hooked up to the same carbon. So you can get things that still fit the bill but that don’t fit into proteins. And one such example is creatine.
Our bodies make it from the amino acids arginine (Arg, R) and glycine (Gly, G) in 2 steps. First, an enzyme called arginine:glycine amidinotransferase (AGAT) transfers the amidino group of Arg onto Gly to make guanidoacetic acid (GAA) and ornithine. Then another enzyme, S-adenosyl-methionine:guanidinoacetate methyltransferase (GAMT) methylates (adds a -CH₃ to) the amidino group to form creatine. As the name suggests, this methyl transfer involves the cofactor S-adenosyl-methionine (SAM)(aka AdoMet), which is a derivative of the amino acid methionine (Met, M) as I talked about here: http://bit.ly/methioninemethyl So creatine-making requires 3 of the “normal” amino acids – Arg, Gly, and Met.
In mammals, the main route of Cr synthesis involves the first step happening in the kidneys and the second step happening in the liver, which then releases Cr into the bloodstream for cells that need it to take in through special membrane-spanning transporter proteins those cells express. This includes cells in the muscle (skeletal and cardiac), brain, retina, and sperm (only where applicable, obviously). But the muscle is the main Cr-user (~94% of our body’s Cr is located there).
An average-sized adult male has ~120g of total creatine, so that’s a lot Cr in your muscles. But you need to keep sending more in because about 2% of that muscular Cr and PCr is nonenzymatically converted to creatinine every day through spontaneous cyclization. This ringification doesn’t require help and this creatinine doesn’t need help getting out of cells either – it just diffuses out and gets excreted by the kidneys into urine. So ~2g “fresh” Cr per day needs to be either made from scratch (de-novo) using that AGAT/GAMT pathway we talked about, or gotten from the diet. Stat source: http://bit.ly/37DJpZY
If the kidney has a hard time getting rid of it, creatinine levels can build up in the blood, so doctors sometimes order blood creatinine tests to check for kidney problems. Sometimes doctors will measure creatine in urine collected over 24 hours and compare it to blood levels to check the “creatinine clearance rate” – how well your kidneys can clear creatine from your blood and into your pee – from which they can estimate the rate of blood flow through the kidneys (glomerular filtration rate (GFR) to get an idea about overall kidney function.
In keeping with the amino acid post tradition, here’s a little history on the discovery process. Creatine was discovered in skeletal muscle in 1832 by French scientist Michel Eugene Chevreul, who named it with inspiration from the Greek word for meat, kreas. In 1912, Otto Folin & Wiley Glover Denis found that if you give animals a lot of creatine, it mostly gets stored in their muscles. In 1927, The phosphorylated version, PCr, was discovered in 1927 by groups at Cambridge & Harvard. And the source of that phosphorylation, the creatine kinase reaction, was discovered in 1934 by German biochemist Karl Lohmann (so this reaction is sometimes called the Lohmann Reaction)
This post is part of my weekly “broadcasts from the bench” for The International Union of Biochemistry and Molecular Biology (@theIUBMB), whose president-elect Dr. Alexandra Newton agrees with me that it’s perfectly normal to dream that you’re a protein. Be sure to follow the IUBMB if you’re interested in biochemistry! They’re a really great international organization for biochemistry.