Asparagine biochemistry, asparagine synthetase, and asparaginase treatment for ALL

The amino acid asparagine is considered nonessential in the dietary sense of the word - our bodies can make it. But they can also break it, and they can use it to build other molecules. So the levels of free Asn in your body at any time depend on how much of it you eat, how much you make, how much you break, & how much you “siphon away.” Your body keeps a nice balance by regulating the making, breaking, and other-purpose-using.

much more in blog form: http://bit.ly/asparagineasparaginase

In a type of blood cancer called childhood Acute Lymphoblastic Leukemia (ALL), the balance gets thrown off because the cells lose their ability to make it as much, but they still need it, so they rely on a steady source of “pre-made” Asn from the bloodstream. This has led to a therapeutic strategy of depriving these cells of what little Asp they do have by adding extra “breakers” into the bloodstream, depleting the Asn supply - this strategy selectively affects the cancer cells because the other cells have enough “makers” to counterbalance these “breakers”⠀
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The “maker” is an enzyme called Asparagine Synthetase (ASNS) and it makes Asn through a “transamination” reaction which transfers an amino group from the amino acid glutamine to aspartate, turning aspartate into asparagine and glutamine into glutamate. It has a pretty cool reaction - I won’t bore you too much, but it has a mechanism involving 2 “active sites” (places where reactions occur). It requires spending a molecule of ATP to “activate” the carboxylate group in Asp’s side chain that you want to turn into an amide. First the carboxylate attacks ATP’s innermost phosphate group, kicking off 2 of the phosphates as inorganic (non-carbon-attached) pyrophosphate (PPi), and leaving you with a β-aspartyl-AMP intermediate where Asp has AMP stuck to it as a big ole, energetically unstable, “attack here” sign. Meanwhile, in the enzyme’s second active site, the amine group is cut off from glutamine as ammonia (NH₃), which travels through a secret tunnel between the active sites to attack, releasing AMP & swapping it out for an amino group.⠀
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The “breaker” is an enzyme called Asparaginase (ASNase). It catalyzes the splitting up of Asn into aspartate and ammonia. When Asn is broken down for energy, it follows the usual “remove amino parts & process them and the carbon skeletons separately” routine we’ve seen. But, unlike most of the other amino acids, we have 2 amino groups to remove. ASNase does the first removal - that of the “extra” amino group - the side chain one, to give you aspartate. And then that aspartate gets broken down the same way aspartate does - because it is aspartate now! The backbone amino group is removed by aspartate aminotransferase to give you OXALOACETATE, which is nitrogen-free and can get processed in the citric acid cycle (aka TCA, aka Krebs cycle) to get energy. Alternatively, the Asp can be used as Asp. ⠀
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It’s the ASNase that’s used as an anti-cancer treatment - and the version that’s used as a drug comes from bacteria.

Both normal and cancer lymphocytes need Asn - and they get their supply by taking it in from circulating blood plasma (plasma is just the non-cell part of the blood). When you introduce ASNase into the blood, it goes to work breaking the Asn down, depleting this supply. This causes leukemic blasts to actually start shipping their cytoplasmic stock out (which gets broken down), so they get depleted intracellularly. And, unlike normal cells, these cancer cells aren’t able to make much if any of their own because, for reasons still not fully understood, they don’t make much of the “maker,” ASNS. But they need Asn to build proteins and survive - and now they can’t get it from the blood & they can’t make it, so they die off. But the normal cells are fine because they do express ASNS & therefore they can make enough to compensate for what they can’t get “pre-made” from the bloodstream. ⠀
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ASNase also depletes the glutamine pool because, even though ASNase prefers breaking down Asn, it can also deaminate glutamine (into glutamate & ammonia). So now you’re messing with levels of multiple amino acids. One of the coolest things about metabolism is that it’s like the “7 degrees to Kevin Bacon” thing - all molecules seem to be somehow connected. But the non-cool thing about this is that messing up one or a couple of the players (especially central hub ones) can send the whole metabolism out of whack. So, among other things, ASNase treatment can lead to global metabolic changes with less glycolysis (sugar breakdown) and increased use of fats for energy. Sensing amino acid deprivation, cells shut down protein-making and even activate autophagy (selective breakdown of intracellular parts) & apoptosis (programmed cell death). ⠀

good review article: https://www.ncbi.nlm.nih.gov/pmc/arti...