Would you like a phosphate cherry on top or on the side of your protein? Kinases are like “presentation chefs” that embellish the protein products your “line cooks” (ribosomes) make – but they do *charge* – the “cherries” are bulky, negatively-charged phosphate groups, which can change not just what the protein looks like, but also how the protein acts. So today let’s talk about kinases and phosphorylation

note: some kinases phosphorylate things other than proteins, but today I’m going to focus on the protein ones. 

text adapted from past post, video new

shorter version of video focused on mechanism: https://youtu.be/lBqPOAeI2d0

You can think of your genome as a collection of recipes for all the things your cells will ever need to make. Some of these recipes are for making protein & RNA “chefs” that help “bake” (translate) other proteins or RNAs from other recipes. 

There’s only so much you can tell by reading a recipe, which is 1 of many reasons why simply sequencing the DNA of everything won’t give us all the answers, which is one of the reasons why structural biology (where we look at the shape of molecules like proteins and how that shape affects how they function and how they interact with one another) is so important

The “line cooks” initially putting together the protein are protein/RNA complexes called ribosomes and they’re pretty restricted to following the recipe word for word. But even if you have that recipe in front of you, it’s hard to “visualize” the final product until it’s complete 

The recipe tells you what amino acids (protein building blocks) to add in what order. These amino acids will be linked together through their generic backbone by strong, covalent, peptide bonds so we know that amino acids next to each other in the recipe will be next to each other in the products (like linked chains in a charm bracelet). But what we don’t know is what else they’ll be near because proteins get folded up into (beautiful) 3D structures

Different amino acid “ingredients” or “letters” have different properties because in addition to the generic backbone that makes them easy to link together any 2, they have unique “charms” called “side groups” sticking out. Some charms are big and bulky, others are small and flexible, some are hydrophobic (water-avoided), some are hydrophilic (water-loving), some are charged (+ or -) others are neutral. 

These properties influence how the protein gets folded (e.g. hide the hydrophobic parts in the center, put the hydrophilic ones on the outside). 

You know how some people fold their pizza in half (not judging, just analogizing…) – parts of the pizza that were once on opposite sides are now close together. Similarly, just because 2 amino acids are far apart in the recipe doesn’t mean that they’ll be far apart in the final structure

BUT if 2 amino acids are far apart (or close together) in the “final” structure it doesn’t mean they *always* are because there’s often not just “1” final structure. Proteins can change shape subtly or dramatically & such “conformational changes” can be caused by post-translational modifications (changes to the protein after they’re made)

Unlike the line cooks, “presentation chefs” can add embellishments (parsley sprig anyone?) that can change the protein’s shape and appearance. One common type of display cook are KINASES. They modify proteins after they’re made (post-translationally) by transferring a phosphoryl group (-PO₃²⁻) group to one of a protein’s charms -> phosphorylation

Usually the phosphoryl group comes from the end of a nucleotide, most commonly from Adenosine TriPhopshatte (ATP). Nucleotides? Like the ones that make up RNA & DNA? Yep – the very same! ATP gives you the adenine “A” of RNA (and if you remove an oxygen from ATP’s sugar to get deoxy adenosine triphosphate (dATP) you get the precursor to DNA’s deoxyadenine “A”)

Although it’s technically a phosphoryl group being added (the 4th oxygen comes from the protein), we normally just say that kinases add phosphates. Phosphate (PO₄³⁻) has a central phosphorous(P) atom connected to 4 oxygen(O) atoms. It has “extra” electrons (e⁻) so it’s ➖ charged.  Putting so many negative charges right next to each other gives you an “unstable” high-energy molecule because the negative charges repel and it’s hard for the phosphate bonds to hold on. Enter the “escorts” – positively charged metals like Magnesium (Mg²⁺) complex with the phosphates and chaperone them as they move from place to place. And proteins like kinases that need to bind them have metals in their binding pocket as well as charged amino acids sticking into the binding pocket that can do a “changing of the guards.” 

Kinases are a type of enzyme – they catalyze (speed up) reactions without getting used up (they can add a phosphate to 1 protein then turn around & add 1 to another protein). Kinases catalyze the phosphoryl group transfer by helping hold together the reactants (ATP & protein substrate) in the right position for the transfer and stabilizing unstable intermediates. They can do this because the amino acid charms sticking out into their “active site” make a perfect environment for the hand-off. But how do they know where to add? 

Different kinases recognize different target consensus sequences – like their “dating profile” describing their “ideal match” sequence around the phosphorylation site – but there’s some wiggle room (and some are more promiscuous than others)(have higher specificity) – the ideality is due to that sequence making the substrate complement the shape, charge, etc. of their binding pocket (like maybe their binding pocket is negatively charged, so they want something with positively charged amino acids near the phosphorylation site)(again, something you wouldn’t know from the linear sequence alone). But what’s the real “phosphorylation site?” which amino acid “charm” is actually getting changed?

Only a few of the charms can be modified like this, the major ones being -> Tyrosine (Tyr or Y), Serine (Ser or S), Threonine (Thr or T) – what do they have in common? A hydroxyl (-OH) group that can ditch the hydrogen and act as a nucleophile and take that phosphoryl group to make a phosphate PO₄³⁻ group. note: Histidine (His or H) can also (less commonly) be phosphorylated, but on a Nitrogen (N) 

But under cellular conditions, there are plenty of H⁺ around (the cellular pH is above their pKa), so these charms don’t really want to give up a H⁺ because that would make them negatively charged (-O⁻) -> enter the kinase -> when the protein substrate binds, there’s a basic amino acid just waiting to pull it off. And when it does, you get a strong nucleophile (the substrate’s -O⁻ really doesn’t like being negative and seeks out something that wants more electrons to share their extra with-> so it attacks the gamma phosphate conveniently sticking out right next to it).

Hold on – didn’t you say phosphate has “extra” electrons and is – charged? Why does it want more? The phosphate group is ➖ overall BUT the central P is actually partly ➕ because the electronegative (electron-hogging) Os pull electrons (e⁻) away from it & metal or ➕ charged charms in the binding site can pull on this charge too -> makes the P electrophilic(electron-loving)

The exposed -O⁻ can therefore attack it -> this forms a temporary pentacovalent intermediate (P bound to 5 Os) and then the bond between Β & γ phosphates is severed -> you end up with ADP (Adenosine DiPhosphate) and a phosphorylated protein

Now these products have fewer favorable contacts with the protein so they’re less tightly bound & can “fall out” so the kinase can do it all again but the enzyme has to “open up” to allow this (another conformational change) & this is often the slowest, “rate-limiting” step 

Many kinases can phosphorylate both Serine AND Threonine (which are both pretty small) but Tyr & His are bulky, so they require “more space” in the active site so they need different kinases. Thankfully, there are lots of different kinases (~2% of the recipes in the cookbook). Thats a lot of kinases and they do a lot of work. ~1/3 of all proteins in your cells likely have at least 1 phosphorylation & proteins can be phosphorylated at multiple sites, by one or multiple kinases. Phosphorylation can be hierarchical -> (e.g. site 2 doesn’t get phosphorylated until site 1 does – sometimes because phosphorylation of site 1 makes it a “match” for a second kinase to phosphorylate site 2) But “what’s the point”? 

When proteins are phosphorylated, they now have a highly concentrated, bulky, charge -> this can cause the protein to change its shape (conformation) (e.g. maybe the negative charge is repulsive so the atoms around it try to “move away” – but because all the protein’s amino acids are linked together, changes in one can have a “ripple effect” with parts of the protein that have a more defined shape (structural domains) often moving as a group and more flexible “linker” regions serving as hinges. You can also have the opposite – if there are + charged amino acids around, they’ll be glad to hang out around the phosphate and can “clamp down” around it & oxygens in the phosphoryl group can hydrogen-bond (H-bond) with group(s) on the protein for a similar “clamping” effect

These conformational changes can activate or inactivate the proteins (e.g. they might bring the catalytic amino acids together to form an active site that wasn’t there before)

The added phosphate also changes a protein’s “appearance” to other proteins and molecules, potentially opening up new binding opportunities. Lots of proteins involved in cell signaling have specific domains that seek out & bind to phosphorylated residues.

Sometimes it’s another kinase that seeks it out -> kinases can phosphorylate other kinases, activating those kinases which then phosphorylate other kinases activating those kinases that then… leads to a phosphorylation cascade that can quickly transmit signals -> and it amplifies the signal along the way because each kinase can activate multiple copies of the kinase it activates. 

That can make way too strong a signal! You need activation regulation! Some kinases have regulatory regions “built in” as “regulatory domains” attached to the “catalytic domain” where the transfer takes place. Others have a separate regulatory subunit (a whole nother protein as opposed to 2 parts of the same protein)

And the signal can be “turned off” by other proteins called phosphatases that remove the phosphates

If the regulation is impaired, kinases can go wild modifying proteins -> tons of proteins end up always on or always off; signal’s on all the time, etc. which can clearly cause problems – in fact, it can cause cancer. which is why kinases are a frequent drug target.

An example of this is Gleevec™️ (imatinib) – a targeted cancer treatment that revolutionized the treatment of a form of leukemia called chronic myeloid leukemia (CML) which causes excess immature white blood cells. Gleevec is a tyrosine kinase inhibitor (TKI) – specifically it inhibits a mutant kinase called Bcr-Abl . More on that in a past post: http://bit.ly/kinases

more about enzymes: http://bit.ly/enzymecatalysis

Leave a Reply

Your email address will not be published.