Understanding Protein Kinases: Functionality, Specificity, and the Role of Protein Kinase A

protein kinases are these enzymes found inside our body which are responsible for catalyzing for sporulation reactions

and as we discussed previously phosphorylation is an example of covalent modification and this is a

mechanism that our cells use to basically regulate and control the activity of enzymes and the

functionality of proteins now we can categorize protein kinases into two groups on one side we have a group

called dedicated protein kinases and on the other side we have a group known as multi functional protein kinases now

what exactly is the difference between these two groups well dedicated protein kinase is basically phosphorylate either

a single substrate molecule or a set of closely related substrate molecules on the other hand multifunctional protein

kinases have the capability of actually catalyzing the phosphorylation of many different types of enzymes and many

different types of protein molecules now the first question I'd like to address is what exactly determines the specific

nature of protein kinase is what exactly determines the ability of a protein kinase to actually bind and catalyze a

specific location on some enzyme on some substrate molecule well for a catalyzation reaction to actually take

place logically speaking that substrate molecule has to be able to fit into the active side of that protein kinase and

not only got the subject molecule has to have a high enough affinity so that it remains long enough in the active side

for that phosphorylation reaction to actually take place in the first place and so what that means is to have a high

affinity between the substrate molecule and the protein kinase the sequence of amino acids around the side around the

amino acid which is a balanced ebbr which is about to be phosphorylated has to be correct because if the SI

of amino acids on the substrate molecule is not correct that active side will not be able to bind on to that substrate

molecule so we conclude that the specificity of protein kinases depends on the amino acid sequence that directly

surrounds that target residue that is about to be force for elated now to actually see what that means let's

discuss a specific example of a specific a specific protein kinase that exists inside our body and this is protein

kinase a or simply PKA so what exactly is the importance what is the functionality of protein kinase a when

does our body actually use protein kinase a well in dangerous or exciting or stressful situations recall that it's

the sympathetic division of our nervous system that kicks in and initiates the flight-or-fight response and what

happens is the sympathetic nervous system basically stimulates the adrenal medulla to release a hormone we call

epinephrine and as epinephrine travels through our cardiovascular system it basically stimulates our cells to

transform ATP molecules into another molecule known as cyclic adenosine monophosphate or simply C A&P now what

CA and P does is it's an allosteric regulator of protein kinase a and as we'll see in just the moment it binds on

to the inactive version of protein kinase a and it activates protein kinase a and protein kinase a once it's

activated it becomes responsible for activating many different types of enzymes via the process of

phosphorylation and it phosphorylates one of two types of residues either the serine residue or the threonine residue

now the question is on any given substrate enzyme molecule we have many different types of

Arina and threonine amino acids so let's suppose we have a sequence of 200 amino acids and let's say 20 of them are

actually serine or threonine amino acids the question is how this protein kinase a actually know which serine or

threonine amino acid to actually bind to what determines the specificity of that protein kinase a so it's exactly what we

said before it's the sequence of amino acids that is found around the side that residue that is about to be forcefully

that determines the ability of that protein kinase a PKA to actually bind onto that substrate molecule now what is

the sequence or the sequence known as the consensus sequence is shown on the board so basically if the substrate

molecule contains arginine arginine X serine 3 or threonine and Y where X is basically any small amino acid for

instance glycine and Y is basically any large hydrophobic amino acid this is basically where that protein kinase will

by 2 and what it will phosphorylate this target side the serine or threonine now of course we can also change the

arginine to lysine and that will also allow the protein kinase to bind on to the sequence but if these are changed to

lysine the affinity will not be as will not be as good as in the case where these two are arginine molecule arginine

amino acid so we see that the consensus sequence that is recognized by protein kinase a is shown to the left and this

means that enzymes substrate molecules that contain serine or threonine surrounded by this specific sequence

will be recognized by the protein kinase a and the protein kinase a will bind onto this section and will fast for

allayed this target side and by phosphorylated it basically change the activity and the

functionality of that target substrate molecule now earlier we said that it's the cyclic a and P molecule that

actually binds and activates protein kinase a now the first question is what exactly is the quaternary structure of

protein kinase a when it is non bound to the cyclic a and P what is the inactive coronary structure protein kinase a so

this is basically shown on the board so in its inactive form PKA consists of two types of subunits so just like aspartate

transcarbamoylase atcs consists of catalytic and regulatory subunits our PKA also consists of

catalytic and regulatory subunits now the catalytic subunit contains the active side while the regulatory subunit

contains that allosteric site that binds onto the cyclic a and P so in the absence of the allosteric effect that

the cyclic adenosine monophosphate molecule the quaternary structure consists of two catalytic sites and

these are shown in green as well as two regulatory sites and these are shown in light brown so this is basically one of

these regulatory subunits and this is the other regulatory subunit and so we have two catalytic and two regulatory

subunits and so we where are we represent the inactive form of PKA with the following format so R 2 C 2 complex

basically means we have two regulatory subunits 1 2 & 2 catalytic subunits 1 & 2 so as we mentioned previously on their

stressful situations the adrenal medulla stimulated and reached and it releases the epinephrine hormone and the

epinephrine hormone basically stimulates the production of cyclic adenosine monophosphate CMP and it's the C

and P that is the allosteric regulator of PKA the question is how exactly does regulate the activity and how does it

activate this enzyme so what happens is if we examine a single one of these regulatory chains each regulatory chains

contains one two allosteric sites so we have one two and two here so we have one two three four of these regulatory sites

that the cyclic adenosine monophosphate can actually bind to now once that cmp binds onto all of these regulatory sites

so we have four CMP molecules binding to four of these sites what happens is that creates a conformational change that

allows the are too complex so this entire Brown section to actually dissociate from these two green sections

and so what happens is these active sites which are occupied in the inactive form basically become unoccupied they

become free and once the active sites are free these catalytic subunits and we have two of them can basically go on and

catalyze all these different types of target enzymes via the process of phosphorylation so once again cyclic

adenosine monophosphate binds to allosteric sites found on the regulatory chains this stimulates the dissociation

of the regulatory subunits from the catalytic subunits and once the catalytic subunits basically dissociate

those active sites become free and now the substrate molecules can go on and by onto those active sites and by the way

this is the active site here and this is the active site here for this second green subunit and in this inactive form

both of these are both of these active sites are basically an occupied by regions of the regulatory subunits and

the sequence of amino acids found on the rats for a subunit that binds until the

active side is known as the pseudo substrate sequence so in this r2 c2 complex if the pseudo substrate sequence

of the are subunits that binds unto the active side and occupies that active side and because that active side is

occupied it cannot bind to substrate molecules and so it is not active but if our concentration of cyclic a and P

inside our body begins to increase what happens is those CMP molecules will begin to bind onto these allosteric

sites and once bound to all four allosteric sites that creates conformational changes that causes these

sewer substrate sequences to the sociate from the active sites and that produces this are too complex so notice that

these two regulatory are subunits do not actually dissociate they remain bound together but these two green catalytic

subunits basically dissociate and so now they're activated because these active sites are free they can go on and bind

to all different all sorts of different types of substrate enzyme molecules and once the substrate molecule binds on to

the active side of that catalytic subunit what happens is this catalytic subunit which actually consists of these

two lobes so we have lobe number one lobe number two once that active side becomes bound onto the substrate

molecule those two lobes essentially close off and as they close off they basically create a perfect conformation

between the substrate molecule and this catalytic subunit and that allows the phosphorylation reaction to actually

take place