inside the cells of our body a process we call aerobic cellular respiration uses oxygen to produce ATP molecules and these ATP molecules are used by ourselves as an energy source now what exactly delivers the oxygen to the cells of our body well we have two proteins inside our body that play this role they deliver oxygen to the cells of our body and these two proteins are myoglobin and hemoglobin globin is a protein that consists of a single polypeptide chain and it is found in the muscle cells of our body it is used by our muscle cells to store oxygen and give the muscle cells the oxygen when the concentration of oxygen becomes very very low on the other hand hemoglobin is a protein that consists of four individual polypeptide chains we have alpha 1 and Alpha 2 which are two identical Alpha chains and we have beta 1 and beta 2 subunits which are also identical subunits and so these four polypeptide chains give the hemoglobin molecule quinary structure and that's exactly what gives that hemoglobin the ability to bind oxygen cooperatively and we'll see what that means in the next lecture so what hemoglobin does is it essentially continually delivers the oxygen from the lungs and the tissues of cells of our body and it also binds CO2 and brings the carbon dioxide back to the lungs so that the carbon dioxide can be expelled by our body now in this lecture what I'd like to focus on is how these two proteins actually are capable of binding to oxygen in the first place so these two proteins contain a special prosthetic group known as the heem group that assists the protein in actually binding the oxygen and the heem group has the following structure so the heem group consists of two components it has the organic component known as protor that contains the carbon atoms the nitrogen atoms the hydrogen atoms and the oxygen atoms and this entire region shown in black is the protop porer it's the organic component of that heem group now at the center of that protopine is an inorganic atom a metal atom the iron atom and this is what makes up the inorganic component of that heem group and it's this Fe atom that is actually responsible to not only binding to the protein but also to binding to that oxygen as we'll see in just a moment now notice as shown in this diagram this Fe atom is bound to four nitrogen atoms we have 1 2 3 4 now Fe can have an oxidation state of postive 6 and in this particular case because we have four bonds what that means is this Fe is in its fairest State and what that means is it has a state an oxidation number of positive2 and so our Fe atom at the center of the heem group can form two other bonds now one of the bonds is formed between one of the amino acids of that polypeptide chain and this is shown in the following diagram so if we take the heem group and we flip it this way so if the heem group lies on the plane of the board and we take it and we flip it this way then at the bottom portion of that heem group we have an amino acid more specifically we have a histadine amino acid that is part of the protein either myoglobin or that hemoglobin that is bound onto that Fe atom so this purple circle is the Fe atom the green circle are the nitrogen atoms the blue circles are the oxygen atoms and the purple circle is that metal atom that Fe atom so on one side of the protor and plane the iron atom is boun to the histadine residue of that polypeptide chain that polypeptide chain can can be part of the myoglobin or it can be part of the hemoglobin molecule so because each polypeptide chain contains a single heem group myoglobin contains a single heem group but hemoglobin contains four different heem groups and that means myoglobin can only bind onto one oxygen while hemoglobin can bind four different oxygen atoms as we'll see in just a moment so on the bottom of that Fe we have the bond form between the nitrogen of this histadine residue and this metal atom found in that heem group now in this particular State the electron density around that Fe is simply too large for that Fe to actually fit inside the center of that plane and that's exactly why this Fe atom will be found slightly below the protop porer plane as shown in the following diagram so in deoxyhemoglobin or deoxy myoglobin when the protein is not Bal to the oxygen the iron adal remains unbal to oxygen and in this case the F metal atom is simply too large it has too large of an electron density around that proton nucleus for that entire metal atom to actually fit Snuggly in the center of that protop porphin and so what that means is this Fe atom will be found slightly below now in this particular case this Fe atom has 1 2 3 four five bonds and what that means is it can form one more bond with some other atom and so on the top portion of that Fe that is exactly where that Bond will be formed between the datomic oxygen and that metal atom remember it's this metal iron atom of the heem group that binds directly and holds onto that datomic oxygen so if this is the Unbound state of our protor then when the oxygen actually binds what happens is this datomic oxygen moves from the top position of that Fe and it begins to pull away some of that electron density from the that metal atom remember oxygen is the second most electronegative atom on the periodic table and it is much more electr negative than this metal iron atom and so what that means is the electron density will be pulled away from that metal atom decreasing the radius and the size of that metal atom and so what happens is because the size of this iron atom decreases it now is able to fit at the center of that protor Forin group and so this is what it will look like when it will be bound to that datomic oxygen so this is the datomic oxygen that is bound to this metal atom and it pulls away the electron density decreasing the size of the metal atom and now the metal atom is able to fit into the center of that protop porer plane now what exactly is the structure that describes this complex here well this complex between the metal atom and the datomic oxygen can be described by a resonance stabilized structure as shown on the board so these are the two electron structures that describe the resonance stabilized complex between our Fe atom and the datomic oxygen so on this side on this electron configuration we have a DI oxygen that contains a neutral charge and we have the iron that contains a positive two a positive two charge so it is in its fairest state but what happens is because the dioxygen because the datomic oxygen consists of these two electronegative atoms they can pull away an electron readily from that feris atom and create a feric ion that contains a positive three charge and so one of the electrons will be pulled away from the metal atom and onto the oxygen giving this datomic oxygen a negative charge and this is called a super oxide ion in fact this super oxide feric ion complex is the resonance structure that more closely describes what the structure is between these uh two atoms the two oxygen atoms and the single metal atom so we see that when the datomic oxygen binds onto that Fe from the top that datomic oxygen develops a negative charge because it is more electr negative and it pulls away those electrons that electron from the metal atom and that's precisely what moves this entire residue up and what allows this metal atom to fit into the center portion of that protor group now because the datomic oxygen gains a negative charge it becomes slightly less stable and so we have to be able to somehow stabilize this datomic oxygen inside that heem group and in fact what happens is we have another histadine residue that is part of the polypeptide chain inside that protein either myoglobin or hemoglobin that binds creates a hydrogen bond with that negative charge and this is shown on the follow in the following diagram so the actual structure of the iron oxygen complex is resonance stabilized as shown in this diagram one of it consists of this structure and the other one consists of this structure here and notice that the super oxide oxygen form has a negative charge on that oxygen and that destabilizes it and to stabilize this structure we see that a region of the protein another histadine amino acid forms a hydrogen bond with this oxygen as shown in the following diagram and this residue is known as the disal histadine so this here is known as the proximal histadine and the proximal histadine is the amino acid of that polypeptide chain that forms a bond with that Fe atom that holds the he group to that protein and it's the disal histadine that is found on the opposite side on the opposite plane of the protopine that is responsible in forming a hydrogen bond between this oxygen here of this datomic oxygen and this nitrogen of that histadine and this stabilizes that diatomic oxygen so we see that if we're talking about myoglobin or or hemoglobin both of these proteins contain a heem group that are that is responsible for binding that datomic oxygen and it's the iron atom the metal atom at the center of that heem group that is actually responsible for directly binding onto that datomic oxygen now the other side of that iron atom is actually bound onto the amino acid found in that protein and it's that bond that hold the he gr inside that protein in the first place we see that we have the distal histadine which is another Amino Acid found inside that protein that is able to actually interact and stabilize that negative charge on the super oxide ion and as we'll discuss in a future lecture it's this stabilization that also allows the unloading the release of this datomic oxygen in its dioxygen form to the cells and the tissues of our body