hemoglobin and myoglobin are oxygen binding proteins hemoglobin is found in blood and myoglobin is abundant in skeletal and cardiac muscle hemoglobin is an oxygen transporter and myoglobin is an oxygen store myoglobin is a globular protein made up of a single polypeptide chain 153 amino acids long hemoglobin is also a globular protein roughly spherical in shape but it is a tetra mer and is composed of four polypeptide chains specifically hemoglobin is an alpha 2 beta 2 type tetra mer meaning it has two identical alpha chains and two identical beta chains each of hemoglobins four subunits is very similar to the polypeptide chain making up myoglobin the alpha chains are 141 amino acids long while the beta chains are 146 amino acids long the myoglobin polypeptide chain consists of eight alpha helix sections which are denoted a 2h each polypeptide chain of the four haemoglobin subunits also consists of these eight alpha helix sections between these alpha helixes are connecting regions named after the HeLa C's they connect for example the a B region or bc region amino acids in each helix section are numbered for instance histidine F 8 both myoglobin and hemoglobin contain a prosthetic group which is a non protein group forming part of or which is combined with a protein the prosthetic group found in both myoglobin and hemoglobin is the heme group the heme group is made up of a proto porphyrin ring and a central iron atom there is a heme group in each of him a globin subunits as well as in myoglobin z' polypeptide chain nestled and the cleft between the E and F HeLa sees the irony each shame group is the part of both proteins that binds to oxygen Iren can interact with six ligands and four of these are provided by the nitrogen atoms of the P roles in the porphyrin ring a fifth is provided by the imidazoles sidechain of histidine f8 when oxygen binds to the iron that is a sixth ligand note that when oxygen is added on it is tilted at 60 degrees to the perpendicular a really cool conformational change happens when oxygen binds to the iron in the Hebrew this cool phenomenon is of no consequence in myoglobin but hemoglobins biological function depends on it before the binding of oxygen steric constraints result in the ferrous iron lying 0.05 5 nano meters above the porphyrin plane the binding of oxygen causes the iron to be drawn into the plane of the porphyrin ring so that it is only 0.02 6 nano meters above it the movement of the iron drags histidine afaid along with it and sets off a chain of conformational changes in hemoglobin that results in increased affinity of the heme groups of adjacent subunits for oxygen in hemoglobin the four subunits the two alpha subunits in the two beta subunits are arranged into two dimeric halves 1 alpha 1 beta 1 subunit pair and 1 alpha 2 beta 2 subunit pair each of these dimeric halves moves as one rigidbody subunits interact mostly with dissimilar chains in other words alpha subunits interact with beta subunits but not alpha subunits and beta subunits interact with alpha subunits but not beta subunits there are two types of contacts between the two dimeric halves of hemoglobin packing contacts and sliding contacts packing contacts do not shift during the conformational changes that occur after the binding of oxygen while sliding contacts do when oxygen binds the conformational change results in the dimeric halves rotating 15 degrees relative to one another hemoglobins two confirmations are called the T 4 tensor taught and R 4 relaxed forms when hemoglobin is in the t form oxygen is only accessible to the heme group of the alpha chains steric hindrance prevents it from binding to the beta chains this steric hindrance is not present in the r conformational state hemoglobin resists oxygenation because it's deoxygenated form the t form is stabilized by certain hydrogen bonds and interchain salt links these interactions are broken in the oxygenated form the our forum where hemoglobin is stabilized in a different conformation meanwhile myoglobin does not easily release oxygen when myoglobin binds oxygen it becomes oxy myoglobin oxy myoglobin releases oxygen during times of extreme oxygen deprivation like when you're exercising while myoglobin is oxygen binding interaction displays classical Michaelis Menten type saturation behavior hemoglobins interaction results in a sigmoid shaped curve rather than a hyperbolic one the sigmoid shape allows us to draw some conclusions binding of oxygen to one subunit of hemoglobin strongly enhances binding of oxygen to other subunits a phenomenon called cooperativity hemoglobin binds oxygen in the lungs where the partial pressure of oxygen is around 100 tor here 98% of hemoglobin has oxygen bound to it in the capillaries of some tissues the partial pressure of oxygen is 40 Torr and the hemoglobin releases oxygen here 6% of hemoglobin has oxygen bound to it the 92% difference is thanks to cooperativity if hemoglobins curve was hyperbolic instead of sigmoidal then only 79% of hemoglobin in the lungs would have oxygen bound to it and 28% of hemoglobin in the capillaries would have oxygen bound to it for a difference of 51% so the cooperativity means that hemoglobin is 92 over 51% or 1.8 times more efficient at delivering oxygen if you liked this video like and subscribe you can also support me by following the link to my patreon if you have any topics you'd like me to cover please leave a comment