L1 Mechanism of O2 Transport PDF

PHM142 Erythrocytes and the Mechanics of Oxygen Transport The primary function of RBCs (erythrocytes) is to transport O2 and CO2. Composed of: 1. Erythrocytes, Leukocytes, Platelets 2. Free proteins (albumin, globulin, ferritins, enzymes, hormones) 3. Electrolytes - In tissues where the...

PHM142 Erythrocytes and the Mechanics of Oxygen Transport The primary function of RBCs (erythrocytes) is to transport O2 and CO2. Composed of: 1. Erythrocytes, Leukocytes, Platelets 2. Free proteins (albumin, globulin, ferritins, enzymes, hormones) 3. Electrolytes - In tissues where the pO2 is low (20-30 mmHg, 5mmHg for active) RBCs deliver and drop off oxygen. The RBCs then travel to the pulmonary system where it enters the lungs with a high pO2 of 100mmHg to pick up oxygen. Cycle continues. Erythrocytes - 95% cell protein is hemoglobin. - 300\*10\^6 Hb molecules per RBC. - Hematocrit (the ratio of RBCs to total blood volume) 35-47% (female) or 40-52% (male) - RBC have a variety of membrane transporters on cell surface. - 8 microns large (needs to squeeze through capillaries to tissue) Erythrocyte Development - The stem cell that gives rise to RBCs is the hemocytoblast. - To commit to becoming a RBC, the hemocytoblast differentiates into the proerythroblast. At this stage the cell is committed into becoming an RBC. - The proerythroblast differentiates to the early erythroblast. This is where ribosome synthesis occurs and proteins are being made within the cell. - The early erythroblasts develop into the late erythroblast where Hb accumulation starts. In this stage, there is an upregulation of globin genes and the cell visibly turns redder. - The late erythroblast then develops into the nomoblast and at this stage, the cell will start to eject organelles out of its cytoplasm to maximize the amount of Hb and therefore O2 it can carry. This process will turn it into the reticulocyte. - The nucleus and other organelles are ejected, with ribosomes being the last to leave (so translation can happen until the very last minute). - The mature erythrocyte does not contain a nucleus, mitochondria or ribosomes. - Having no nucleus means that any damage done to the RBC cannot be repaired and is permanent. Instead, the spleen captures injured or old RBCs and destroys them. - Too many reticulocytes in a blood sample can suggest that there is an increased number of RBC destruction, as the body is trying to compensate for the loss by trying to make more RBCs but have not had sufficient time or resources to mature. Erythrocyte Biology - In fetal development, RBCs are produced in the liver. - Bone marrow production starts at 4 months of age. - In the adult, RBC production occurs only in bone marrow. - Because erythrocytes lack nucleus, ER, ribosomes and mitochondria, there is no aerobic respiration (oxidative phosphorylation). Instead, the RBC is entirely anaerobic to produce its ATP sources (glycolysis). This is evolutionarily useful because the role of RBCs is to deliver O2 to tissues that require them the most and thus not using up any O2 for its own use can be good. - No gene transcription and translation occurs at the erythrocyte. All proteins had to be made during the genesis stage of the cell. Globin Synthesis - During fetal development, Hb is made up of 2 alpha chains and 2 gamma chains as opposed to the mature Hb of 2 alpha and 2 beta. This is because the gamma chains are upregulated and beta chains downregulated until after birth. This makes fetal hemoglobin (HbF) have higher affinity to O2, ensuring it has priority for O2 supply. - After birth, beta globins are upregulated and gamma downregulated and developed Hb is made instead. - The sites of globin production occur at the yolk sac, liver, spleen and bone marrow (majority is bone marrow). Myoglobin - Monomer, contains high amount of alpha helices which increases stability. - Heme group found in the middle. - Heme is made up of a series of pyrrole rings with Fe conjugated in the middle. The Fe atom can form 6 bonds. - Catabolism of myoglobin: Fe gets reused (via transferrin), Globin gets broken down to AA by peptidase and heme can get used in different biomolecules like bilirubin. - O2 binding is reversible. - Pyrrole rings are conjugated giving it a vivid red colour, shifts in the conjugation of pyrrole rings can alter colour visible to the human eye. Myoglobin Binding Curve - Myoglobin does not exhibit any allosterism or cooperativity. - The curve is hyperbolic function, displaying fast saturation at relatively low levels of pO2. - The P50 (which is a measure of O2 affinity) occurs at pO2 2.8 torr which means that myoglobin has a high oxygen affinity. - Makes myoglobin an excellent O2 carrier for distributing in active tissues like muscles because it is able to be saturated when O2 levels are low. - Used as secondary oxygen carrier/supply. Hemoglobin - Tetrameric protein. 4 heme groups per protein so each molecule of Hb can hold up to 4 O2 molecules. - Exhibits cooperativity and allosterism. - Binding of O2 at one of the sites will increase the affinity of the other binding sides. - Positive cooperativity. Affinity of Hb for 4^th^ O2 is 100x greater than the first due to conformational changes of Hb when O2 binds to it each time.  - Hb binding curve is sigmoidal which reflects the allosterism and positive cooperativity. - Hb is more hesitant to bind to O2 (lower affinity = higher p50) and so it saturates at a much higher pO2 level compared to myoglobin (p50=26 torr) - However, after the first O2 has bound, the affinity increases which is the reason why the graph rises quickly in the middle section -- accounting for the positive cooperativity. Hill Plot - A measure of cooperativity. - Y = number of binding sites occupied / total number of binding sites. - Y/1-Y = pO2/pO250 - Log Y/1-Y = log (pO2-pO2 50) - 1 means no cooperativity, negative means negative cooperativity and positive means positive cooperativity. Perutz Mechanism - States that Hb exists in two stable conformations: T and R state. - T state = tense state this is DEOXY-Hb. When O2 binds, triggers a conformational change to (out of phase) - R state = relaxed state this is OXY-Hb (in phase). - The switch from T R state is thought to straighten the coordination linkage of the Fe atom to the distal His. - The binding site of Hb is crowded by the E7 His which increases selectivity of molecules to bind to it. - CO is a linear molecule which can easily bind to the site (25 000x affinity to O2), however the E7 His through steric hindrance decreases the affinity of CO over O2 to only 200x more. - D94 and H14 have an electrical attraction and breaking this attraction requires energy which fuels the conformational change from T R state. The Bohr Effect - This is the body's response to local pH effect, and how O2 can be more conveniently off-loaded in active tissues. - In active tissues, CO2 is produced at a larger rate, CO2 reacts with water via the reaction: CO2 + H2O H2CO3 HCO3- + H+ (via carbonic anhydrase) - The decreased local pH of the surrounding tissue will protonate AA residues of T-Hb which will decrease its affinity for O2. Decreasing the affinity for O2 in these tissues would result in better off loading providing active tissues with more oxygen. - In the Hb binding curve, decreased O2 affinity by the Bohr-effect will cause a rightward shift as P50 will have increased.