Lecture 11: Respiratory Pigments 2023 PDF

Biology 2601: Organismal Physiology Respiratory Pigments Pigments and diffusion of gases Gases that combine chemically with other molecules no longer contribute to partial pressure CO2 + H2O H2CO3 Implications for gas diffusion? O2 Transport- respiratory pigments Heme is a porphyrin ring comp...

Biology 2601: Organismal Physiology Respiratory Pigments Pigments and diffusion of gases Gases that combine chemically with other molecules no longer contribute to partial pressure CO2 + H2O H2CO3 Implications for gas diffusion? O2 Transport- respiratory pigments Heme is a porphyrin ring complexed with an iron atom. Myoglobin is a monomeric protein with a single heme molecule. O2 Transport- respiratory pigments Hemocyanins are blue, copper containing, non-porphyrin proteins that have multiple O2 binding sites (up to 160). They are found extracellularly in some molluscs and arthropods. Deacapod crustacean (lobster) Cephalopod mollusc (squid) O2 Transport- respiratory pigments Chlorocruorins have a modified porphyrin ring with iron to bind O2 but they are green. They are found in four types of marine annelid worms. Feather duster worms O2 Transport- respiratory pigments Hemerythrins are non-porphyrin, iron-containing proteins that occur intracellularly in muscle and blood cells. They are found in some types of worms and brachiopods. Sipunculid worm Brachiopod O2 Transport- respiratory pigments Hemoglobin is found in blood and may have up to four heme-globin subunits. The oxygen dissociation curve Binding of oxygen at one subunit of multimeric hemoglobin facilitates binding at other subunits, producing a sigmoid relationship between PO2 and % O2 saturation (or O2 concentration). The oxygen dissociation curve The plateau phase determines O2 loading at the gills or lungs. The steep phase determines O2 unloading at the tissues. The oxygen dissociation curve As PO2 decreases, less and less of a drop results in greater off-loading of O2 . The oxygen dissociation curve Myoglobin shows no cooperativity, and always has greater O2 affinity than hemoglobin. The oxygen dissociation curve P50 is a measure of the binding affinity of a respiratory pigment for O2. The PO2 needed to reach 50 % saturation. The oxygen dissociation curve % Saturation P50 is a measure of the binding affinity of a respiratory pigment for O2. The PO2 needed to reach 50 % saturation. Duck P50 ≈ 6kPa PO2 The oxygen dissociation curve % Saturation P50 is a measure of the binding affinity of a respiratory pigment for O2. The PO2 needed to reach 50 % saturation. Trout P50 ≈ 4kPa PO2 Hematocrit The fraction of blood volume made up by red blood cells can vary and is maximal at around 55%. The oxygen dissociation curve The amount of O2 in blood depends on PO2 and hematocrit. Factors that affect P50 - pH The Bohr effect or shift Factors that affect P50 – PCO 2 The Bohr effect or shift Factors that affect P50 – temperature Organophosphate modulators A variety of organic phosphates in blood cells bind to hemoglobin and reduce O2 affinity. Fish: ATP, GTP, Inositol pentaphosphate (IPP) Amphibians: ATP, 2,3-diphosphoglycerate (DPG) some GTP Reptiles: ATP, GTP Birds: DPG (embryo), IPP (adult) Mammals: DPG Organophosphate modulators Fetal hemoglobin In mammals fetal blood has higher O2 affinity than adult blood due to different globin subunits, lower DPG levels or less sensitivity to DPG. Non-pregnant Schmidt-Nielsen. 1997 Animal Physiol. Fetal hemoglobin Human fetuses express gamma globin rather than beta globin. Environmental O2 Active animals living in high O2 environments generally have high P50 and a strong Bohr shift. Animals in low O2 conditions often Have a low P50. Wilmer, Stone, Johnston. 2000. Environ. Physiol. of Animals Environmental O2 Burrows can have low O2 (6 %) and high CO2 (6 %) concentrations. Burrowing mammals generally have a low P50 and a blunted panic response to high CO2. Prairie dog (burrower) Flying squirrel (non-burrower) McNab. 2001. Physiol Ecol of Verts. Environmental O2 Evolutionary adaptation to high altitude is characterized by high affinity hemoglobin. Body size Small mammals generally have a higher P50 and a stronger Bohr shift that favors O2 unloading at the tissues. Schmidt-Nielsen. 1997 Animal Physiol. Body size Acclimatization Animals may shift globin gene expression and the amount of respiratory pigments in response to changes in O2 availability. Carbon dioxide transport Carbon dioxide is transported in blood mainly as bicarbonate ion (HCO3-). [HCO3-] [H+]/[CO2] = K Since H+ is rapidly buffered by terminal aminos and imidazole side groups of histidine (mainly on hemoglobin) and [CO2] is constant, [HCO3-] can become very high. Carbon dioxide transport Carbon dioxide transport Carbon dioxide transport Carbonic anhydrase reversibly catalyzes the reaction CO2 + H20 HCO3- + H+