Zoology 344 O2 Dissociation

Hemoglobin and blood pigments.

Respiratory Pigments • Among animals, there are 4 classes of respiratory pigments which bind reversibly with O2 at specific bindings sites. • Binding site to O2 is coupled to metal ions. • Hemoglobins • Among all vertebrates and some invertebrates. • Red in colour due to iron (Fe) • Hemocyanins • Two classes – Arthropod hemocyanin (crabs, lobsters, crayfish) – Mollusc hemocyanin (squid, octopuses, snails) • Blue in color due to copper (Cu) • Chlorocruoins • Among some marine annelid worms • Greenish/red in color due to different iron porphyrin group. • Hemerythrins • Among brachiopods and one family of marine annelid. • Colorless when deoxygenated but reddish/violet when oxygenated due to iron (Fe3+).

Structure of a Heme group

All heme groups are metalloporphyrins. It consists of ferrous hexahydrate and protoporphyrin components. It is a ferrous (Fe2+) ion with 6 linkages. 4 with N- groups of the protoporphyrin, another with water, and the last one to either water of a N- of an amino acid (usually histidine) of a polypeptide. Among Vertebrates, there are 2 primary O2 carrying molecules, myoglobin and hemoglobin.

Myoglobin is a monomeric protein found in muscle tissue of higher vertebrates and in the blood of lampreys. In higher vertebrates, it is responsible for uptake and storage of O2 and serves as a reserve store of O2. In hagfish, there is a dimeric form of the protein. Myoglobin consists of a single polypeptide chain whereas hemoglobin consists of 4 subunits . Myoglobin plays an important role in diving mammals. • - In the human tetrameric form, each subunit can bind 1 O2, therefore up to 4 O2 can be bound in each hemoglobin. • Hemoglobins that are oxygenated are called oxyhemoglobins. - Those that are not oxygenated are called deoxyhemoglobins. • The “reduced ferrous form” of hemoglobin is called methemoglobin and does not bind oxygen. • - In normal blood, there is an enzyme, methemoglobin reductase, that reduces the ferric form to the ferrous form -. This prevents methemoglobin from forming. • - Hemoglobin has a 200 times greater affinity for CO than for O2. Therefore, carbon monoxide poisoning can occur even at very high O2 partial pressures. • Other compounds (e.g., nitrites and chlorates) can act to oxidize hemoglobin or to inactivate methemoglobin reductase, thus impairing O2 transport. inside tissue lung cell

Note: differences in P50 between hemo- & myoglobin Hemoglobin- Hb

• Hb molecule consists of 4 subunits – 2 alpha subunits – 2 beta subunits • Each subunit binds 1 molecule of oxygen. • Binding of one molecule of O2 enhances the binding affinity of the next molecule which enhances the binding affinity of the next… This is called cooperative or facilitative binding P50 • Partial Pressure of Oxygen, PO2 • the portion of the air pressure that is exerted by the O2 present • e.g., normal atmospheric pressure = 760 mm Hg • % O2 in air ~ 21% • PO2 under normal atmospheric pressure • would be ~ 150-160 mm Hg • - in the lab, you will need to calculate the exact PO2 values under different conditions, accounting for differences in humidity, latitude and temperature

• P50 Values for O2 Carriers • the partial pressure of O2 that causes 50% saturation of O2 binding • a useful way of comparing the O2 binding affinity of various O2 carriers HbO2 dissociation curve.

• HbO2 curve is best understood % saturation as an unloading curve as at pO2 of the lungs, Hb is almost completely saturated. Normal Increased affinity (decreased unloading- leftward shift (lower p50)) Decreased affinity tissues lungs pO2 (increased unloading- rightward shift (higher p50)

Fetal Hb has a higher affinity for O2 than maternal Hb.

Why? From Hill Fig 22.2 pH effects on Hb binding

• Acidification decreases the carrying capacity of hemoglobin. •Immediate effect is unloading of O2 into the local blood solution (caused by lactic acid). •O2 is rapidly available for tissue use. •Used by many teleosts to fill swim bladder with a high oxygen gas mixture or provide O2 to metabolically very active tissues (retina) Increasing Temperature Causes rightward shift (decreased affinity, increased unloading) Other Factors affecting the oxygen dissociation curve

Temperature • Increasing temperature shifts the curve to the right. Other metabolites • 2,3-diphosphoglycerate concentrations. This is a by- product of glycolysis in the RBC. This tends to bind to the beta chain and reduces the affinity of the heme group for oxygen. • This tends to shift the curve to the right. In exercise situation, the right shift is beneficial as this helps to unload oxygen. • Other phosphates - increases in IP6 in bird eurythrocytes and ATP in fishes also causes the right shift. Things to watch out for in this lab:

• Corrections for partial pressure readings • Avoid leaks in system • Have enough hemoglobin solution to cover the light path of the spectrophotometer • Foaming of blood preparation is bad • Appropriate time for equilibration • Know which valve to turn and when to open the valve; don’t let out the vacuum at the wrong time • Make use of both “Absorbance” and “Transmission” scales