While most humans are able to hold their breath for only a minute or two, crocodiles can stay submerged under water for an hour or longer. This adaptation allows crocodiles to kill their prey by drowning. In 1995, Nagai and colleagues suggested that bicarbonate (HCO3[-]) binds to crocodile hemoglobin to promote oxygen dissociation in a manner similar to BPG in humans and, in so doing, promotes delivery of a large fraction of bound oxygen to tissues. Information gathered in these experiments may allow scientists to design effective blood replacements.
A. Consider the hypothesis that bicarbonate serves as an allosteric modulator of oxygen binding to hemoglobin in crocodiles. What is the source of (HCO3[-]) in crocodile tissues?
B. Draw oxygen-binding curves for crocodile hemoglobin in the presence and absence of bicarbonate. Which conditions increase the p50 value for crocodile hemoglobin? What does this tell you about the oxygen-binding affinity of hemoglobin under those conditions?
C. The investigators found that the bicarbonate binding site on crocodile hemoglobin was located at the alpha-1 beta-2 subunit interface, where the two subunits slide with respect to each other during the oxy <--> deoxy transition. Based on their results, the researchers modeled a stereochemically plausible binding site that included the phenolate anions of Tyr 41-beta and Tyr 41-alpha and the epsilon-amino group of Lys 38-beta. What interactions might form between these amino acid side chains and bicarbonate?
D. Fish use ATP or GTP as allosteric effectors to encourage hemoglobin to release its bound oxygen. What structural characteristics do all of these different allosteric effectors have in common and how would the molecules bind to hemoglobin?