Question

Naked mole rats were recently discovered to be able to survive 18 minutes of an anoxic (0% oxygen) environment without any difficulty. In 1976, the naked mole rat hemoglobin was studied and compared to that of mice and found to have higher oxygen affinity. The data below are representative of this study.

Table 1: Whole blood from each organism was assayed at pH 7.4 for oxygen affinity at two different temperatures. The P₅₀ is reported at each condition in the table.

Species	30°C	37°C
Heterocephalus glaber	18.8mmHg	23.3mmHg
Mus musculus	24.9mmHg	33.0mmHg

Naked mole rats have a body temperature of 30-32°C, which is lower than the 37°C body temperature of most other mammals including Mus musculus.

A. Quickly look up data regarding the solubility of oxygen in water versus temperature. Does body temperature alone explain the greater oxygen binding capacity of Heterocephalus glaber compared to Mus musculus?

B. What do these p50 values indicate about the oxygen binding capacity of Heterocephalus glaber hemoglobin compared to Mus musculus hemoglobin?

C. Furthermore, the amount of 2,3-BPG in each organism’s bloodstream was assayed. [2,3-BPG] in Heterocephalus glaber was found to be 7.3mM and for Mus musculus, 7.4mM. Additionally, when hemoglobin from each organism was stripped of any ligands and then assayed for oxygen binding, the p50 for Heterocephalus glaber was found to be 8.0mmHg and the p50 for Mus musculus was found to be 11.3mmHg. What do these data tell you about the mechanism by which the affinity of each organism’s hemoglobin differs?

D. The protein sequence for the b-subunit of the naked mole rat hemoglobin is as follows:

HLSNE EKAAV TSLWG KVNVE ETGGE ALGRL LVVYP WTQRF FEHFG DLSSP SAIMG NPKVK AHGAK VLASF SEGLN HLDNL KGTFA KLSEL HCDKL HVDPE NFRLL GNVIV IVLAH HHGHD LTPTV QAAFQ KVVAG VAHAL GHKYH

Note that the actual structure of hemoglobin or the actual protein and mRNA sequences of other subunits of hemoglobin in the naked mole rat have not been studied. The mRNA information available is predicted from genome sequencing efforts, but has not been experimentally verified or studied.

What is the significance of the shaded amino acids? Hint, use your book. Are there any major differences in the two sequences that might explain the phenotype? Do not spend too much time agonizing over this.

Answer 1