Structural basis for salinity-induced alteration in oxygen binding by haemocyanin from the estuarine amphipod Chaetogammarus marinus (L.).

In the estuarine amphipod Chaetogammarus marinus, differences in O(2) binding by haemocyanins (Hc) could be related to natural and salinity-related quantitative variation in just one polypeptide subunit (band 2) and not to variations in any of the other seven bands present. Band 2 was always present, irrespective of salinity treatment, and naturally makes up 6%-36% of the total Hc. However, low salinity exposure (S=4 per thousand for 48-49 h, T=15 degrees C) was accompanied by an increase in the prevalence of greater concentrations of band 2 (i.e., range, 20%-36% of total Hc present) and a concomitant increase in Hc-O(2) affinity (half saturation [P(50)] decreased from 1.38 to 1.12 kPa at pH=7.81, T=15 degrees C). A similar salinity-related mechanism (that is, one band altering) has been shown previously for the blue crab Callinectes sapidus, although the functional consequences were different. In contrast with C. marinus, an increase in the proportion of one polypeptide subunit in C. sapidus resulted in a decrease in Hc-O(2) affinity. This study has confirmed that between-individual variation (quantitative rather than qualitative) in just one Hc subunit may have functional consequences, although the significance of such variation is difficult to interpret.