- 期刊
Molecular Respiratory Mechanisms in the Cambrian Metazoans
並列摘要
Oxygen content in the atmosphere has gradually increased during the geological history, owing to the increasing oxygen-producing photosynthetic activities during the early evolutionary history of life on the earth. Sufficient concentration of free oxygen (O2) in the environment is necessary for the rise of animals in Late Proterozoic and their adaptive radiation at the dawn of the Phanerozoic (i.e., the Cambrian “explosion”). And, the mechanism for utilizing oxygen by organisms should be one of the most essential factors affecting their success and diversification in the later evolutionary cause. Animals intake oxygen in their metabolism through different ways both analytically and at the molecular level. Specialized molecules (proteins) that transport O2 to different parts of the animal body are referred to as respiratory proteins or “molecular lung”. Majority of vertebrate animals has the most advanced molecular lung – hemoglobins and myoglobins – which is able to exchange O2 and CO2 very efficiently between the animal and its environment. The respiratory protein of siphunculids, priapulids, brachiopods and some annelids is an Fe2+-binding protein hemerythrin (Holmes and Stenkamp, 1991). Because arthropods and mollusks comprise /he most abundant and diversified fossil groups in the Early Cambrian fauna, their respiratory proteins, hemocyanin and related copper proteins, are of the primary interest in this study. For most other metazoan phyla, including Cnidaria, Ctenophora, Gnathostomulida, Rotifera, Kinorhyncha, Gastrotricha, Acanthocephala, Onycophora, Linguatulida, Bryozoa, Pentastomida, Entoprocta, Chaetognatha, Hemichordata and the subphyla Cephalochordata and Urochordata of Chordata, no specialized respiratory proteins are discovered (Lu, 1998); therefore, these animals presumably transport O2 and CO2 within their body through the relatively slower process of diffusion. Hemocyanin has been found in today’s crustaceans and chelicerates of Arthropoda and cephalopods of Molluscs. Arthropod hemocyanin has heterogeneous subunits each with a molecular weight of about 75 kDa; the subunits form hexamers and hexamer compounds. Each subunit has two Cu-binding sites and each site can reversibly bind one molecule of oxygen. Molluscan hemocyanin is made of 7 to 8 functional subunits each of 50 kDa and also with two Cu-binding sites that can reversibly bind oxygen. The efficiency for oxygen transportation by hemocyanin is lower than for globins (most vertebrates) but much higher than that by hemerythrin (molecular lung in brachiopods, siphunculids and priapulids) which has 8 functional units each possessing two Fe2+-binding sites, of which only one site binds oxygen. Hemocyanin and related copper proteins are ancient molecules (Durstewitz and Terwilliger, 1997) and could have arisen during the Proterozoic when the atmosphere’s oxygen content reached a considerable high as compared with the modern atmosphere. Possession of highly efficient respiratory proteins could help metazoans acquire better mobility in highly oxygenated environment and, thus, higher predatory capability, which is essential of their own survival and dominance. Considering the modern distribution of the types of respiratory proteins and the composition of the Cambrian fauna, it is suggested here that hemerythrin and hemocyanin were the most probable molecular lungs in the Early Cambrian metazoans. The Cambrian arthropods (including trilobites, crustaceans and related taxa) and mollusks most likely used hemocyanin for transporting O2 and CO2 which was the most effective molecular lung at that time, for hemoglobins and myoglobins did not occur or dominate until the vertebrate animals abundantly occurred on the earth in the Silurian Period). Other major metazoan groups that occurred during the Early Cambrian radiation, such as the lobopodians, hyolithes, xandarellids, tegopeltids, naroiids, fuxianhuids, opabinids and anomalocarids, might also have used hemocyanin as their molecular lung. Because of its relatively high efficiency in O2 transportation, hemocyanin is considered essential for the arthropods to become the primary predators (such as Anomalocaris, Chen et al. 1994) and dominate the Cambrian Ocean. This effective molecular respiratory mechanism is also significant for the diversification and later evolution for arthropods and mollusks. It is believed that the rise of a new and more efficient respiration mechanism (i.e.,the occurrence of hemocyanin) among metazoans probably in the Late Proterozoic, following the increased oxygen concentration in the atmosphere, was one of the most important foundations laid down for the Cambrian”explosion”.