RESUMO
Ferritin is important in iron homeostasis. Its twenty-four chains of two types, H and L, assemble as a hollow shell providing an iron-storage cavity. Ferritin molecules in cells containing high levels of iron tend to be rich in L chains, and may have a long-term storage function, whereas H-rich ferritins are more active in iron metabolism. The molecular basis for the greater activity of H-rich ferritins has until now been obscure, largely because the structure of H-chain ferritin has remained unknown owing to the difficulties in obtaining crystals ordered enough for X-ray crystallographic analysis. Here we report the three-dimensional structure of a human ferritin H-chain homopolymer. By genetically engineering a change in the sequence of the intermolecular contact region, we obtained crystals isomorphous with the homologous rat L ferritin and of high enough quality for X-ray diffraction analysis. The X-ray structure of human H ferritin shows a novel metal site embedded within each of its four-helix bundles and we suggest that ferroxidase activity associated with this site accounts for its rapid uptake of iron.
Assuntos
Ferritinas/ultraestrutura , Sequência de Aminoácidos , Animais , Sítios de Ligação , Gráficos por Computador , Cristalografia , Ferritinas/genética , Humanos , Ferro/metabolismo , Modelos Moleculares , Dados de Sequência Molecular , Peso Molecular , Ratos , Proteínas Recombinantes , Difração de Raios XRESUMO
A protein factor has been found in serum which converts the M form of placental alkaline phosphatase (PLAP) to the A and B forms. The identity of the conversion products has been confirmed by analysis of their dimers and polypeptides. Proteolysis is not implicated in this phenomenon. This report establishes microvillous M-PLAP as the precursor of the A and B forms.
Assuntos
Fosfatase Alcalina/isolamento & purificação , Isoenzimas/isolamento & purificação , Placenta/enzimologia , Fosfatase Alcalina/sangue , Feminino , Humanos , Isoenzimas/sangue , Microvilosidades/enzimologia , Peso Molecular , Gravidez , Conformação ProteicaRESUMO
RNA prepared from human placental syncytiotrophoblast tissue was analysed by in vitro translation and immunoprecipitation. The major polypeptide synthesized from syncytiotrophoblast mRNA was human placental lactogen, accounting for 15-20 per cent of the translated polypeptides. Immunoprecipitation using antibodies to placental alkaline phosphatase (PLAP) specifically precipitated polypeptides of 56 kD and 58 kD. The possible relationship between these polypeptides and those of placental microvillus PLAP is discussed.
