Porin (protein)  From Wikipedia, the free encyclopedia Jump to:
navigation, search A single monomer of the same protein in side
view, illustrating the antiparallel beta barrel structure.
Porins are beta barrel proteins that cross a cellular membrane
and act as a pore through which molecules can diffuse.[1] Unlike
other membrane transport proteins, porins are large enough to
allow passive diffusion, i.e., they act as channels that are
specific to different types of molecules. They are present in
the outer membrane of Gram-negative bacteria and some Gram-
positive bacteria of the group Mycolata (mycolic acid-containing
actinomycetes), the mitochondria, and the chloroplast.   Porins
are composed of β strands, which are, in general, linked
together by beta turns on the cytoplasmic side and long loops of
amino acids on the other. The β strands lie in an antiparallel
fashion and form a cylindrical tube, called a β barrel.[2] The
amino acid composition of the porin β strands are unique in that
polar and nonpolar res