PorinsAs areporinas composedestán ofcompostas por [[betafolla sheetbeta|β strand]]s, which are, in general, linked together by beta turns on the [[cytoplasm]]ic side and long loops of [[amino acid]]s on the other. The [[beta sheet|β strand]]s lie in an antiparallel fashion and form a cylindrical tube, called a [[beta barrel|β barrel]].<ref name="pmid9615433">{{cite journal |author=Schirmer T |title=General and specific porins from bacterial outer membranes |journal=J. Struct. Biol. |volume=121 |issue=2 |pages=101–9 |year=1998 |pmid=9615433 |doi=10.1006/jsbi.1997.3946 |url=}}</ref> The amino acid composition of the porin β strands are unique in that polar and nonpolar residues alternate along them. This means that the nonpolar residues face outward so as to interact with the [[nonpolar]] [[lipid]] membrane, whereas the polar residues face inwards into the center of the beta barrel to interact with the [[aqueous]] channel. The [[phospholipid]]s that compose the [[Bacterial outer membrane|outer membrane]] give it the same semi-permeable characteristics as the [[cytoplasmic membrane]].{{Citation needed|date=November 2010}}
The porin channel is partially blocked by a loop, called the eyelet, which projects into the cavity. In general, it is found between strands 5 and 6 of each barrel, and it defines the size of solute that can traverse the channel. It is lined almost exclusively with charged amino acids arranged on opposite sides of the channel, creating a transversal electric field across the pore. The eyelet has a local surplus of negative charges from four [[glutamic acid]] and seven [[aspartic acid]] residues (in contrast to one [[histidine]], two [[lysine]] and three [[arginine]] residues) is partially compensated for by two bound calcium atoms, and this asymmetric arrangement of molecules is thought to have an influence in the selection of molecules that can pass through the channel.<ref>Branden and Tooze, Introduction to Protein Structure, second edition</ref>
