Polar porins

The amino acid compositions and sequences of the /3-strands in porin proteins are novel. Polar and nonpolar residues alternate along the /3-strands, with polar residues facing the central pore or cavity of the barrel and nonpolar residues facing out from the barrel where they can interact with the hydrophobic lipid milieu of the membrane. The smallest diameter of the porin channel is about 5 A. Thus, a maltodextrin polymer (composed of two or more glucose units) must pass through the porin in an extended conformation (like a spaghetti strand). 

Channel proteins have a polar pore through which ions and other hydrophilic compounds can pass. For example, there are channels that allow selected ions to pass (ion channels see p. 222) and porins that allow molecules below a specific size to pass in a more or less nonspecific fashion (see p. 212). 

The lipid membranes are a few nanometers thick. They contain proteins whose role is to actively transport particular target chemicals across these nonpolar barriers. The outer membranes of gram negative bacteria also have protein channels called porins that allow passage of small polar and charged substrates. 

Nabedryk, E., Garavito, R. M., and Breton, J. (1988). The orientation of /3-sheets in porin. A polarized Fourier transform infrared spectroscopic investigation. Biophys.J. 53, 671-676. 

Figure 3.46. "Inside Out" Amino Acid Distribution in Poiin. The outside of porin (which contacts hydrophobic groups in membranes) is covered largely with hydrophobic residues, whereas the center includes a water-filled channel lined with charged and polar amino acids.

VDAC plays a role in the regulated flux of metabolites—usually anionic species such as phosphate, chloride, organic anions, and the adenine nucleotides—across the outer membrane. VDAC appears to form an open p -barrel structure similar to that of the bacterial porins (Section 12.5.2). although mitochondrial porins and bacterial porins may have evolved independently. Some cytoplasmic kinases bind to VDAC, thereby obtaining preferential access to the exported ATP. In contrast, the inner membrane is intrinsically impermeable to nearly all ions and polar molecules. A large family of transporters shuttles metabolites such as ATP, pyruvate, and citrate across the inner mitochondrial membrane. The two faces of this membrane will be referred to as the matrix side and the cytosolic side (the latter because it is freely accessible to most small molecules in the cytosol). They are also called the N and P sides, respectively, because the membrane potential is negative on the matrix side and positive on the cytosolic side. 

The porin monomers associate to form trimeric channels as is shown in Fig. 8-20B. They all have a central water-filled, elliptical channel that is constricted in the center to an "eye" -0.8 x 1.1 nm in size. In this restriction zone the channel is lined with polar residues that provide the substrate discrimination and gating. For example, in OmpF and FhoE there are many positively and negatively charged side chains that form the edge of the eye (Fig. 8-20C). The electrostatic potential difference across the outer membrane is small, but apparently determines whether the porins are in an open or a closed state. The voltage difference has opposite effects on OmpF and FhoE, apparently as a result of the differing distribution of charged... 

The amino acid sequences of porins are predominantly polar and contain no long hydrophobic segments typical of integral proteins with ct-helical membrane-spanning domains. X-ray crystallography has revealed that porins are trimers of identical subunits. In each subunit, 16 (3 strands form a barrel-shaped structure with a pore in the center (Fig-... 

Porin A protein in the outer membrane of Gram-negative bacteria that non-selectively transports polar molecules into the periplasmic space. 

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