Python Programming for Biology: Bioinformatics and Beyond

Membrane proteins

As  already  hinted  at,  there  are  some  proteins  that  do  not  reside  wholly  in  water,  instead

they  lie  within  lipid  membranes

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 of  cells.  In  some  sense  the  lipid  membrane  can  be

viewed as an alternative solvent to water, and thus it is a somewhat different environment

for protein folding. It is a very hydrophobic (water-hating) environment, without polar or

hydrogen  bonding  groups  that  could  interact  with  water,  and  thus  excludes  water

molecules. This exclusion of water is part of the reason why membranes exist in cells: to

define different compartments to isolate different biochemical environments, including the

limits  of  the  cell  itself.  Although  cell  membranes  are  distinct  layers,  made  of  fatty

molecules,  they  are  not  entirely  static;  their  component  lipid  molecules,  and  other

embedded  molecules  such  as  membrane  proteins,  can  move  about  (diffuse),  albeit  in  a

mostly  two-dimensional  manner.  While  the  lipids  form  a  barrier,  membrane  proteins  are

inserted  to  give  the  membrane  biological  function,  e.g.  to  transport  specific  compounds

across the barrier.