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Subject - Biochemistry
Porins are transmembrane proteins, that are found in the of gram-negative bacteria and the organelles of eukaryotic organisms. In mitochondria they occur in the outer membrane, and in chloroplasts they are found in both the outer and inner membranes (also referred to as the envelope). Atypically for membrane proteins, porins (German learning unit) fold over the membranes by means of . Each polypeptide chain consists of 300-420 amino acids and forms a β-barrel made of 16- or 18-stranded β-sheets in an antiparallel arrangement (example: OmpF-Porin, German learning unit). The quaternary structure of the protein is a homotrimer.
Unspecific and specific porins differ in their functionality. The former allow molecules of up to about 600 Da to pass, regardless of their structure, while the latter are specialized in areas such as sugar transport. Porins are thus indispensable for material exchange. Between the cytoplasm and the (aqueous intermembrane space) this is regulated by highly specific transporters, while exchange between the periplasm and the outside world occurs through the porins.