Periplasmic

Periplasmic refers to the space or components located in the periplasm, the region between the cytoplasmic (inner) membrane and the outer membrane in most Gram-negative bacteria. Some uses extend to describe proteins that reside in this compartment. The term emphasizes a location rather than a membrane-bound organelle, since the periplasm is not a separate cellular compartment bounded by its own membrane.

The periplasm contains a gel-like matrix of solutes, enzymes, binding proteins, and transport components. It houses enzymes involved in nutrient acquisition, cell wall remodeling, and detoxification, and it includes periplasmic chaperones that assist protein folding. The environment of the periplasm is distinct from the cytoplasm, often with different redox potential and composition, which supports specific biochemical processes such as disulfide bond formation in exported proteins.

Many essential transport processes involve periplasmic proteins. Substrate-binding proteins in the periplasm capture nutrients and deliver them to inner-m membrane transporters, particularly in ABC transport systems. Periplasmic enzymes participate in breakdown and processing of substrates, and some components of the electron transport chain extend into or associate with the periplasm, facilitating respiration.

Protein targeting to the periplasm typically uses signal peptides that direct polypeptides across the inner membrane via the Sec or Tat pathways. After translocation, signal peptides are cleaved and mature proteins reside in the periplasm or perform functions at the inner face of the membrane. In biotechnology, periplasmic expression is exploited to simplify purification and to promote proper disulfide bond formation in recombinant proteins.

In Gram-positive bacteria, a classic periplasmic space is not present; terms referring to less-defined extracytoplasmic regions are sometimes used, reflecting differences in cell envelope structure.