In a broader sense , transport proteins are proteins that carry out or facilitate the transport of certain substances, the target location being within the same cell, directly outside the cell or even another location within a multicellular organism. In the narrower sense, transport protein refers to a protein that sits stationary in the cell membrane and passively facilitates transport through the cell membrane (example: ion channel ) or actively (example ion pump ).
In contrast to transport processes based on diffusion , protein-dependent transport shows typical kinetic parameters that are analogous to those of enzyme-catalyzed reactions. Transport proteins, for example, show saturation behavior with increasing substrate concentrations and, where appropriate, the various forms of inhibition .
Transport across the cell membrane
Transmembrane transport proteins (sometimes also called permeases ) transport substances out of and into the cell, either passively with the concentration gradient , or against the concentration gradient with direct or indirect energy consumption ( active transport ). In the past, mobile carriers in biological membranes were assumed ( valinomycin model), but today we know that most membrane transport proteins are stationary in the membrane.
Proteins in organelle membranes also belong to this class of transport proteins , as they do not differ fundamentally from those in the cell membrane. In particular, enzymes participating in shuttle systems , such as. B. the malate dehydrogenase of the malate sluice , referred to as a transport protein.
The subdivision of the membrane transport proteins takes place according to the subdivision of the membrane transport (see there) into:
Transporter (carrier) for the
- Uniport, catalyzed permeation (individual molecules are transported in one direction).
- Symport, cotransport (simultaneous transport of two / three different particles in the same direction)
- Antiport, counter-transport (simultaneous transport of two / three different particles in opposite directions by antiporter )
- Ion pumps , such as B. the plasma membrane ATPases or proton pumps
- Ion channels , such as the sodium channel
- Aquaporins , i.e. channels that promote water exchange
Transport outside cells
Most of these substances are also bound to specific transport proteins for the transport of substances in the bloodstream.
Transport proteins in the blood are for example:
- Hemoglobin : binds oxygen and carbon monoxide
- Haptoglobin : binds hemoglobin
- High density lipoprotein : binds fats
- Low density lipoprotein : binds fats
- Thyroxine-binding globulin : binds the thyroid hormone thyroxine
- Transcortin : binds steroid hormones
- Transferrin : binds iron
- Vitamin D binding protein : binds vitamin D.
Transport within cells
- P. Schopfer and A. Brennicke: Plant Physiology. 6th edition Elsevier, 2005. ISBN 3-8274-1561-6
- Schmidt, Lang, Thews: Physiologie des Menschen 29th edition Springer, 2005. ISBN 3-540-21882-3