Integral Proteins vs Peripheral Proteins
Proteins are considered as macro molecules, which consist of one or more polypeptide chains. The polypeptide chains are made up of amino acids bonded together by peptide bonds. The primary structure of a protein can be determined by the amino acid sequence. Certain genes code for many proteins. These genes determine the sequence of amino acid, thereby determining their primary structure. Integral and peripheral proteins are considered as ‘plasma membrane proteins’ due to their occurrence. These proteins are generally responsible for a cell’s ability to interact with the external environment.
Integral proteins are mainly found either fully or partially submerged in the phospholipids bilayer of the plasma membrane. These proteins have both polar and non-polar regions on them. Polar heads protrude from the surface of the bilayer while non-polar regions are embedded in it. Usually only the non-polar regions interact with the hydrophobic core of the plasma membrane by making hydrophobic bonds with the fatty acid tails of the phospholipids.
The integral proteins that span the entire membrane from the inner surface to the outer surface are called transmembrane proteins. In transmembrane proteins, both ends that project out of the lipid layer are polar or hydrophilic regions. The middle regions are non-polar and have hydrophobic amino acids on their surface. Three types of interactions help to embed these proteins in the lipid bilayer, namely, ionic interactions with the polar heads of phospholipid molecules, hydrophobic interactions with the hydrophobic tails of phospholipid molecules and specific interactions with certain regions of lipids, glycolipids or oligosaccharides.
Peripheral proteins (extrinsic proteins) are present on the innermost and outermost of phospholipids bilayer. These proteins are loosely bound to the plasma membrane either directly by interactions with polar heads of phospholipids bilayer or indirectly by interactions with integral proteins. These proteins constitute about 20-30 % of total membrane proteins.
Most of the peripheral proteins are found on the innermost surface or cytoplasmic surface of the membrane. These proteins remain bounded by either through covalent bonds with fatty chains or through an oligosaccharide to phospholipids.
What is the difference between Integral and Peripheral Protein?
• Peripheral proteins occur on the surface of plasma membrane whereas integral proteins occur either fully or partially submerged in the lipid layer of plasma membrane.
• Peripheral proteins are loosely bound to the lipid bilayer and do not interact with the hydrophobic core in between two layers of phospholipids. In contrast, integral proteins are tightly bound and are directly interacting with the hydrophobic core of the plasma membrane. Due to these reasons, integral protein dissociation is more difficult than peripheral proteins.
• Mild treatments can be used to isolate peripheral proteins from the plasma membrane, but for isolation of integral proteins, mild treatments are not enough. To break the hydrophobic bonds, detergents are required. Thus, integral proteins can be isolated from the plasma membrane.
• After isolation of these two proteins from the plasma membrane, peripheral proteins can be dissolved in neutral aqueous buffers while integral proteins cannot be dissolved in neutral aqueous buffers or aggregates.
• Unlike peripheral proteins, integral proteins are associated with lipid when solubilized.
• Examples of peripheral proteins are spectrin of erythrocytes, cytochrome C and ATP-ase of mitochondria and acetylcholinesterase in electroplax membranes. Examples of integral proteins are membrane bounded enzymes, drug and hormone receptors, antigen and rhodopsin.
• Integral proteins represent around 70% while peripheral proteins represent the remaining portion of plasma membrane proteins.