Proteins in Cells Membranes

Proteins in Cells Membranes The proteins that make up membranes can be classified into three categories, namely 1) integral proteins, 2) proteins bound to fat, and 3) peripheral. The first type, integral membrane proteins, also called transmembrane proteins, stretches across the phospholipid bilayer and is composed of three parts. The cytosolic part and the exoplasmic part have a hydrophilic part on their surface which directly interacts with the water environment in the cytosol and the outer part of the membrane. These proteins consist of one or many types with an α helix or β sheet structure. In fact, many transmembrane proteins are glycosylated with branched sugars attached to one or many amino acid chains. The sugar chains are always located in the exoplasmic part of the cell membrane. The second type, membrane proteins that are bound to fat are covalently bound to one or many fat molecules. Hydrophobic carbon chains are fats that are attached and bound together to form a membrane to which proteins are bound. The polypeptide chain does not penetrate the two layers of the phospholipid bilayer biomembrane. The third type of membrane protein is peripheral which does not interact with the hydrophobic part of the phospholipid bilayer. These proteins are usually not directly bound to the membrane but interact with integral proteins or can also be directly bound to lipid head groups. Peripheral proteins are located on the outside of the cytosol or exoplasmic part of the plasma membrane. One type of transmembrane protein is glycoporin A, which in the picture shows a hydrophilic part with an α helix-shaped amino acid chain that also binds to the hydrophobic part of the amino acid chain (red amino acids). Through negative bonds in the phospholipid head group, the positive part consisting of a group of arginine and lysine (blue) is attached to the cytosol and helps bind the glycoporin to the membrane. Image b is a type of protein that is bound to fat on one side of the membrane layer. There are examples of three types of proteins, namely prenylation, namely proteins found in the cytosol with amino acids bound to a layer of fat with the final part containing a residue or collection of glycine (Gly). The second type is acylation in which the amino acid is bound to one or two cysteines (Cys). The third type is an example of a protein that is bound to fat on the outer surface of the plasma membrane and is called glycosylphospatidylnositol (GTI). Carbohydrates located in cell membranes are usually branched oligosaccharides with an amount of less than 15 sugar units. These oligosaccharides will be covalently bound to lipids to form glycolipids, or bound to proteins to form glycoproteins. Many transmembrane proteins covalently bond to carbohydrate chains to form glycoproteins, and are usually located in the outer membrane. The same condition is glycolipids, where carbohydrate chains in the form of glycerol or sphingosine are always on the surface membrane. Specifically, glycoproteins and glycolipids are only found in eukaryotic cell membranes, but not in mitochondrial membranes, chloroplast lamellae, and cell organelle membranes. This is because the presence of carbohydrate chains is needed to interact and communicate with other components in the extracellular matrix such as lectins, growth hormones and antibodies. Often the presence of carbohydrates is important in cell membranes because they function to recognize neighboring cells and also differentiate one type of neighboring cell from another. This condition is important, for example, in terms of sorting cells into tissues and organs in animal embryos. Apart from that, recognition is also the basis for rejecting foreign cells (including graft organ cells) by the immune system, as an important step in defense in vertebrate animals.