Genes encoding membrane proteins constitute approximately 30% of genome. Membrane proteins act as major drug targets, affecting almost all the biological system in human body. These proteins are among the most important proteins biologically as they are responsible for cell's communication with other cells and within its environment by cell adhesion, structure formation and, recognition of foreign particles, such as antigens. The G-protein coupled receptors (GPCRs) account for about 26.8% of major drug targets in market today. Membrane proteins have been implicated in a number of important processes such as maintaining the electrolyte balance, energy production and transmission, and photosynthesis in plants, and control of development. The importance of membrane proteins is well acknowledged across the fields of medicine and agriculture.
Based on interactions of the membrane layer and the protein, membrane proteins are classified into two categories, the integral or intrinsic membrane proteins and the peripheral or extrinsic membrane proteins.
Intrinsic membrane proteins
Integral membrane proteins are the transmembrane (TM) proteins. Integral proteins have hydrophobic residues with one or more segments that are embedded in the phospholipid bilayer shown in. These hydrophobic side chains interact with fatty acyl groups of phospholipids within the membrane, thereby anchoring the protein to the membrane.
The TM proteins contain membrane spanning domain as well as domains that are present in the cytosolic environment and would be soluble. The membrane-spanning domains mainly are a helices or β sheets. The alpha helical domains contain anywhere from one to fourteen helices. For example ion-channels have four or five helices, GPCRs have seven helices, and photosynthetic reaction center fourteen helices.
The β sheets are less common than the alpha helical structures among the TM proteins. Beta strand forms hydrogen bond with its neighbor in an anti-parallel arrangement that form a single beta sheet rolled into a barrel. An example of the beta proteins is the porin that contains a hydrophilic and a hydrophobic exterior, which makes them reside in the membrane.
Peripheral membrane proteins
Peripheral membrane proteins or extrinsic proteins, as the name suggests, are not present within the membrane and do not interact with the hydrophobic core of the phospholipid bilayer. They attach temporarily to the membrane via their interactions with integral membrane proteins or sometimes, by interacting with lipid polar head groups. They may be anchored through covalent links to membrane buried hydrophobic cofactors.
Peripheral membrane proteins often constitute the regulatory subunit of membrane proteins such as ion channels or other transmembrane receptors. These proteins are known to be functional as enzymes, membrane targeting domains, structural domains, transporters of small hydrophobic molecules, and electron carriers. They can also act as polypeptide hormones, toxins, and anti-microbial peptide. Some peripheral membrane proteins shuttle between the cytosol and the cytosolic face proteins, and those that are localized to the cytosolic face of the plasma membrane include the cytoskeletal proteins spectrin and actin in erythrocyte and the enzyme protein kinase C, which plays a role in signal transduction.
SEVEN HELICAL MEMBRANE PROTEINS
Seven helical transmembrane proteins also known as seven spanning membrane proteins are one of the most important categories of membrane proteins. There are two classes of the seven helical membrane proteins. In the prokaryotic cell, the principle example is the bacteriorhodopsin, which binds to retinal as its ligand. While in the mammalian cells they are known as the G- protein coupled receptors (GPCRs). Unlike bacteriorhodpsins, GPCRs bind to a number of ligands such as carazolol, retinal, etc.
In bacterial cells, bacteriorhodopsins are used to derive chemical energy from light; the basic reaction pathway is depicted in Figure. 1.5. Upon absorption of a photon the retinal present in the bacteriorhodopsins undergoes conformational changes. The 11-cis conformation, which is the dark stage, changes to all-trans after undergoing protonation and changes back to all-cis after deprotonation. The protonation and deprotonation reaction are vectorial and create a gradient of protons that is converted to the chemical energy in the prokaryotes.
In GPCRs, the amino terminus is located extracellularly while the carboxyl terminus is located intracellularly. Each individual helix contains around 22 amino acids to stretch through the membrane. GPCRs fold similarly to bacteriorhodopsin in that each helix alternates direction and the seven helices lie next to each other following the path of flattened circle. GPCRs can be divided into three major classes based on the similarity of the transmembrane region and the nature of their ligand. Class 1 includes rhodopsin-like receptors, and is activated by ligands such as biogenic amines, chemokines, and neuropeptides. Class 2 includes secretin-like receptors, which are activated by ligands including secretin, hormones, glucagons etc. Class 3 includes metabotropic-glutamate receptors and calcium sensing receptors. All these GPCRs follow quite similar receptor activation mechanism as is depicted in. Once a ligand binds to the GPCR, the GPCR/G-protein complex breaks, releasing the α subunit from the βγ subunit of the G-protein. These α and βγ units then interact with secondary messengers to relay a variety of effects.