Fc?RIIa N64 in D1 and N145 in D2, away

Fc?RIIa receptor (CD32a) is a FC receptor (as it FC region of antibody binds to it) found on the plasma membrane of leukocytes and is the key activating effectors of the Immunoglobin G (IgG)-mediated immune functions. Although Fc?RIIa is the only FC receptor found on human platelets(McKenzie et al., 1999b) , it is evidently shown that it is not found in murine creatures such as mice but primarily found in higher primates due to evolution. However, transgenic mice are used in laboratories to study the biology of Fc?RIIa in vivo (McKenzie et al., 1999a) . Activation of this receptor initiate different response depending on the cell. Neutrophils and macrophages will produce greater L-selectin expression and phagocytosis may occur (L A Sanders et al., 1995) ; basophils will enhance its cytokine production.These receptors have very low affinity (KD ?10-6 M) (Katsumi Maenaka et al., 2001)  towards an open confirmation (Figure ???) monomeric IgG but forms high avidity with multimeric antigen-antibody complexes (Ag-Ab, where Ab is in this case IgG) or immune complexes (ICs). Since signalling immunoreceptor tyrosine-based activation motifs (ITAM) is on the same ligand binding chain (Daëron, 1997) , if affinity for IgG was high, there is no other way to control the signalling which will result immune response being triggered constantly. Fc?RIIa is a type I glycoprotein which its N-terminus is facing the extracellular environment (McKenzie et al., 1999). Type I proteins are single-pass with their N-terminus targeted to the ER lumen first during synthesis. Its extracellular region has two globular domains, D1 at the N-terminus following D2, which are 52° apart (Figure ???) (Maxwell et al., 1999). This arrangement is maintained by hydrogen bonding (H-bonding) between hydrophilic amino acid residues at the surface of D1 and D2 with water molecules acting as a bridge between two residues which are further apart, forming a large network of hydrogen bonding (Maxwell et al., 1999). Further down is the transmembrane domain of the protein which anchors it in the plasma membrane. ITAM lies in the cytoplasm towards the C-terminus of protein (McKenzie et al., 1999) . Unlike the peptide chain, the glycan part of this glycoprotein is not encoded within the genome. Its structure is determined by environmental factors such as the availability of metabolites and enzymes in the Golgi apparatus. The glycans were N-linked to N64 in D1 and N145 in D2, away from the binding site (Paul A Ramsland et al., 2011). Besides stabilizing the protein in aqueous environment by forming H-bonds with water molecules, it also functions as a barrier, preventing lateral interaction with other proteins on the membrane surface (Paul A Ramsland et al., 2011). This receptor shows polymorphism. The amino acid lying in D2 at position 134 (based on N-terminal sequence) can be either arginine (R134) or histidine (H134) which are high-responder/low-responder respectively (HR/LR). (Paul A Ramsland et al., 2011)  The names were given as so due to their affinity to bind to Immunoglobin G1 (IgG1). HR having the higher affinity for IgG1 and LR lower. Due to its higher affinity, binding domain will be based on Fc?RIIa-HR.The binding surface of FC is located on the surface of D2, which is closer to D1, facing away from the plasma membrane. It is divided into 3 regions. First is the protruding ridge which is made up of the FG loop (Y160 as direct interaction with ligand), BC loop (P117, V119, and K120), and C’ strand (F132, R134, L135) (Paul A Ramsland et al., 2011). It was evidently that this region was having direct contact with ligand. When alanine substitutions were introduced to these regions, binding decreased significantly, or ligands were unable to bind (Hulett et al., 1995) . Y160 interacts with L234-L235-G236-G237 on chain B of IgG. Based on its structure (figure ???), the interaction between this residue and region is based of hydrophobic effect. Before binding, water is well organised as it surrounds the non-polar face forming a clathrate. A clathrate is form due to the inability to interact with non-polar surface. This causes water molecule at the surface to form stronger interaction with each other, making it more densely packed and unable to move around freely. This restriction of freedom reduces the permutation of water molecules leading to reduction in entropy. Based on this equation, ?G=?H-T?S, (G = Gibbs free energy, H = enthalpy, T= temperature and S = entropy), a lower ?S will result a less negative or more positive ?G. Reaction is less favourable (Voet & Voet, 2011). However, when there are two non-polar surfaces the water molecule moves out of way between the non-polar surfaces, bringing the non-polar surfaces close to one another. This reduces the non-polar surface area exposed to water molecules. Water molecules are more disorganised as they are no longer forming the clathrate. Permutation increases, entropy increases, ?G becomes more negative hence favourable (Berg et al., 2015a). At the BC loop, it binds to a larger area on B chain (S239, D265, S267, V266 and N297)(Paul A Ramsland et al., 2011) .The second region involves trapping P329 on chain A of IgG by W90 and W113 (Paul A Ramsland et al., 2011) on receptor through CH/? interactions (Zondlo, 2013) . Proline can favourably interact with the aromatic ring on tryptophan due to hydrophobic effect and the attraction between polarised C-H bonds proline with the electron dense aromatic ring. The third region is a small patch (S129 and K128) on receptor interacting with the N-glycosylated oligosaccharide on N297 of chain B (Paul A Ramsland et al., 2011). Its transmembrane domain is composed of hydrophobic amino acids, which acts as a stop transfer sequence during integration into ER membrane (Alberts et al., 2015). The hydrophobic nature of these amino acid residues makes it energetically favourable to exist in the hydrophobic region of the lipid membrane. Due to the absence of hydrophilic molecules such as water in the lipid membrane, the C=O and N-H in the backbone of the peptide chain forms H-bonds among themselves as it would be more energetically favourable to do so (Berg et al., 2015b).Although having the same number of electron shells, C and O have different electronegativity due to the different proton number. O having more protons creates a more positive nucleus, attracting electrons more strongly compared to C. Electrons are not distributed evenly within the bond with electrons density is shifted towards O, forming a bond dipole, C being ?+ and O ?-. Same goes to N-H, N is more electronegative than H. The ?- of O is attracted to ?+ C. Internal H-bonding is formed by C=O of the first amino acid forms H-bond with the N-H of the fourth amino acid. Second amino acid forms with the fifth and so on. Basically, nth amino acid forms H-bonds with (n+3) th amino acid. This arrangement forms an ?-helix (Berg et al., 2015). In my opinion, ?-helix is formed firstly by hydrophobic residues outside of the helix (hydrophobic effect); secondly, favorable torsion angles in peptide backbone to have favorable van der Waals interaction and to prevent clashing of atoms; and lastly linear H-bonding between C=O and H (it forms strong H-bonds).Unlike other FC receptors, a significant portion of Fc?RIIa forms a non-covalent dimer without the binding of IC complexes. This is possible as the interface between the two monomer and biding sites are not near one another (Paul A Ramsland et al., 2011) . Dimerization is necessary for efficient signal transduction. Mutation towards amino acids involved in dimerization affected both the magnitude and kinetics in phosphorylating the tyrosine in ITAM (Powell et al., 2006). Although necessary, dimerization alone is not enough to produce effective intracellular signalling. Aggregation of receptor induced by ligand binding is require trigger ITAM signalling and to produce a strong signal which can increase concentration of signalling molecules at that part of the cell (Underhill & Goodridge, 2007). However, it is not known how ligand binding induces clustering of receptors.Located in the cytoplasmic tail, ITAM is composed of two YXXL, where X can be any amino acid. Fc?RIIa’s the two YXXL is separated by 12 amino acids. Clustering of receptor being ITAM close to one another, allowing activated Src family kinase (a family of non-receptor tyrosine kinases) to phosphorylate the duo tyrosine. This becomes a docking site for Spleen tyrosine kinase (Syk) at Src homology domain 2 (SH2). Syk is now activated (Getahun & Cambier, 2015).  This triggers a series of reaction which leads to cellular response.