BIOCHEMISTRY 2

Q1. Define the term Posttranslational modifications and give at least two examples of different types of posttranslational modifications. Synthesisof protein molecules occur through the process of translation which is a m-RNA mediated mechanism. The process encompasses various enzymatic methods in order to certify that proteins formed in the process of translation are accurate, with proper folding and are ready to perform their assigned functions namely as enzymes, antibodies, hormones etc. The process of translation is followed by numerous modifications in the covalent bonds leading to the alteration in the properties of proteins. such alterations are known as post-translational modifications (PTMs). The process involves either an addition of a transforming group to one of the amino acids of the polypeptide chain or its proteolytic cleavage. The modifications comprise glycosylation, phosphorylation, nitrosylation, ubiquitination, acetylation, methylation, proteolysis lipidation. The process of PTM is governed by enzymes such as transferases, kinases, ligases and phosphatises which act upon distinctive amino acid side chains or on the peptide linkages. The mechanism of action of these enzymes involves addition or removal of distinct functional groups, sugars, lipids, proteins etc. either from or to the amino acid side chains.PTMs can occur at any stage of protein synthesis to enhance the complexity of proteome:1. During the process of translation (e.g. disulfide bonding). Disulfide bonds are formed between the two sulphur molecules. Amino acid Cysteine contains sulphur, as a result disulfide bond is formed between two residues of cysteine (R-S-S-R). The bond formed is strong and plays an imperative role in accurate folding as well as stability of protein molecules.2. When folding of protein molecule is completed its modification occurs during the process of localization (e.g. collagens) (Web. DNA and protein synthesis: Post-translation).Q2. Explain briefly how a protein is made starting from its DNA coding sequence. DNA is the basic genetic material which is made up of four nucleotides Adenine (A), Guanine (G), Cytosine (C) and Thiamine (T). Adenine always pair with Thiamine with a double Hydrogen bond while Guanine pairs with Cytosine with a triple Hydrogen bond. The process of transcription is responsible for the formation of messenger RNA or mRNA. This process takes place inside the nucleus in eukaryotes. The sequence of nucleotides in DNA is responsible for the amino acid sequence of mRNAs which are translated into proteins. Transcription and translation involves the processes such as initiation, elongation and termination. The process of transcription involves RNA polymerase which is attached to the promoter sequence. When the protein is desired, the gene is switched on and when protein is not required the gene is switched off. Elongation is carried out after the assembly of PIC at the promoter site, TFIIH is similar to helicase which unwinds the DNA. Termination of RNA involves poly-A signal gene which is required to add adenylated residue during RNA processing (Web. Transcription Mechanisms in Eukaryotes). The onset of translation is with the binding of smaller subunit through complementary bases. The bases are read as triplet by the tRNA which is called the transporter. These triplets are called codons while the tRNA carries anti-codon. The AUG is the initiation codon. Initiation process begins when the smaller subunit binds with the specific tRNA called N-formyl methionine or fMet. This binds with the initiation codon methionine. The process of elongation involves the formation of peptide bonds, as a result the tRNA in the A site becomes peptidyl RNA and moves to the P site. Termination of the translation is dependent on the occurrence of three stop signals also called nonsense codons at the A site. These stop codons are UAG, UGA and UAA (Web: Molecular Biology: Translation). Q3. Explain the steps in globin chain synthesis. Genetic alterations may result in diseases or genetic anomaly. To understand this, genes involved in globin (Hemoglobin) chain synthesis are taken into consideration. The hemoglobin molecule consists of two subunits, α subunit and β subunit, genes of these subunits are present on two different chromosomes. Genes of α subunit are present on chromosome 16 while genes of β subunit are present on chromosome 11. The combination of two α chains and two β chains forms a hemoglobin molecule (a tetramer). The synthesis of the globin molecule onsets in the nucleus of the RBC, where the DNA forms the messenger RNA by the process of transcription. The mRNA so formed is the precursor molecule which contains the coding exon sequences and non-coding intron sequences. Splicing removes the introns and all the three exons combine in a linear manner to serve as a template for the formation of protein molecule. This mRNA now moves to the cytoplasm where it is translated to the globin protein molecule. Work CitedMolecular Biology: Translation. Web. 31 March 2015. .Omajali, J. DNA and protein synthesis: Post-translation. Web. 31 March 2015. .Transcription Mechanisms in Eukaryotes. Web. 31 March 2015. .