Abstract
Proteins are made up of amino acids and have major structural as well as functional roles in the body. Amino acid chains are synthesized from DNA sequences through transcription and translation. Proteins become fully functional only after co-translational or post-translational modification of the amino acid side chains and folding into appropriate three-dimensional conformations. Even slight changes in the DNA code could drastically alter the protein folding, alter its functioning, and eventually lead to certain diseases. Inherent cellular repair mechanisms, protein degradation pathways and molecular facilitators such as chaperones try to keep proteins in their native, stable conformation by acting at the DNA level as well as post-translational level. But, misfolded proteins escape these mechanisms and manifest themselves in diseases like Alzheimer’s, Parkinson’s and sickle cell anemia. Sickle cell anemia is caused by a single mutation in the HBB gene sequence that leads to substitution of valine in place of glutamic acid in the beta subunit of hemoglobin. Mutant hemoglobin thus formed leads to clumping of RBCs, obstruction of blood flow and anemia. Treatment options for protein misfolding diseases include gene therapy and amyloid motif targeted therapy but a real cure is yet to be found.
Protein Structure and Diseases Caused by Misfolding and Mutations
Introduction
Proteins are biomolecules that play major structural and functional roles in the human body. Proteins are made of monomer units called amino acids. The amino acid chains or peptides become functional proteins only after undergoing certain modifications and folding into specific three-dimensional structures. While, structural proteins such as keratin, collagen and myosin form structural body parts, functional proteins such as enzymes and antibodies take part in vital biochemical reactions in the body. The function of a protein is determined by its structural modification or folding, and any misfolding might result in complete destruction of the protein structure and loss of its functionality. As the amino acid sequence is formed only after transcription and translation of the DNA sequence, mutations in the coding DNA sequence lead to protein malformations. Defective or non-functional proteins cannot perform their respective functions and lead to major diseases.
Protein Structure and conformation
The characteristics and functionality of a protein is determined by its amino acid composition, side chain modification and folding. Amino acids are organic compounds with a amino group on one side (-NH2) and an acid group (-COOH) on the other side. However, each amino acid has a different type of side chain. Based on the type of side chain, amino acids can be hydrophilic or hydrophobic. Hydrophilic amino acids can in turn be acidic, basic or neutral based on the polar nature of the side chains. Amino acids are linked to each other through a peptide bond (Saxena, 2014).
The peptides thus formed undergo modifications in side chains to become functional. Side chain modifications include acetylation, phosphorylation, carboxylation, methylation, glycosylation, etc. Fate of the proteins is partially determined by the side chain modifications. For e.g. cellular membranes are formed by peptides that have undergone glycosylation, and acetylation is essential for proteins such as Acetyl-CoA to be functional enzymes. These modifications can occur along with translation, which is called co-translational modification or they can happen after the translation of mRNA to amino acid sequence, which is called post-translational modification (Saxena, 2014).
The peptide sequence with a linear, primary structure thus formed can assume a alpha helical or beta sheet secondary structure through hydrogen bonds. Structural proteins like keratin have only secondary level organization. However, functional proteins become active only after undergoing tertiary or quaternary level of organization. Tertiary structure of proteins forms through formation of hydrogen bonds, sulphur bridges as well as hydrophobic interactions. Quaternary structures form from the association of two or more polypeptide subunits. Hemoglobin is a typical example of protein with quaternary structure (Saxena, 2014).
Mutation in DNA to Misfolding in Protein
Amino acids are linked to each other based on the specific messenger RNA (mRNA) sequence that is transcribed from the DNA. Transfer RNAs (tRNAs) with specific anti-codons, serve as carriers for the amino acids and translate the amino acid sequence from the mRNA codons. Codon Anti-codon triplet pairing is the key to proper protein synthesis and the physicochemical properties of the amino acids are responsible for correct protein folding. Usually proteins fold in the correct conformation that is most stable and it is also known as the minimal energy or native conformation. Mutations or defects in the DNA coding sequence will lead to synthesis of altered or misfolded proteins that no longer have their appropriate conformation (Reynaud, 2010).
Usually the DNA repair mechanism in cells detects and corrects mutations in the coding sequences. Specialized proteins called molecular chaperones help proteins achieve their stable three-dimensional conformation. In case these mechanisms are overridden, a newly formed, misfolded protein is trapped by quality control systems such as Endoplasmic reticulum associated degradation (ERAD) pathway, modified with ubiquitin side chain addition and marked for destruction (Reynaud, 2010).
However, misfolded proteins do escape these repair mechanisms and lead to malfunctions in biochemical processes within the body. Even a single amino acid change in a vital motif of a protein, could drastically alter its function. Mutations thus translate into diseases if the altered protein takes part in a vital metabolic activity. Protein misfolding diseases include Alzheimer’s disease, Parkinson’s disease, Mad Cow disease, sickle cell anemia, etc (Neurophage pharmaceuticals, 2015). The sickle cell anemia case is elaborated below.
Sickle Cell Anemia
Sickle cell anemia is caused by a mutation in the gene sequence that codes for hemoglobin, the protein that carries oxygen in the blood (Albright College, 2014). Hemoglobin is a tetramer with 2 alpha-globin subunits of 141 amino acids and 2 beta-globin subunits with 146 amino acids each. The HBB gene codes for the beta-globin subunit. A single nucleotide mutation i.e. thymidine replacing adenine in the HBB gene, leads to substitution of valine in place of glutamic acid in the beta-globin peptide sequence. Valine is a hydrophobic amino acid and glutamic acid is hydrophilic and the amino acid substitution occurs at a critical position of folding. The beta subunit thus misfolds, and the mutated hemoglobin forms insoluble aggregates. Red blood cells (RBCs) with clumped hemoglobin become sickle shaped instead of normal round shape, and further cycles of oxygenation and de-oxygenation promote more sickle formations. These sickle RBCs block blood vessels, cause severe pain, anemia and even damage organs (US National Library of Medicine [NLM], 2009).
Conclusion
Misfolded proteins have certain specific characteristic features such as the amyloid fold and they tend to bind together and form fibers. Hence recent research focus is on developing drugs that could interact and bind to the amyloid motifs and promote disintegration of the misfolded proteins (Neurophage pharmaceuticals, 2015). Gene therapy is also being studied widely to treat protein-misfolding diseases at the initial protein synthesis stage itself. However, it might take some time for these strategies to become actual treatment procedures.
References
Albright College. (2014). Sickle Cell Anemia Fact Sheet. Retrived Jul. 2, 2015, from http://albright.edu/resources/healthcenter/Sickle_Cell_Packet.pdf
Neurophage Pharmaseuticals. (2015). Protein Misfolding Diseases. Retrieved Jul. 1, 2015, from http://neurophage.com/science/protein-misfolding-d
NLM,. (2009). HBB. Retrieved Jul 1, 2015, from Genetics Home Reference. Retrieved Jul. 1, 2015, from http://ghr.nlm.nih.gov/gene/HBB
Reynaud, E. (2010). Protein Misfolding and Degenerative Diseases. Retrieved Jul. 1, 2015, from http://www.nature.com/scitable/topicpage/protein-m
Saxena, I. M. (2014). Protein Structure. (chap. Cell Biology) Retrieved Jul. 1, 2015, from http://www.pai.utexas.edu/faculty/isaxena/BIO320
