Types of synthesis : 2. Chemical synthesis

Chemical synthesis Short proteins can also be synthesized chemically by a family of methods known as peptide synthesis, which rely on organic synthesistechniques such as chemical ligationto produce peptides in high yield. [ 9 ]Chemical synthesis allows for the introduction of non-natural amino acids into polypeptide chains, such as attachment of fluorescentprobes to amino acid side chains. [ 10 ]These methods are useful in laboratory biochemistryand cell biology, though generally not for commercial applications. Chemical synthesis is inefficient for polypeptides longer than about 300 amino acids, and the synthesized proteins may not readily assume their native tertiary structure. Most chemical synthesis methods proceed from C-terminus to N-terminus, opposite the biological reaction. [ 11 ] Structure Main article: Protein structure Further information: Protein structure prediction The crystal structure of the chaperonin. Chaperonins assist protein folding. Three possible representations of the three-dimensional structure of the protein triose phosphate isomerase. Left: all-atom representation colored by atom type. Middle: Simplified representation illustrating the backbone conformation, colored by secondary structure. Right: Solvent-accessible surface representation colored by residue type (acidic residues red, basic residues blue, polar residues green, nonpolar residues white) Most proteins foldinto unique 3-dimensional structures. The shape into which a protein naturally folds is known as its native conformation. [ 12 ]Although many proteins can fold unassisted, simply through the chemical properties of their amino acids, others require the aid of molecular chaperonesto fold into their native states. [ 13 ]Biochemists often refer to four distinct aspects of a protein's structure: [ 14 ] *. Primary structure: the amino acid sequence. A protein is a polyamide. *. Secondary structure: regularly repeating local structures stabilized by hydrogen bonds. The most common examples are the alpha helix, beta sheetand turns. Because secondary structures are local, many regions of different secondary structure can be present in the same protein molecule. *. Tertiary structure: the overall shape of a single protein molecule; the spatial relationship of the secondary structures to one another. Tertiary structure is generally stabilized by nonlocal interactions, most commonly the formation of a hydrophobic core, but also through salt bridges, hydrogen bonds, disulfide bonds, and even posttranslational modifications. The term "tertiary structure" is often used as synonymous with the termfold. The tertiary structure is what controls the basic function of the protein. *. Quaternary structure: the structure formed by several protein molecules (polypeptide chains), usually called protein subunitsin this context, which function as a single protein complex. Proteins are not entirely rigid molecules. In addition to these levels of structure, proteins may shift between several related structures while they perform their functions. In the context of these functional rearrangements, these tertiary or quaternary structures are usually referred to as " conformations", and transitions between them are calledconformational changes.Such changes are often induced by the binding of a substratemolecule to an enzyme's active site, or the physical region of the protein that participates in chemical catalysis. In solution proteins also undergo variation in structure through thermal vibration and the collision with other molecules. [ 15 ] Molecular surface of several proteins showing their comparative sizes. From left to right are: immunoglobulin G(IgG, an antibody), hemoglobin, insulin(a hormone), adenylate kinase(an enzyme), and glutamine synthetase(an enzyme). Proteins can be informally divided into three main classes, which correlate with typical tertiary structures: globular proteins, fibrous proteins, and membrane proteins. Almost all globular proteins are solubleand many are enzymes. Fibrous proteins are often structural, such as collagen, the major component of connective tissue, or keratin, the protein component of hair and nails. Membrane proteins often serve as receptorsor provide channels for polar or charged molecules to pass through the cell membrane. [ 16 ] A special case of intramolecular hydrogen bonds within proteins, poorly shielded from water attack and hence promoting their own dehydration, are called dehydrons.