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The proteins and nucleic acids Proteins The real importance of proteins is mainly provided by the various growth and support roles they play with regard to cell structures. Moreover, metabolic reactions take place with sufficient speed in the cellular environment for the presence of enzymes. Proteins with a specific catalytic function. The proteins and nucleic acids > Proteins Amino acids Amino acids are molecules that contain •a functional amine group (–NH2) •a carboxyl group (–COOH) The proteins and nucleic acids > Amino acids The α carbon atom In all the amino acids the α carbon atom is asymmetric, with a highly differentiated residual group. Two molecular structures, specular to each other, indicated with letters L and D. The amino acids naturally present in proteins all belong to the L-series, called the “natural series”. The proteins and nucleic acids > The α carbon atom The residual –R groups of amino acids The chemical nature of the residual –R groups of amino acids allow the protein they form to have a very specific structure and thus to perform specific functions. Alanine or phenylalanine are NON-POLAR in character and constitute areas of hydrophobicity in the protein chain. The proteins and nucleic acids > The residual –R groups of amino acids The –CH2–OH in serine are polar in character an d participate in the formation of hydrogen bonds. The –CH2–SH in cysteine is responsible for actual covalent bonds, known as disulphide bridges. The peptide bond Amino acids can link together using a strong (peptide) bond through a condensation reaction with the elimination of a molecule of water between the carboxyl group of one amino acid and the amino group of another. The proteins and nucleic acids > The peptide bond The structural organization of proteins Structure of a protein depends on 1.the chain’s amino acid sequence; 2.how the chains are arranged in three dimensions in space; 3.by further twists and folds; 4.by the presence of either a single block or the association of several subunits. The proteins and nucleic acids > The structural organization of proteins Corresponding to these factors, there are 4 ‘levels’ in the structural organization of proteins. The primary structure 1. The chain’s amino acid sequence The primary structure of a protein is the exact sequence of the amino acids which it is made of. This also affects other factors upon which the overall structure of the protein depends and determines its shape and behaviour. The proteins and nucleic acids > The primary structure The secondary structure 2. The secondary structures of the most common polypeptide chains are the twists: • α-helix globular proteins: keratin in hair. • β-sheet fibrous proteins: silk. • triple helix collagen, a protein which is found in the tendons The proteins and nucleic acids > The secondary structure The tertiarity structure 3. The spatial structure that it assumes as a result of the twisting of the protein chains due to the formation of bonds between amino acid residual groups that are distant from each other and in association with the presence of nontwisted sections that form the pivot for any folding. 3D structure characteristic of globular proteins The proteins and nucleic acids > The tertiarity structure The quaternary structure 4. The spatial structure of its molecule resulting from the association of polypeptide subunits to form biologically active complexes that may also be associated with non-protein elements. The haemoglobin molecule consists of 4 subunits (two α and two β chains, of 141 and 146 amino acids respectively) The proteins and nucleic acids > The quaternary structure Denaturation of proteins The forces that hold the subunits of a protein together are weak and very sensitive to changes in the environment. Changes in temperature or pH Modify secondary and tertiary structure and thus change 3D configuration The proteins and nucleic acids > Denaturation of proteins Denatured protein can return spontaneously to its ternary 3D configuration if its primary structure has not been changed. Only the primary sequence determines the over-riding structures. Functions of proteins in organisms Proteins perform functions of growth and of support, but also more sophisticated roles, linked for example to defence, regulation and catalysis. The main functions • enzymatic catalysis • contraction • defence • transport • regulation • structure The proteins and nucleic acids > Functions of proteins in organisms Enzymes Enzymes are proteins of varying complexity, able to change the speed of biological reactions under conditions compatible with the life of the organisms. Thanks to enzyme, a reaction may require less time or lower activation energy to take place. The proteins and nucleic acids > Enzymes Substrates and products Enzymes as biocatalysts, greatly accelerating the speed of reactions which otherwise would take place far more slowly. The substances that are transformed by the action of enzymes are called substrates, While those that are formed by the reaction are called products. The proteins and nucleic acids > Substrates and products Nucleic acids Are polymers and their monomers are nucleotides. Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) are the main elements responsible for the cellular characteristics. The histones, structures of 8 subunits, each of which wraps around two turns of the nucleic acid strand The proteins and nucleic acids > Nucleic acids The nucleotides of RNA and DNA Each nucleotide is composed of three parts: a phosphate group (orthophosphoric acid), a sugar with 5 carbon atoms (a pentose) and a nitrogenous base. Of these there are 2 types: one group with two rings, the purines, and another with only a single ring called the pyrimidines. The proteins and nucleic acids > The nucleotides of RNA and DNA The structure TWO CHAINS DNA is presented as a linear molecule with 2 chains of nucleotides coupled with opposite spatial orientation. The proteins and nucleic acids > The structure OPPOSITE DIRECTION The bases are oriented towards the interior and each base of a filament binds by means of hydrogen bonds with the complementary base of the other strand, which develops in the opposite direction. The binding THE DNA BINDING As a result of the special structure of the bases, the 2 DNA molecules can only bind to each other in a single manner, with 2 hydrogen bonds allowing only the union of adenine with thymine (AT), while 3 hydrogen bonds only allow guanine to bind with cytosine (GC). The pairing also permits the maintenance of a regular distance between the 2 strands. The proteins and nucleic acids > The binding The double helix In 1953 J.D. Watson and F. Crick hypothesize the double-helix model for DNA. The helical structure repeats itself every 340 nm, which correspond to 10 nucleobases. The double helix of DNA is a self-replicating molecule: it is in fact able to reproduce chains identical to itself in the preparative phase before cell division. Biomolecules: carbohydrates and lipids / Metabolism: the role of energy