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Protein Structure Review
1. What are proteins? How can the protein diversity of living beings be explained? Proteins are molecules made of sequences of amino acids bound by a peptide bond. The genetic code codifies twenty different amino acids that can compose proteins. So there are numerous combinations of amino acid which can form polypeptide chains and for this reason protein molecules can be immensely diverse. 2. What is the importance of proteins for living beings? Proteins play a fundamental role in nearly all biological processes. Due to their diversity they can assume many different configurations and they can play varied roles in cells and tissues. Some protein functions are noteworthy: the structural function (cell membrane proteins, cytoskeleton proteins, proteins of the connective tissue), the enzymatic function (enzymes are proteins), the energy storage function (proteins can be degraded into acetyl-CoA and “cycle” the Krebs cycle), the osmotic regulation function (albumin), the transportation function (membrane channels, respiratory pigments), the immune protection function (antibodies), the movement function (contractile proteins), the endocrine integration function (hormones) and the informative function (membrane receptors, intracellular signalers). There are also many proteins whose biological functions are not yet known. 3. What is the constitutional unit of proteins? The constitutional units of proteins are the amino acids. Protein Structure Review - Image Diversity: amino acid structure 4. What is an oligopeptide? How is it different from a polypeptide? Peptide is the molecule formed by the union of amino acids through the peptide bond. Oligopeptide is a peptide made of few amino acids (oligo = few). Polypeptides are peptides with many amino acids (poli = many), in general more than 50. 5. How many are the known amino acids that form proteins in living beings? There are twenty different known amino acids that form proteins related to the genetic code of the living beings. There are still many other amino acids as yet not known. Protein Structure Review - Image Diversity: types of amino acids 6. Does every amino acid have a central carbon? To which organic group is that central carbon bound? A carboxyl group –COOH, an amine group – NH2, an atom of hydrogen –H and a variable radical -R necessarily are bound to the central carbon of an amino acid. 7. How can amine groups be classified? Amines can be classified into primary amines, those to which one –R (variable radical) is attached to a –NH2, secondary amines, those where one hydrogen of NH2 is substituted by another –R, thus having two –R, and tertiary amines, those with no hydrogen bound to the nitrogen and with three –R. primary amine secondary amine tertiary amine 8. What is the structural representation of a carboxyl group? Carboxyl groups have a carbon attached to one hydroxyl group by a simple bond and to one oxygen by a double bond. The other site of binding in the carbon is available to other chemical entities. 9. What is the structural flat representation of an amino acid molecule? An amino acid has a central carbon to which a carboxyl group binds on a side and to which a –R (variable radical) binds on the opposite side. In the perpendicular direction of those ligands an amine group binds the central carbon on one side and a hydrogen binds on the opposite side. The bind of the carboxyl group to a carbon where a hydrogen is laterally attached is responsible for the name “acid” in amino acids. The bound of an amine group in the central carbon provides the name “amino”. Protein Structure Review - Image Diversity: carboxyl group 10. What is the importance of the –R group (variable radical) in an amino acid molecule? The –R group, also called a lateral chain, is the variable part of the amino acid molecule. The –R group can be a complex carbonic chain, a substituting methyl group (forming then the amino acid alanine) or even only a hydrogen (forming glycine, the simplest amino acid). So the –R group is important because it is the differentiation factor of amino acids. 11. How can the binding of two amino acids for the peptide formation be described? A peptide is formed when a carbon from the carboxyl group of one amino acid is connected to the nitrogen of the amine group of another amino acid. During that binding the hydroxyl of the carboxyl and one hydrogen of the amine is lost resulting in the liberation of one water molecule. 12. What is the binding between two amino acids called? The chemical bond between two amino acids is called a peptide bond. Protein Structure Review - Image Diversity: peptide bond 13. Do the –R groups bound to the central carbons participate in the union between amino acids? The peptide bond attaches the nitrogen of the amine group of one amino acid to the carbon of the carboxyl group of another amino acid liberating one molecule of water. So the –R groups do not participate in that bond. 14. Do the –H groups bound to the central carbons participate in the peptide bond? The central carbons themselves, the –R groups and the hydrogens attached to the central carbons do not participate in the peptide bond. 15. Do the amine and the carboxyl groups attached to central carbons participate in the union between amino acids? Yes. The nitrogen of the amine group of one amino acid binds to the carbon of the carboxyl group of the other amino acid. The water molecule liberated from the formation of the peptide bond thus has a hydrogen from the amine and an oxygen and another hydrogen from the carboxyl. 16. Does the chemical reaction to unite amino acids incorporate or liberate atoms? What are the chemical entities incorporated or liberated in this reaction? The union of amino acids by peptide bond liberates atoms. They are liberated as constituents of one molecule of water. 17. Are there different proteins made by the same total number of amino acids? Different proteins with the same total number of amino acids may exist. In such cases the differentiation is given by the types of amino acids or by the sequence in which they form the protein. 18. Are proteins with the same number of each different amino acid that form them necessarily identical proteins? Even if many proteins have the same number of each different amino acid that form them, for example, 50 alanines, 70 glycines and 20 histidines, the sequences in which these amino acids are connected may be very different. So if two or more proteins are in such condition of numeric similarity for each type of their constituent amino acids, they are not necessarily identical. 19. What is the essential condition for a protein to be identical to another protein? For a protein to be identical to another protein it is necessary for the sequence of amino acids that form them to be identical. 20. What is the primary structure of a protein? What is the importance of the primary structure? The primary protein structure is the linear sequence of amino acids that form the molecule. The primary structure is the basis of the protein identity. Modification of only one amino acid of the primary structure creates a different protein. This different protein can be inactive or can even have other biological functions. Protein Structure Review - Image Diversity: protein primary structure 21. What is the secondary structure of a protein? The secondary protein structure is generated by the manner its amino acids interact through the intermolecular bond. These interactions create a spatial conformation of the polypeptide filament. The two most studied secondary conformations of proteins are the alpha-helix and the beta-sheet. Protein Structure Review - Image Diversity: protein secondary structure 22. What is the difference between the alpha-helix and the beta-sheet protein conformations? Alpha-helix and beta-sheet conformations are the two main types of secondary structure of a protein molecule. According to the primary protein structure its secondary structure can be of one type or the other. In the alpha-helix structure the polypeptide curls longitudinally by the action of hydrogen bonds forming a spiral, or helix. In the beta-sheet conformation the protein is more distended and the hydrogen bonds form a zig-zag-shaped protein structure called B-strand. Many assembled beta-strands make a beta-sheet. 23. What is the tertiary structure of a protein? What are the main types of tertiary structure? The tertiary protein structure is a spatial conformation additional to the secondary structure in which the alpha-helix or the beta-sheet folds itself up. The forces that keep the tertiary structure generally are interactions between the –R groups of the amino acids and between other parts of the protein and water molecules of the solution. The main types of tertiary structure of proteins are the globular proteins and the fibrous proteins. Protein Structure Review - Image Diversity: protein tertiary structure 24. What is the quaternary structure of a protein? Do all proteins have quaternary structure? The quaternary protein structure is the spatial conformation due to interactions among polypeptide chains that form the protein. Only those proteins made of two or more polypeptide chains have quaternary structure. Insulin (two chains), hemoglobin (four chains) and the immunoglobulins (antibodies, four chains) are some examples of protein having quaternary structure. Protein Structure Review - Image Diversity: protein quaternary structure 25. What is protein denaturation? Is there any change in the primary structure when a protein is denatured? Secondary, tertiary and quaternary structures of proteins are spatial structures. Denaturation is modification in any of these spatial structures that makes the protein deficient or biologically inactive. After denaturation the primary protein structure is not affected. Protein Structure Review - Image Diversity: denatured protein 26. How can denaturation be classified regarding its reversibility? Protein denaturation can be a reversible or an irreversible process, i.e., it may be possible or impossible to make the protein regain its original spatial conformation. 27. What are some factors that can lead to protein denaturation? Protein denaturation can be caused by temperature variation, pH change, changes in the concentration of surrounding solutes and by other processes. Most proteins denature after certain elevation of temperature or when in very acid or very basic solutions. This is one of the main reasons that it is necessary for the organisms to keep stable temperature and pH. 28. Is it expected that a change in the primary, in the secondary or in the tertiary structure of a protein will produce more functional consequences? Any change of the protein structure is relevant if it alters its biological activity. Changes in the primary protein structure are more important because they are modifications in the composition of the molecule and such composition determines all other structures of the protein. 29. In sickle cell anemia, a hereditary disease, there is substitution of one amino acid by another in one of the four polypeptide chains of hemoglobin. In this case are all of the structural levels of the protein modified? In sickle cell disease there is a change in the primary protein structure of one of the polypeptide chains that form hemoglobin: the amino acid glutamic acid is substituted by the amino acid valine in the β chain. The spatial conformation of the molecule in addition is also affected and modified by this primary “mistake” and the modification also creates a different (sickle) shape to the red blood cells. Modified, sickled, red blood cells sometimes aggregate and obstruct the peripheral circulation causing tissue hypoxia and the pain crisis typical of sickle cell anemia. 30. What is the difference between essential and natural amino acids? Essential amino acids are those that the organism is not able to synthesize and that need to be ingested by the individual. Natural amino acids are those that are produced by the organism. There are living species that produce every amino acid they need, for example, the bacteria Escherichia coli, that does not have essential amino acids. Other species, like humans, need to obtain essential amino acids from the diet. Among the twenty different known amino acids that form proteins humans can make twelve of them and the remaining eight need to be taken from the proteins they ingest with food. The essential amino acids for humans are phenylalanine, histidine, isoleucine, lysine, methionine, threonine, tryptophane and valine. 31. What are respectively some remarkable functions of myosin, CD4, albumin, keratin, immunoglobulin, reverse transcriptase, hemoglobin and insulin? Myosin is a protein that when associated with actin produces a muscular contraction. CD4 is a membrane protein of some lymphocytes, the cells that are infected by __________. Albumin is an energy storage protein and also an important regulator of the blood osmolarity. Keratin is a protein with structural function present in the epidermis and skin appendages of vertebrates. Immunoglobulins are the antibodies, specific proteins that attack and inactivate strange agents that enter the body. Reverse transcriptase is the enzyme responsible for the transcription of RNA and formation of DNA in the life cycle of retroviruses. Hemoglobin is the protein that carries oxygen from the lungs to the cells. Insulin is a hormone secreted by the pancreas that participates in the metabolism of glucose. |