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4 proteins structure and function q and a

  1. 1. Q. How proteins are made up of?A. Proteins are made by polymerization of amino acids through peptide bonds.Q. What is a peptide bond?A. Alpha carboxyl group of one amino acid reacts with alpha amino group of anotheramino acid to form a peptide bond or CO-NH bridgeQ. What is a dipeptide?A. Two amino acids are combined to form a dipeptide.Q. How many peptide bonds are present in a tripeptide?A. A tripeptide is a combination of three amino acids; so there are two peptide bonds.Q. What is a polypeptide?A. A combination of 10 to 50 amino acids is called as a polypeptide.Q. What is the difference between a polypeptide and a protein?A. A combination of 10 to 50 amino acids is called a polypeptide. By convention,chains containing more than 50 amino acids are called proteins.Q. What are the levels of organizations of proteins?A. Proteins have primary, secondary, tertiary and quaternary levels of organization.Q. What is meant by primary structure of a protein?A. It denotes the number and sequence of amino acids in the protein.Q. What is the force that maintains the primary structure?A. The primary structure is maintained by the covalent bonds of the peptide linkagesQ. What are the salient features of a peptide bond?A. The peptide bond is a partial double bond. The C-N bond is ‘ trans’ in nature andthere is no freedom of rotation because of the partial double bond character.Q. What is the N-terminal end of a protein?A. In a protein, at one end there will be one free alpha amino group. This end is calledthe amino terminal (N-terminal) end and the amino acid contributing the amino groupis named as the first amino acid. Dr. Ehab Aboueladab, Associate Prof.Dr. Of Biochemistry and Nutrition, email:ehab10f@gmail.com
  2. 2. Q. What are the names for the end amino acids of proteins?A. The end where there is a free alpha amino group is called the amino terminal (N-terminal) end. The other end of the polypeptide chain is called the carboxy terminalend (C-terminal) where there is a free alpha carboxyl group.Q. Can you give an example of a pseudopeptide?A. Glutathione (gamma-glutamyl-cysteinyl-glycine). The pseudopeptide a peptidebond formed by carboxyl group, other than that of alpha position.Q. What are the salient structural features of insulin?A. It has two polypeptide chains. These chains are held together by disulfide bridges.Insulin has total 51 amino acids.Q. What is pro-insulin?A. Insulin is synthesised by the beta cells of pancreas as a prohormone, proinsulin is asingle poly-peptide chain with 86 amino acids.Q. What is mutation?A. Amino acid change in the linear sequence is called a mutation.Q. Can you give an example?A. sickle cell anemia due to Hemoglobin S,Q. What is the defect in HbS?A. Normally the 6 th amino acid in the beta chain is glutamic acid, this is replaced byvaline in the HbS molecule.Q. Which are the forces that maintain the secondary, tertiary and quaternarystructures of a protein?A. Hydrogen bonds, Electrostatic bonds, Van der Waals forces and Hydrophobicbonds.Q. What are the salient features of alpha structure of proteins?A. It is a right-handed spiral structure; each turn is formed by 3.6 amino acid residues;it is major structural motif in globular proteins.Q. Which will inhibit the formation of alpha helix?A. Proline. Dr. Ehab Aboueladab, Associate Prof.Dr. Of Biochemistry and Nutrition, email:ehab10f@gmail.com
  3. 3. Q. What is meant by secondary structure of a protein?A. Secondary structure denotes the configurational relationship between residueswhich are about 3-4 amino acids apart. In other words, secondary level defines theorganisation at immediate vicinity of amino acids.Q. What is meant by tertiary structure of a protein?A. The tertiary structure denotes three dimensional structure of the whole protein. Itdefines the steric relationship of amino acids which are far apart from each other inthe linear sequence.Q. What is meant by a domain of a protein?A. It is the term used to denote a compact unit of a protein. It generally represents afunctional unit.Q. What is meant by quaternary structure of a protein?A. Certain polypeptides will aggregate to form one functional protein. This is referredto as the quaternary structure.Q. Give some examples of proteins having quaternary structure.A. Hemoglobin, lactate dehydrogenase, immunoglobulin.Q. What are the reagents that are used for identifying the first amino acid in aprotein?A. Fluorodinitro benzene, dansyl chloride, phenylisothiocyanate.Q. Protein chains may be separated by what reagent?A. 8 molar urea.Q. What is iso-electric point of a protein?A. At the iso-electric point, the number of anions and cations present on the proteinmolecule will be equal and the net charge is zero.Q. What are the characteristic features of iso-electric point?A. At the pI value, the proteins will not migrate in an electrical field; solubility,buffering capacity and viscosity will be minimum and precipitation will be maximum.Q. What is the iso-electric pH of human albumin?A. It is 4.7. Dr. Ehab Aboueladab, Associate Prof.Dr. Of Biochemistry and Nutrition, email:ehab10f@gmail.com
  4. 4. Q. How proteins are precipitated from solution?A. Any factor which neutralises the charge or removes water of hydration will causeprecipitation of proteins.Q. How albumin is precipitated?A. By full saturation of ammonium sulfate or 28 % sodium sulfate.Q. What will be precipitated by half-saturation of ammonium sulfate?A. Globulins are precipitated by half-saturation of ammonium sulfate.Q. Give an example of precipitation at iso-electric point.A. Casein is precipitated when the solution is brought to iso-electric pH.Q. What is the iso-electric pH of casein?A. 4.6.Q. Give some examples of anionic precipitating agents.A. Tungstic acid, phosphotungstic acid, trichloroacetic acid, picric acid,sulphosalicylic acid and tannic acid are protein precipitating agents.Q. What are the features of denaturation?A. The secondary, tertiary and quaternary structures are lost, but primary structure ispreserved. The functional activity is lost. The denature proteins are insoluble andeasily precipitated.Q. What are the usual agents that cause denaturation of proteins?A. Brief heating, urea, X-ray, ultraviolet ray, high pressure, vigorous shaking.Q. What is heat coagulation?A. When heated at iso-electric point, some proteins will denature irreversibly toproduce thick floating conglomerates called coagulum. This is called heatcoagulation.Q. Give examples of proteins that coagulate easily.A. Albumin is easily coagulated, and globulins to a lesser extent.Q. How proteins are classified?A. They may be classified depending on the function or based on the physicochemicalcharacteristics or based on their nutritional value. Dr. Ehab Aboueladab, Associate Prof.Dr. Of Biochemistry and Nutrition, email:ehab10f@gmail.com
  5. 5. Q. What is the functional classification of proteins?A. 1. Catalytic proteins, 2. Structural proteins, 3. Contractile proteins, 4. Transportproteins, 5. Regulatory proteins or hormones, 6. Genetic proteins and 7. Protectiveproteins.Q. Based on physiochemical properties, how are they classified?A. Simple proteins conjugated proteins and derived proteins.Q. Give examples of simple proteins.A. Albumins, globulins, protamines, prolamines, lectins, scleroproteins.Q. Give examples of scleroproteins.A. Collagen of bone, cartilage and tendon, keratin of hair, horn, nail and hoof.Q. What are conjugated proteins?A. Combinations of protein with a non-protein part is called prosthetic group.Q. How are conjugated group subclassified?A. Glycoproteins, lipoproteins, nucleoproteins, chromoproteins, phospho-proteins andmetallo-proteins.Q. Give examples of phosphoproteins.A. Casein of milk and vitellin of egg yolk.Q. Where is this phosphate attached to proteins?A. The phosphoric acid is added to the hydroxyl groups of serine and threonineresidues of proteins.Q. What are lectins?A. Plant proteins having specific carbohydrate binding site.Q. Give an example of a nutritionally rich protein (first class protein).A. Casein.Q. Some proteins are called as poor proteins; why?A. They lack in many essential amino acids and a diet based on these proteins will noteven sustain the body weight.Q. Which method of protein estimation is dependent on the intact peptide bond?A. Biuret method. Dr. Ehab Aboueladab, Associate Prof.Dr. Of Biochemistry and Nutrition, email:ehab10f@gmail.com
  6. 6. Q. What is the advantage of biuret method?A. The biuret method is simple one step process, and is the most widely used methodfor plasma protein estimations.Q. What is the disadvantage of biuret method?A. The sensitivity of the method is less and is unsuitable for estimation of proteins inmilligram or microgram quantities.Q. What is the basis of Lowrys method of protein estimation?A. This is based on the reduction of folin-ciocalteauphenol reagent (phosphomolybdicacid and phosphotungstic acid) by the tyrosine and tryptophan residues of protein.Q. Which component of the protein absorb UV light at 280 nm?A. Indole ring of tryptophan.Q. What is Nephelometry?A. Nephelometry is defined as the detection of light scattered by turbid particles insolution. Dr. Ehab Aboueladab, Associate Prof.Dr. Of Biochemistry and Nutrition, email:ehab10f@gmail.com