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All questions of Amino Acids and Proteins for MCAT Exam

The pI for a polyelectrolyte that contains three carboxyl groups and three amino groups whose pKa values are 4.0, 4.6, 6.3, 7.7, 8.9, and 10.2 is _______________
    Correct answer is '7'. Can you explain this answer?

    Jay Nambiar answered
    Example the pI is a pH midway between the 3rd and 4th pKa values: pI = (6.3 + 7.7)/2 = 7.0. To confirm this conclusion, imagine how the net charge on the molecule will change as the solution is adjusted from strongly acidic to strongly basic pH. As the carboxylate groups and subsequently the ammonium groups begin to ionize, the net charge will change successively as follows: +3, +2, +1, 0, “1, “2, “3.

    The net charge on the following peptide at pH 1 is _________________.
      Correct answer is '2'. Can you explain this answer?

      Vandana Chopra answered
      At pH 1 the charge distribution on the peptide will be as follows:
      NH3+-Ala-Asp-Asn-Ile-Pro-Gln-Thr-Agr-COOH
      Note that every peptide is written from amino to carboxy terminal, and at a very low pH such as 1 every side chain and main chain ionizable groups will be protonated. Hence net change will be decided by amino groups, as the sequence contain one basic amino acid (arginine) and only alanine contain free amino group, so the  charge is due to  positively charged amino group.

      Which of the following amino acids will have Diastereomers.
      • a)
        Isoleucine
      • b)
        Threonine
      • c)
        both (a) and (b)
      • d)
        None
      Correct answer is option 'C'. Can you explain this answer?

      Aashna Shah answered
      Stereoisomers that are mirror images of each other are called enantiomers.    
      Stereoisomers that are not mirror images of each other are called diastereomers.    
      Diastereomers are present only in those molecules which have more than one chiral carbon atoms, and isoleucine as well as threonine both have two chiral centers. Since both threonine and isoleucine have two chiral centres each, maximum number of stereoisomers in each case will be 2n = 22 = 4 (n is the number of chiral centres).

      The isoelectric point of lysine which has pKb 9.06, pKa 2.16 and pKR as 10.54 is ____________[Answer upto two decimal places]
      • a)
         
      • b)
         
      • c)
         
      • d)
         
      • e)
         
      Correct answer is '9.8'. Can you explain this answer?

      Isoelectric point: In proteins the isoelectric point (pI) is defined as the pH at which a protein has no net charge.
      In case of a basic amino acid, pI lies in between the pKb and pKR and is equal to the average of pKb and pKR(10.54+9.06)/2 = 9.77 = ~ 9.8.
      In case of an acidic amino acid, pI lies in between the pKa and pKR and is equal to the average of pKa and pKR.
      In case of a neutral amino acid, pI lies in between the pKb and pKa and is equal to the average of pKa and pKb.

      Which of the following represents the two-dimensional structure of proteins?
      • a)
        Quaternary
      • b)
        Tertiary
      • c)
        Secondary
      • d)
        Primary
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      The primary structure of Protein represents the two-dimensional structure of proteins. The primary structure of proteins just contains amino acids linked together to form a long chain of a polypeptide. Quaternary, tertiary, and secondary structure refers to the three-dimensional structure of proteins.

      Total number of known amino acids in biological systems is
      • a)
        20
      • b)
        21
      • c)
        22
      • d)
        More than 900
      Correct answer is option 'D'. Can you explain this answer?

      Juhi Sen answered
      There are various groups of amino acids:
      • 20 standard amino acids
      • 22 proteinogenic amino acids
      • Over 80 amino acids created abiotically in high concentrations
      • About 900 are produced by natural pathways
      • Over 118 engineered amino acids have been placed into protein

      If an amino acid with neutral side chain has pKa value of 3.4 and pKb value of 9.6 at which of the following pH the amino acid will migrate towards anode
      • a)
        6
      • b)
        7.4
      • c)
        2.3
      • d)
        3.4
      Correct answer is option 'B'. Can you explain this answer?

      Anisha Pillai answered
      The isoelectric point here is the average of pKa and pKb which is (3.4+9.6)/2 = 6.5.
      Every amino acid is negatively charged at pH above its pI and move towards Anode (+). So at pH 7.4 the overall charge on amino acid will be negative and it will move towards Anode. Only pH 7.4 is above the isoelectric part of amino acid (isoelectric point = 6.5)

      Amino acids that are most likely to be phosphorylated in a protein during a signalling event are
      • a)
        Serine and threonine
      • b)
        Leucine and Isoleucine
      • c)
        Tryptophan and Tyrosine
      • d)
        Threonine and Arginine
      Correct answer is option 'A'. Can you explain this answer?

      Phosphorylation is most likely to occur at hydroxyl group of side chain of amino acids i.e. serine and threonine. Hydroxyl group is not present in leucine, isoleucine, arginine and tryptophan.

      Which of the following names do not represent the name of a peptide
      • a)
        WATSON
      • b)
        EINSTEIN
      • c)
        KENDREW
      • d)
        LIPMAN
      Correct answer is option 'A'. Can you explain this answer?

      Maitri Sen answered
      There are 20 amino acids and 26 letters in English alphabet, hence 6 letters viz B, J, O, U, X , Z do not represent any amino acids, which means the name containing these amino acids will not be representing a peptide O is present in option (a), hence option (a), does not represent a peptide.

      Which of the following amino acid shows highest absorbance at 280 nm
      • a)
        Phenylalanine 
      • b)
        Tyrosine 
      • c)
        Alanine
      • d)
        Tryptophan
      Correct answer is option 'D'. Can you explain this answer?

      Shivam Khanna answered
      Three aromatic amino acids (Tyr, Trp, and Phe) absorb most of the UV light in a protein. Tyrosine and tryptophan both absorb more than phenylalanine. Tryptophan shows four times more absorbance than tyrosine.

      Identify the amino acids containing nonpolar, aliphatic R groups.
      • a)
        Phenylalanine, tyrosine, and tryptophan
      • b)
        Glycine, alanine, leucine
      • c)
        Lysine, arginine, histidine
      • d)
        Serine, threonine, cysteine
      Correct answer is option 'B'. Can you explain this answer?

      Ava Brown answered
      Understanding Nonpolar, Aliphatic Amino Acids
      Amino acids are the building blocks of proteins, and their properties are determined by their R groups (side chains). Nonpolar, aliphatic amino acids have hydrophobic side chains that do not interact favorably with water.
      Identifying Nonpolar, Aliphatic Amino Acids
      The correct answer to the question is option 'B', which includes the following amino acids:
      • Glycine - The simplest amino acid with a hydrogen as its side chain, making it nonpolar.
      • Alanine - Contains a methyl group as its R group, also classified as nonpolar.
      • Leucine - Has a branched-chain structure, contributing to its nonpolar nature.

      Why Options A, C, and D Are Incorrect
      • Option A (Phenylalanine, Tyrosine, Tryptophan) - Contains aromatic rings, making them more polar than purely aliphatic amino acids.
      • Option C (Lysine, Arginine, Histidine) - These are polar and positively charged amino acids due to their side chains containing nitrogen.
      • Option D (Serine, Threonine, Cysteine) - These amino acids contain hydroxyl or thiol groups, which are polar and can form hydrogen bonds.

      Conclusion
      In summary, the amino acids listed in option 'B' are characterized by their nonpolar, aliphatic side chains, distinguishing them from the other options that include polar or aromatic amino acids. Understanding these classifications is essential for grasping protein structure and function in biochemistry.

      Which of the following is a true statement?
      • a)
        Tryptophan and tyrosine are significantly more polar than phenylalanine
      • b)
        Leucine is commonly used as an ingredient in the buffers of SDS page
      • c)
        Aspartate is an essential amino acid
      • d)
        Lysine is a non-essential amino acid
      Correct answer is option 'A'. Can you explain this answer?

      Elizabeth Lee answered
      Explanation:

      Tryptophan and tyrosine are significantly more polar than phenylalanine:
      - Tryptophan and tyrosine contain polar functional groups such as hydroxyl and amino groups, making them more polar compared to phenylalanine which lacks such groups.
      - The presence of polar groups in tryptophan and tyrosine allows for interactions with water molecules, leading to their higher polarity.
      This statement is true because tryptophan and tyrosine have more polar side chains compared to phenylalanine, which is a nonpolar amino acid. The polarity of an amino acid side chain is determined by the presence of polar functional groups such as hydroxyl (-OH) and amino (-NH2) groups. Tryptophan and tyrosine both contain these polar functional groups in their side chains, making them significantly more polar than phenylalanine, which lacks such groups. This increased polarity allows tryptophan and tyrosine to interact more readily with water molecules and other polar substances, influencing their behavior in biological systems.

      Which of the following amino acid is/are not encoded by standard codons
      • a)
        Serine
      • b)
        Selenocysteine
      • c)
        Pyrolysine
      • d)
        both b and c
      Correct answer is option 'D'. Can you explain this answer?

      Akshat Saini answered
      Amino acids Selenocysteine and Pyrolysine are not encoded by standard codons but by stop codons.Selenocysteine is coded by UGA and Pyrolysine is coded by UAG.
      UAG and UGA both are stop codon. Serine is encoded by 6 different codons.  

      Which of the following proteins primarily have alpha helix in their structure
      • a)
        Myoglobin    
      • b)
        Silk fibroin
      • c)
        Ribonuclease
      • d)
        Porin
      Correct answer is option 'A'. Can you explain this answer?

      Shreya Chauhan answered
      Myoglobin contains only α helix. Around 75% of Myoglobin is α helix. Myoglobin consists of eight alpha helixes connected through turns. Silk fibroin and Porin contain only β sheets. Ribonuclease contains a combination of alpha helix and beta sheets.

      The correct statement about the biosynthesis of amino acids are
      • a)
        Amino acids are synthesised from glycolytic and citric acid cycle intermediates
      • b)
        Glutamate can be formed by amination of alpha ketogluterate
      • c)
        Pyruvate on amination can form alanine
      • d)
        There are 9 families of amino acids based on the metabolic precursors.
      Correct answer is option 'A,B,C'. Can you explain this answer?

      Saranya Mehta answered
      Only statement d is incorrect, there are 6 families of amino acids in which they can be grouped on the basis of their metabolic precursors.
      1. α -Ketoglutarate
      Glutamate →  Glutamine/Proline (also form Ornithine)/Arginine
      2. 3-Phosphoglycerate
      Serine → Glycine/Cysteine (also form Methionine SAM-sulfur donar in mammals)
      3. Oxaloacetate
      Aspaitate/Asparagine/Methionine/Threonine/Lysine
      4. Pyruvate
      Alanine/Valine/Leucine/Isoleucine
      5. Phosphoenolypyruvate and erythrose 4-phosphate Tryptophan/Phenylalanine Tyrosine (from Phenylalanine by hydroxylation)
      6. Ribose 5-phosphate
      Histidine

      Which of the following is not true about the secondary conformation of proteins
      • a)
        Secondary conformations arise from folding of backbone
      • b)
        Random coils are most abundant secondary conformations
      • c)
        Pi helix and 2.27 are alternate names of alpha helix
      • d)
        Ramachandran plot explains the formation of peptide bond.
      Correct answer is option 'A,C,D'. Can you explain this answer?

      Sahana Sharma answered
      (a) is coll ect as formation of a secondary structure is the first step in the folding process that a protein takes to assume its native structure, (b) is incorrect as the most common secondary structures are alpha helices and beta sheets, (c) is incorrect as pi helix (or π -helix) is believed to be an evolutionary adaptation derived by the insertion of a single amino acid into a α -helix and 2.27 and d is incorrect as a Ramachandran plot is a way to visualize backbone dihedral angles ψ against φ of amino acid residues in protein structure. A Ramachandran plot can be used in two somewhat different ways. One is to show in theory which values, or conformations, of the ψ and φ angles are possible for an amino-acid residue in a protein.

      For a protein with 100 amino acids, how many possible sequences are there?
      • a)
        (100)20
      • b)
        (2)100
      • c)
        (100)2
      • d)
        (20)100
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      The general rule is, for a protein having ‘n’ residues, there are (20)n possible combinations. There are 20 different types of amino acids occurring naturally in proteins. Therefore, (20)100 is the correct answer.

      The peptide C=O bond of the nth residue of the backbone of alpha-helix points along and forms a hydrogen bond with the peptide N—H group of which among the following residue?
      • a)
        (n+1)th
      • b)
        (n+2)th
      • c)
        (n+3)th
      • d)
        (n+4)th
      Correct answer is option 'D'. Can you explain this answer?

      Aiden Davis answered

      Explanation:

      Hydrogen bonding in alpha-helix structure:
      - In an alpha-helix structure, the peptide C=O bond of the nth residue forms a hydrogen bond with the peptide N-H group of the (n+4)th residue.
      - This hydrogen bonding pattern helps stabilize the alpha-helix structure.

      Specific interaction:
      - In this case, the peptide C=O bond of the nth residue points along and forms a hydrogen bond with the peptide N-H group of the (n+4)th residue.
      - This specific interaction is a characteristic feature of the alpha-helix structure.

      Correct answer:
      - The correct option is (n+4)th residue, as the hydrogen bonding occurs between the C=O bond of the nth residue and the N-H group of the (n+4)th residue.

      Who discovered the alpha-helix structure in a protein molecule?
      • a)
        Lynn Margulis
      • b)
        Francis Collins
      • c)
        Louis Pasteur
      • d)
        Linus Pauling
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      Linus Pauling discovered the alpha-helix structure in a protein molecule. Lynn Margulis gave the theory of endosymbiosis. Francis Collins discovered the gene for Cystic Fibrosis. Louis Pasteur is also known as the father of immunology.

      Which disease results from the dietary deficiency of vitamin C?
      • a)
        Jaundice
      • b)
        Malaria
      • c)
        Cancer
      • d)
        Scurvy
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      The deficiency of vitamin C in diet results in Scurvy. Collagen is a triple helix and contains some hydroxylated residues. The enzyme (prolyl hydroxylase) that catalyzes this reaction requires vitamin C to maintain its activity.

      Which type of interactions are involved in the quaternary structure?
      • a)
        Only hydrogen bonds
      • b)
        Hydrogen bonds and hydrophobic interactions
      • c)
        Hydrogen bonds, hydrophobic interactions, and disulfide bonds
      • d)
        Hydrogen bonds, hydrophobic interactions, disulfide bonds, and ionic interactions
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      Quaternary structure of a protein is made of a combination of any of the following interactions: i) Hydrogen bonds, ii) hydrophobic interactions, iii) disulfide bonds, iv) ionic interactions. Thus, the option containing all of the above options is correct.

      In metal chelate affinity chromatography, divalent cations such as Zn+2 or Ni+2 is attached on the electrophoretic matrix and it binds to ___________
      • a)
        Trp tag
      • b)
        Ala tag
      • c)
        Gly tag
      • d)
        His tag
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      When 6 histidine residues are inserted consecutively through recombinant DNA technology, it is called as His tag. It has a high affinity for nickel. This strategy is used for the purification of recombinant proteins.

      Which of the following interactions is shown in the figure below?
      • a)
        Disulfide bond
      • b)
        Hydrophobic interactions
      • c)
        Hydrogen bonds
      • d)
        Ionic interactions
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      The interaction shown in the above figure is ionic interactions. Ionic interaction is the interaction between two charged groups. If the two charged groups are oppositely charged then they will attract, while two similar charges repel each other.

      Which of the following interactions is crucial for the primary structure of proteins?
      • a)
        Hydrogen bond
      • b)
        Di-sulfide bond
      • c)
        Vander Waals interactions
      • d)
        Peptide bond
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      Protein’s primary structure is made of amino acids linked together by a peptide bond. Thus, the peptide bond is crucial for the primary structure of proteins. Hydrogen bond, disulfide bond, Vander Waals interactions are all required in a higher level of organization.

      ______ is used as a reducing agent in SDS PAGE.
      • a)
        Sodium dodecyl sulfate (SDS)
      • b)
        Ammonium persulphate (APS)
      • c)
        Bisacrylamide
      • d)
        2-mercaptoethanol
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      2-mercaptoethanol (beta-mercaptoethanol) is used as a reducing agent in SDS PAGE. It breaks the disulfide bonds and denatures the protein molecule. SDS is not a reducing agent, it disrupts non-covalent interactions between peptides. Ammonium persulphate (APS) and Bisacrylamide do not show any reducing properties.

      Which structure of a protein is the arrangement of protein subunits in a multi-subunit complex?
      • a)
        Primary
      • b)
        Secondary
      • c)
        Tertiary
      • d)
        Quaternary
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      Quaternary structure of a protein is the arrangement of protein subunits in a multi-subunit complex. Many proteins are composed of more than one polypeptide chain. These polypeptide chains fold to form a subunit of a complex protein. All subunits combine to form a complete functional protein.

      The term “electron density maps” is related to which of the following technique?
      • a)
        Optical microscopy
      • b)
        NMR spectroscopy
      • c)
        cryo-electron microscopy
      • d)
        X-ray crystallography
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      X-ray crystallography is a technique where in electron density maps are obtained. Thus, the term “electron density maps” is related to X-ray crystallography. Other techniques mentioned do not involve electron density maps.

      Which of the following elements is not visible in the maps of X-ray crystallography?
      • a)
        Nitrogen
      • b)
        Carbon
      • c)
        Oxygen
      • d)
        Hydrogen
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      Hydrogen atom is not visible in the electron density maps of X-ray crystallography. The reason behind this is the low electron density of the hydrogen atom. Hydrogen contains a minimum of one electron.

      Which among the following residues is most likely to be present on the surface of a protein?
      • a)
        Val
      • b)
        Leu
      • c)
        Ile
      • d)
        Arg
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      Arg is most likely to be present on the surface of a protein because it is a polar residue. Generally, polar residues reside on the surface of a protein. Val, leu, and Ile are non-polar residues, hence, they are not likely to be present on the surface of a protein.

      What are multi-subunit proteins called when some or all of its subunits are identical?
      • a)
        Monomers
      • b)
        Polymers
      • c)
        Protomers
      • d)
        Oligomers
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      Many proteins have a complex structure i.e. they have a multi-subunit structure. When some or all the sub-units of a complex protein are identical it is called oligomers.

      Which of the following is an imino acid?
      • a)
        Alanine
      • b)
        Glycine
      • c)
        Proline
      • d)
        Serine
      Correct answer is option 'C'. Can you explain this answer?

      Orion Classes answered
      Proline is secondary amino acid also called as an imino acid as it contains –C = NH – OH group.

      Which of the following interactions is shown in the figure below?
      • a)
        Disulfide bond
      • b)
        Ionic interactions
      • c)
        Hydrophobic interactions
      • d)
        Hydrogen bonds
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      The interaction shown in the above figure is the hydrogen bond. A hydrogen bond is the interaction between a hydrogen atom attached to an electronegative atom with another electronegative atom like oxygen in its vicinity. It is non-covalent in nature.

      Which of the following is used for the reduction of disulfide bonds?
      • a)
        Urea
      • b)
        Guanidine hydrochloride
      • c)
        Iodoacetate
      • d)
        Beta-mercaptoethanol/ dithiothreitol
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      Beta-mercaptoethanol/dithiothreitol is used forthe reduction of disulfide bonds. Urea and guanidine hydrochloride are denaturating agents, while Iodoacetate is used for acetylation of sulfhydryl groups.

      Which of the following interactions is shown in the figure below?
      • a)
        Hydrogen bond
      • b)
        Vander Waals interactions
      • c)
        Ionic bond
      • d)
        Disulfide bond
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      The interaction shown in the above figure is a disulphide bond. Two Cysteine residues are involved in the formation of a disulfide bond. It is covalent in nature.

      Which of the following options contain only those amino acids that are likely to be present in the interior of a protein?
      • a)
        Arg, Val, Met
      • b)
        Asp, Met, Phe
      • c)
        Lys, Phe, Val
      • d)
        Val, Met, Phe
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      The option containing Val, Met, and Phe is the correct option. Val, Met, and Phe are all non-polar residues, hence, likely to be present in the interior of the protein. Arg, Asp, and Lys are polar residues.

      A clinician, during the ultrasound of a pregnant lady found that the lady had multiple fracture of in her fetus. She did not report any trauma. Later a genetic abnormality was identified to cause this due to the non-formation of collagen in the fetus as the mutation replaced glycine to serine in collagen. The name of this syndrome is
      • a)
        Collagenopsis    
      • b)
        Collegeniosis
      • c)
        Osteogenesis imperfecta       
      • d)
        Osteomalacia
      Correct answer is option 'C'. Can you explain this answer?

      Aryan Gupta answered
      Osteogenesis imperfecta is also known as brittle bone disease. Most of the mutations that cause osteogenesis imperfecta type I occur in the COL1A1 gene. This gene encodes major component of type 1 collagen. Therefore in this disease, there is a problem with connective tissue due to lack of type 1 collagen. Glycine is the most abundant amino acid in collagen and this genetic change replaces glycine to serine, thereby reducing the amount of type I collagen produced in the body, which causes bones to be brittle and to fracture easily.

      Which of the following statements is/are true about proteins and amino acids.
      • a)
        The typical length of the membrane spanning region of an integral membrane protein is 20-25 amino acids
      • b)
        Methionine and glutamine are optically inactive
      • c)
        Lysine and leucine are the only two amino acids which are exclusively ketogenic in nature
      • d)
        Tyrosine, valine and histidine are both glucogenic and ketogenic in nature
      Correct answer is option 'A,C'. Can you explain this answer?

      Isha Bose answered
      Statement (a) is correct as usually the membrane spanning region of an integral membrane protein is 20-25 amino acid long.
      Statement (b) is incorrect, because glycine is the only amino acid which is optically inactive as it has no chiral centre. All other amino acids have atleast one chiral centre, so all other amino acids except glycine are optically active.
      Statement (c) is correct as lysine and leucine are the only two amino acids which are exclusively ketogenic in nature.
      Statement (d) in incorrect as there are only five amino acids which are both glucogenic and ketogenic in nature. They are tyrosine, isoleucine, phenylalanine, tryptophan and threonine.     

      Which of the following interactions is shown in the figure below?
      • a)
        Disulfide bond
      • b)
        Ionic interactions
      • c)
        Hydrogen bonds
      • d)
        Hydrophobic interactions
      Correct answer is option 'D'. Can you explain this answer?

      Orion Classes answered
      The interaction shown in the above figure is hydrophobic interactions. Hydrophobic interactions involve interaction between two non-polar hydrophobic residues. It is non-covalent in nature.

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