Amino Acids

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1962
  • Structures of the amino acids
    • Most amino acids contain a carboxyl group, an amino group, and a side- chain(R group), all attached to the alpha-carbon.
      • Exceptions
        • Glycine, (MCQ)
          • does not have a side chain
          • Its alpha-carbon contains two hydrogens.
        • Proline
          • nitrogen is part of a ring, is an imino acid.(MCQ)
    • All of the 20 amino acids, except glycine, are of the L configuration. (MCQ)
      • Because glycine does not contain an asymmetric carbon atom, it is not optically active, and thus, it is neither D nor L.
    • The classification of amino acids
      • Hydrophobic amino acids
        • have side chains that contain
          • aliphaticgroups (valine, leucine, and isoleucine) (MCQ)
          • aromatic groups(phenylalanine, tyrosine, and tryptophan) that can form hydrophobic interactions.(MCQ)
            • TyrosinehasaphenolicgroupthatcarriesanegativechargeaboveitspKa(=10.5),soitis not hydrophobic in this pH range.
      • Hydroxyl groupsfound on serine and threonine can form hydrogen bonds(MCQ)
      • Sulfuris present in cysteine and methionine.(MCQ)
        • The oxidation of the sulfhydryl groups of two cysteines can form a disulfide bond, producing cystine.
      • Ionizable groupsare present on the side chains of seven amino acids.
        • They can carry a charge, depending on the pH.
        • When charged, they can form electrostatic interactions.
      • Amides are present on the side chains of asparagine and glutamine.(MCQ)
      • Thesidechainofprolineformsaringwiththenitrogenattachedtothealpha-carbon.(MCQ)
  • Charges on amino acids
  • Chargesonalpha-aminoandalpha-carboxylgroupsat physiologic pH
    • the alpha-amino group is protonated (pKa=9) ,carries a positive charge
    • carboxyl group is dissociated (pKa=2) and carries a negative charge.
  • Chargesonsidechains
    • Positive chargesare present on the side chains of the basic amino acids arginine, lysine,and histidineat pH 7.(MCQ)
    • Negative chargesare present on the side chains of the acidic amino acids aspartate andglutamateat pH 7.(MCQ)
  • The isoelectric point (pI) is the pH at which the number of positive charges equals the number of negative charges such that the molecule has no net charge.

 

Amino Acids

@ The protein albumin is a major buffer of the pH in the blood, which is normally kept between 7.2 and 7.4. Which of the following is an amino acid side chain of albumin that participates in this buffering range?

(A) Histidine

(B) Aspartate

(C) Glutamate

(D) Lysine

(E) Arginine

10 The answer is A:

Histidine. Of the amino acid choices listed only histidine has a side chain which could conceivably buffer in the range of 7.2 to 7.4. The imidazole group of histidine has a pKa of 6.0, but this can be altered by the local environment of the protein. Aspartic acid and glutamic acid have side chain carboxylic acids, each of which has a pKa about 4.0 and would not be able to contribute to buffering at neutral pH. Both lysine and arginine have basic side chains, with pKa values about 9.5, and those too will not be able to buffer near neutral pH.

@ HbA1c differs from unmodifi ed hemoglobin by which one of the following?

(A)         Amino acid sequence

(B)         Serine acylation

(C)         Valine glycosylation

(D) Intracellular location

(E)         Rate of degradation

19 The answer is C:

Valine glycosylation. HbA1c is glycosylated hemoglobin, refl ecting the level of blood glucose over the lifetime of the erythrocyte (120 days). The higher the concentration of HbA1c, the more poorly controlled blood glucose levels are (normal is about 5.5% HbA1c). The glycosylation primarily occurs on the N-terminal valine residues of the β chains (which contain a free amino group). The amino acid sequences of hemoglobin and HbA1c are the same, there is no fatty acid addition (acylation) to the hemoglobin, the red cell contains no intracellular organelles for compartmentation to be an issue, and the rate of degradation of nonmodified hemoglobin and HbA1c are the same.

@ A thin, anxious woman, who is 5 6 tall, weighs 92 lb. Blood work indicates a glucose level of 70 mg/dL under fasting conditions. Her liver is using which of the following as precursors for glucose production under these conditions?

(A)         Glycerol, lactate, and leucine

(B)         Fatty acids, alanine, and glutamine

(C)         Glycerol, lactate, and glutamine

(D)         Glycerol, fatty acids, and glutamine

(E)         Lactate, heme, and lysine

8 The answer is C:

Glycerol, lactate, and glutamine. The woman has a BMI in the unhealthy range (15.7), indicating inadequate nutrient uptake. Since her nutrient uptake is poor, most of the glucose present in her blood is derived from gluconeogenesis, as her glycogen stores are most likely depleted. The substrates for gluconeogenesis are lactate (derived from red blood cell metabolism), glycerol (from triglyceride degradation), and amino acids derived from protein muscle degradation.

  • Glutamine is a glucogenic amino acid, and is also used to transport nitrogen groups from the muscle to the liver for safe excretion via the urea cycle.
  • Leucine is a strictly ketogenic amino acid (giving rise only to acetyl-CoA), and leucine carbons cannot be used to make glucose via gluconeogenesis.

Fatty acids are also strictly ketogenic, and cannot be used for glucose production (fatty acids give rise to acetyl-CoA, which cannot be used to produce net glucose). Lysine is also a strictly ketogenic amino acid, and cannot be used for glucose production. Heme degradation gives rise to bilirubin, which cannot be further degraded, and none of the carbons of heme can be used for glucose production.

23.          All of the following amino acids have a monoamino monocarboxylic group EXCEPT:

A.            Glycine

B.            Valine

C.            Methionine

D.            Phenylalanine

ANS:      23.          D. Glycine (simple amino acid), valine (branched chain amino acid) methionine (sulfur containing amino acid) have mono amino mono carboxylic group but phenyl alanine is aromatic amino acid with benzene group.

1

The figure shown above is:

A.            Phenylalanine

B.            Arginine

C.            Tyrosine

D.            Tryptophan

ANS:      24.          C. Structure shown in the question is that of the amino acid tyrosine. Hydroxyl group of tyrosine participate in hydrogen bonding of protein structure and the side chain is hydrophilic in nature.

25.          The following compounds are derived from tyrosine EXCEPT:

A.            Dopaquinone

B.            Serotonin

C.            Thyroxin

D.            Norepinephrine

ANS:      25.          B. Tyrosine is a non essential amino acid, which is synthesized from phenylalanine. Phenylalanine is an essential amino acid; hydroxylation of phenylalanine by phenylalanine hydroxylase produces tyrosine. Tyrosine is the precursor for the synthesis of dopaquinone. It is an intermediate compound in the synthesis of melanin pigments. Thyroxine and catecholamines (Dopamine, norepinephrine and epinephrine) are synthesised from tyrosine. Serotonin is synthesised from tryptophan.

26.          Example for purely ketogenic amino acid is:

A.            Alanine

B.            Phenylalanine

C.            Leucine

D.            Tryptophan

ANS:      26.          C. The carbon atoms of leucine, purely a ketogenic amino acid. It can not be used to form glucose. Alanine, phenyl alanine, tryptophan can contribute to glucose formation because they are partially glucogenic and partially ketogenic.

27.          Which amino acid does not have any methyl or methylene groups in its side chain:

A.            Valine

B.            Leucine

C.            Isoleucine

D.            Glycine

ANS:      27.          D. Valine, leucine, and isoleucine are all branched chain amino acids. Valine has three methylene groups, while leucine and isoleucine have four. Glycine is the simplest of amino acids, contain only single hydrogen (polar nature) as a side chain.

28.          All of the following are essential amino acids EXCEPT:

A.            Histidine

B.            Isoleucine

C.            Arginine

D.            Serine

ANS:      28.          D. The amino acids which are necessary for the tissue synthesis and growth are said to be essential amino acids. Out of twenty amino acids ten amino acids are essential (Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenyl alanine, Threonine, Tryptophan), they are not synthesized in the body and must be supplied in the diet.

29.          Guanidino group containing amino acid is:

A.            Lysine

B.            Arginine

C.            Histidine

D.            Glutamic acid

ANS:      29.B. Arginine contains guanidine group, lysine contains £-amino acid, histidine contains imidazole ring. Glutamic acid contains y-carboxyl group.

30.          Which amino acid is not optically active:

A.            Glycine

B.            Cysteine

C.            Lysine

D.            Serine

ANS:      30.A. Amino acids having an asymmetric carbon (four different groups around the a-carbon) exhibit optical activity. Two mirror forms of each amino acid are thus possible; L- isomers and D-isomers. Only L-isomers are utilized in proteins. Since glycine has two identical groups on the a-carbon. It is not optically active.

32.          At neutral a pH, the amino acid in a solution exists as:

A.            Dipolar ions

B.            Non-polar molecules

C.            Polar molecules

D.            Positive and monovalent

ANS:      32.A. Most of the amino acids in solution exists as dipolar ions at neutral pH, owing to the pK values of the amino and carboxyl groups so called zwitter ions. The amino group is protonated (NH3+) and the carboxyl group is ionized (COO’). The pH at which amino acid carboxyl groups are half dissociated (pK values) is about pH 9 and pH 2 respectively. Thus, at neutral pH most amino acids contain both the positive and negative charges. Molecules that act as both proton donors and proton acceptors are called ampholytes.

33.          The buffering capacity at physiologic pH is best exhibited by which one of the following amino acid:

A.            Lysine

B.            Histidine

C.            Asparticadd

D.            Valine

ANS:      33.          B. Buffering capacity is dependent upon the presence of amino acids having ionizable side chains with pK’s near physiological pH. The pK value of imidazole group of histidine is 6.1 is close to neutrality. Valine has uncharged, branched chains. Lysine has a 33.    B. Buffering capacity is dependent upon the presence of amino acids having ionizable side chains with pK’s near physiological pH. The pK value of imidazole group of histidine is 6.1 is close to neutrality. Valine has uncharged, branched chains. Lysine has a

34.          All the statements regarding iso-electric pH is correct EXCEPT:

A.            The molecule carries no net charge

B.            Solubility and buffering capacity will be minimum

C.            Some amino acids contain only positive charges

D.            There is no mobility in an electric field

ANS: C  At neutral pH, amino acids in solution exists as zwitter ions containing both arotonated amino group and a dissociated carboxyl group. The charged groups of amino acids are shown below in relation to acid, neutral and basic pH.

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In acidic solution amino acids behave as anions and in alkaline solution they behave as anions. Since some amino acids do have polar side chains (R groups), these amino acids will also contain other charges in addition to the charged carboxyl and amino groups that are common to all amino acids.

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35.          alpha-COOH pK1 and alpha NH3 pK2values for glutamic acid are 2.2,4.3 . it follows that the isoelectric point is:

A.            3.2

B.            6.1

C.            6.0

D.            5.9

ANS:A

                The isoelectric point of an amino acid is that pH at which it has no net charge and does not move in electric field. pK values denote the pH at which a given a-COOH, a-NH3+ or R group is dissociated. It is possible to calculate the pi, for an amino acid with only two dissociating groups (uncharged side groups), pi is half way from point between the alpha-COOH pK1 and alpha NH3 pK2

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36.          If ammonia combines with the -COOH group of dicarboxylic amino acid, which compound will be synthesized:

A.            Aspargine

B.            Glutamate

C.            Histamine

D.            Tyramine

ANS:

5

The amide group of glutamine serves as the source of nitrogen for nucleic acid synthesis.

37.          All the following statements are correct regarding peptide bond EXCEPT:

A.            It is rigid and planar

B.            It has no rotation freedom

C.            It is a bond between amino groups

D.            It has partial double bond character

ANS:      37.          C. Peptide bond is formed between the carboxylic group of one amino acid residue and the amino group of the adjacent amino acid residue. In peptide bond formation one water molecule is eliminated. It is rigid and polar, having no rotational freedom. Both carboxyl oxygen and the amino hydrogen are always in a trans position. Because of this, adjacent bonds to a-carbon of the amino acid residues on either side of the peptide bond have a large degree of freedom of rotation.

38.          Which one of the following is known as Sanger’s reagent:

A.            1-Fluoro 2, 4-dinitro benzene

B.            l-Dimethyl-aminonapthelene-5-sulphonyl chloride

C.            Phenyl isothiocynate

D.            Fluoro thiocyanate

 ANS:     38.          A. Sanger’s reagent is 1- fluoro 2, 4-dinitro benzene (FDNB). It is used to study the sequence of amino acids in protein molecule. FDNB binds with N-terminal amino acid to form a DNP. 1- Dimethyl amino naphthalene -5- sulphonyl chloride (Dansyl chloride) combined with N-terminal group amino acid form a fluorescent dansyl derivative. Phenyl isothiocynate reacts with -NH2 group and forms phenyl thiocarbamyl derivative, which is cleaved off the peptide. On treatment with mild acid, it cyclizes to form phenyl thiohydantoin derivative, chromatography technique is used to identify that derivative.

 

 

 

 

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