Amino Acids And Proteins MCQ MCAT Biochemistry MCQs & solutions - Free

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.  

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.

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.

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.

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

Following figure represent the location of psi and phi angle:

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 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)

α helix is disrupted by the presence of glycine or proline in the chain or successive presence of amino acids with same charge. Glycine, with no side chain, is a helix-breaker because rotation around both F and Y is so unconstrained: recall the large area allowed to glycine on the Ramachandran diagram. Proline also is a helix-breaker because it has no backbone NH to hydrogen bond. Hence b is correct which doesn’t consist of glycine and proline.

All of these molecules are amino acids
1. Gamma-aminobutyric acid (GABA) is a non-essential amino acid formed from glutamic acid.
2. Glutamate refers to the anion of glutamic acid in its role as a neurotransmitter.
3. Citrulline is produced as a byproduct of the enzymatic production of nitric oxide from the amino acid arginine, catalyzed by nitric oxide synthase

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.

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.

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

Only right handed alpha helix and 3₁₀ helix lie in the same quadrant

Ramachandran Plot

The Ehlers-Danlos syndromes (EDS) are a group of related disorders caused by different genetic defects in collagen.Patients also have excessively flexible, loose joints. Ehler danlos syndrome is not lethal and individuals are still present so it is not eradicated from the world.

(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.

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.     

Isoelectric point in case of no ionizable side chain is the average of pKa and pKb, (1.8+9.5)/2 = 5.65

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 pK_b and pK_R and is equal to the average of pK_b and pK_R(10.54+9.06)/2 = 9.77 = ~ 9.8.
In case of an acidic amino acid, pI lies in between the pK_a and pK_R and is equal to the average of pK_a and pK_R.
In case of a neutral amino acid, pI lies in between the pK_b and pK_a and is equal to the average of pK_a and pK_b.

All naturally occurring proteins from all living organisms consist of L amino acids.

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.