Amino Acids and Proteins Questions in English

(D) protein in its native state possesses a unique three-dimensional structure and biological activity. When subjected to physical changes (e.g.,temperature) or chemical changes (e.g.,$pH$),the hydrogen bonds are disrupted,causing the protein to lose its biological activity; this process is known as denaturation.
During denaturation,the secondary and tertiary structures of the protein are destroyed,but the primary structure (the sequence of amino acids) remains intact.
Therefore,the statement that denaturation destroys all $1^{\circ}, 2^{\circ}$ and $3^{\circ}$ structures is false.
Curdling of milk is a classic example of protein denaturation caused by the formation of lactic acid by bacteria.
Thus,$(A)$ is false but $(R)$ is true.

(A) Peptides containing more than $10$ amino acids are called polypeptides.
Polypeptides are formed by the linear sequence of amino acids.
Some proteins are composed of two or more polypeptide chains.
Relatively shorter peptides are known as oligopeptides,whereas longer polymers are called polypeptides.
Polypeptides containing more than $100$ amino acids and having a molecular mass higher than $10,000 \ u$ are generally called proteins.
However,the distinction between a polypeptide and a protein is not sharp.

(C) The given dipeptide is an artificial sweetening agent known as $Aspartame$.
It is the methyl ester of a dipeptide derivative,formed from $L-aspartic$ acid and $L-phenylalanine$.
It is approximately $150-200$ times sweeter than sucrose,but it is unstable at cooking temperatures.
Therefore,it is used as a sweetening agent in cold foods and soft drinks.
Hence,the correct answer is option $(C)$.

(B) Aspartame is the methyl ester of the dipeptide formed from aspartic acid and phenylalanine. Its chemical structure is $H_2N-CH(CH_2COOH)-CONH-CH(CH_2C_6H_5)-COOCH_3$.
By analyzing the structure,we can identify the following functional groups:
$1$. Carboxylic acid group $(-COOH)$
$2$. Amine group $(-NH_2)$
$3$. Amide linkage $(-CONH-)$
$4$. Ester group $(-COOCH_3)$
Thus,the correct set of functional groups is $-COOH, -NH_2, -CONH-, -COOCH_3$.

(D) Zwitter ion is a dipolar ion that contains both a positively charged group and a negatively charged group within the same molecule,resulting in a net neutral charge.
In amino acids,the amino group $(-NH_2)$ acts as a base and accepts a proton to become $-NH_3^+$,while the carboxylic acid group $(-COOH)$ acts as an acid and donates a proton to become $-COO^-$.
Thus,the structure $R-CH(NH_3^+)-COO^-$ represents the Zwitter ion form of an amino acid.

(D) tetrapeptide is formed by the condensation of $4$ amino acid units.
In a linear polypeptide chain,the number of peptide bonds is always one less than the number of amino acid units.
Therefore,for a tetrapeptide,the number of amino acids = $4$ and the number of peptide bonds = $4 - 1 = 3$.

(C) dipeptide is formed by the condensation of two amino acids,where the carboxyl group $(-COOH)$ of one amino acid reacts with the amino group $(-NH_2)$ of another to form a peptide bond $(-CO-NH-)$.
For the dipeptide gly-ala (Glycine-Alanine):
$1$. Glycine $(NH_2-CH_2-COOH)$ is the $N$-terminal amino acid.
$2$. Alanine $(NH_2-CH(CH_3)-COOH)$ is the $C$-terminal amino acid.
$3$. The peptide bond is formed between the $-COOH$ of Glycine and the $-NH_2$ of Alanine.
$4$. The resulting structure is $NH_2-CH_2-CO-NH-CH(CH_3)-COOH$.

(A) The behavior of amino acids in an electric field depends on their net charge at a given $pH$ relative to their isoelectric point $(pI)$.
$1$. For glutamic acid $(pI = 3.2)$,at $pH = 6.0$ $(pH > pI)$,it exists as an anion and migrates to the anode.
$2$. For arginine $(pI = 10.7)$,at $pH = 6.0$ $(pH < pI)$,it exists as a cation and migrates to the cathode.
$3$. For alanine $(pI = 6.0)$,at $pH = 6.0$ $(pH = pI)$,it exists as a zwitterion (dipolar ion) with a net charge of zero and does not migrate to either electrode,remaining uniformly distributed.

(D) When two different amino acids,glycine $(G)$ and alanine $(A)$,react to form dipeptides,they can combine in different sequences to form different products.
The possible combinations are:
$1.$ Glycine + Glycine $\rightarrow$ Glycylglycine $(G-G)$
$2.$ Alanine + Alanine $\rightarrow$ Alanylalanine $(A-A)$
$3.$ Glycine + Alanine $\rightarrow$ Glycylalanine $(G-A)$
$4.$ Alanine + Glycine $\rightarrow$ Alanylglycine $(A-G)$
Since the question asks for dipeptides obtained from glycine and alanine,it implies all combinations involving these two amino acids (including self-condensation). Thus,all four listed dipeptides can be formed.

(D) Alanine is a chiral amino acid with one chiral center. $A$ racemic mixture contains both $(R)$ and $(S)$ enantiomers.
When two alanine molecules combine to form a linear dipeptide,the resulting molecule has two chiral centers (one from each alanine residue).
Each chiral center can exist in either $(R)$ or $(S)$ configuration.
The possible combinations for the two chiral centers in the dipeptide are $(R, R)$,$(R, S)$,$(S, R)$,and $(S, S)$.
Since all these combinations represent distinct stereoisomers,there are $2^2 = 4$ possible isomeric linear dipeptides.
Therefore,the total number of isomeric linear dipeptides is $4$,and the correct option is $(d)$.

(A) Alanine is a neutral amino acid with an isoelectric point $(pI)$ of approximately $6.0$.
In an acidic medium ($pH$ $2-4$),which is below the $pI$,the amino group becomes protonated,and the molecule exists as a cation: $H_{3}N^{+}CH(CH_{3})COOH$ (Structure $I$).
In a basic medium ($pH$ $9-11$),which is above the $pI$,the carboxyl group becomes deprotonated,and the molecule exists as an anion: $H_{2}NCH(CH_{3})COO^{-}$ (Structure $II$).
Therefore,the correct pair is $I$ and $II$.

(A) tripeptide is formed by the combination of $3$ amino acids.
Since we have $3$ distinct amino acids ($ala$,$gly$,$val$) and each is used exactly once,the number of possible arrangements is given by the permutation of $3$ items taken $3$ at a time.
Number of arrangements = $3! = 3 \times 2 \times 1 = 6$.
The possible tripeptides are:
$1. Gly-ala-val$
$2. Gly-val-ala$
$3. Val-gly-ala$
$4. Val-ala-gly$
$5. Ala-val-gly$
$6. Ala-gly-val$
Total tripeptides = $6$.

(C) By analyzing the structure of the pentapeptide,we can identify the individual amino acids by breaking the peptide bonds through hydrolysis. The amino acids present are:
$1$. $N$-terminal: $\text{Threonine} (\text{Thr})$
$2$. Second: $\text{Serine} (\text{Ser})$
$3$. Third: $\text{Aspartic acid} (\text{Asp})$
$4$. Fourth: $\text{Glycine} (\text{Gly})$
$5$. $C$-terminal: $\text{Alanine} (\text{Ala})$
Thus,the sequence is $\text{Thr} - \text{Ser} - \text{Asp} - \text{Gly} - \text{Ala}$.
Among these,$\text{Threonine}$ is an essential amino acid. Therefore,$(Y) = \text{Threonine}$ and the sequence is $\text{Thr} - \text{Ser} - \text{Asp} - \text{Gly} - \text{Ala}$.

(C) . Enzymes lower the activation energy of a reaction,making the hydrolysis of sucrose faster compared to acid catalysis. This statement is correct.
$B$. Denaturation disrupts the hydrogen bonds and other interactions that maintain the secondary and tertiary structures of a protein,but the primary structure (amino acid sequence) remains intact. This statement is correct.
$C$. Nucleotides are joined by phosphodiester linkages between the $C_{3}$ and $C_{5}$ carbons of the pentose sugar,not glycosidic linkages. This statement is incorrect.
$D$. Quaternary structure refers to the arrangement of multiple polypeptide subunits in a protein complex,not just the folding of a single chain. This statement is incorrect.
Therefore,only statements $A$ and $B$ are correct.

(C) The primary structure of a protein is defined by the specific linear sequence of amino acids in the polypeptide chain held together by peptide bonds. This sequence determines the further folding of the protein into higher-level structures (secondary,tertiary,quaternary).

(A) . Glutamine: Contains an amide group $(-CONH_2)$ in the side chain,which can undergo Hoffman bromamide degradation $(IV)$.
$B$. Lysine: Contains a primary amino group $(-NH_2)$ in the side chain,which reacts with benzenesulfonyl chloride in Hinsberg's test $(I)$.
$C$. Tyrosine: Contains a phenolic group ($-OH$ attached to a benzene ring),which gives a positive violet color with neutral $FeCl_3$ solution $(II)$.
$D$. Serine: Contains a primary alcohol group $(-CH_2OH)$ in the side chain,which gives a positive red color with Ceric ammonium nitrate test $(III)$.
Therefore,the correct matching is: $A-IV, B-I, C-II, D-III$.

(C) Amino acids containing a phenolic group give a characteristic violet color with neutral ferric chloride solution.
Tyrosine contains a phenolic side chain ($p-hydroxybenzyl$ group).
Therefore,Tyrosine reacts with neutral ferric chloride to form a violet-coloured complex.

(C) The tertiary structure of a protein refers to the three-dimensional folding of the polypeptide chain.
This structure is sensitive to environmental changes like $pH$,temperature,or solvent polarity,which can lead to denaturation.
Statement $(C)$ claims that the structure remains intact when exposed to $pH$ changes,which is incorrect.
Changes in $pH$ alter the ionization states of side chains,disrupting electrostatic interactions and hydrogen bonds that stabilize the tertiary structure.