Amino Acids and Proteins Questions in English

(A) Aspartic acid is an acidic amino acid,which is represented by the one-letter symbol '$D$'.

(D) Amino acids are classified as acidic,basic,or neutral based on the relative number of amino and carboxyl groups in the molecule.
Basic amino acids contain more amino groups than carboxyl groups.
Among the given options,$Histidine$ contains an imidazole ring in its side chain,which has a nitrogen atom capable of accepting a proton. Therefore,it is classified as a basic amino acid.

(B) $Asp$ stands for Aspartic acid,which is an acidic amino acid.
$Arg$ (Arginine) is basic.
$Asn$ (Asparagine) is neutral.
$Ala$ (Alanine) is neutral.

(C) In a polypeptide chain,when $n$ amino acids are linked together,they form $(n-1)$ peptide (amide) bonds.
This is because each peptide bond is formed by the condensation reaction between the carboxyl group of one amino acid and the amino group of the adjacent amino acid.
Therefore,for $n$ amino acids,there are $(n-1)$ such linkages.

(B) Glycine is the simplest amino acid with the formula $NH_2-CH_2-COOH$.
In an aqueous solution,the acidic carboxyl group $(-COOH)$ loses a proton to the basic amino group $(-NH_2)$.
This internal proton transfer results in the formation of a dipolar ion known as a zwitter ion.
The structure of the glycine zwitter ion is $H_3N^{+}-CH_2-COO^{-}$.

(A) Amino acids are classified based on the relative number of amino and carboxyl groups in their molecule.
$Glu$ (Glutamic acid) contains two carboxyl groups and one amino group,making it an acidic amino acid.
$Gly$ (Glycine) is neutral.
$Gln$ (Glutamine) is neutral.
$Arg$ (Arginine) is basic due to the presence of an extra amino group in its side chain.

(A) Basic amino acids contain more than one amino group and fewer carboxyl groups.
$Lysine$,$Arginine$,and $Histidine$ are basic amino acids.
$Proline$ is an imino acid and is neutral in nature.
Therefore,$Proline$ is not a basic amino acid.

(A) Essential amino acids are those that cannot be synthesized by the human body and must be obtained through the diet.
Among the given options,$Valine$ is an essential amino acid.
The list of essential amino acids includes: $1$. $Leucine$,$2$. $Isoleucine$,$3$. $Lysine$,$4$. $Methionine$,$5$. $Phenylalanine$,$6$. $Threonine$,$7$. $Tryptophan$,$8$. $Valine$,and $9$. $Histidine$.
$Tyrosine$,$Proline$,and $Alanine$ are non-essential amino acids,meaning they can be synthesized by the body.

(D) Among the given options,$Cysteine$ is a sulfur-containing amino acid. Its chemical structure is $HS-CH_2-CH(NH_2)-COOH$.

(D) The amino acids are organic compounds containing an amino group $(-NH_2)$ and a carboxyl group $(-COOH)$ attached to the same carbon atom.
Glycine is the simplest amino acid with the structure $NH_2-CH_2-COOH$.
Therefore,it contains both amino and carboxyl groups.

(D) Essential amino acids are those that cannot be synthesized by the human body and must be obtained through the diet.
There are $10$ essential amino acids in total.

(C) Amino acids that are not synthesized in the human body are called essential amino acids and must be obtained through the diet. $Phenylalanine$ is an essential amino acid,whereas $Cysteine$,$Asparagine$,and $Serine$ are non-essential amino acids that can be synthesized by the body.

(A) In a dipeptide,the amino acid with a free carboxyl group $(-COO^-)$ is called the $C$-terminal residue.
In glycyl alanine,the structure is $NH_3^+-CH_2-CO-NH-CH(CH_3)-COO^-$.
The $N$-terminal residue is glycine,and the $C$-terminal residue is alanine.

(B) An $\alpha$-amino acid is classified as basic if it contains more amino groups than carboxyl groups in its structure.
Lysine has the structure $H_2N-(CH_2)_4-CH(NH_2)-COOH$,which contains two amino groups and one carboxyl group,making it a basic $\alpha$-amino acid.

(B) Amino acids are classified as acidic,basic,or neutral based on the relative number of amino $(-NH_2)$ and carboxyl $(-COOH)$ groups in their structure.
Basic amino acids contain more amino groups than carboxyl groups.
Among the given options,$Histidine$ contains an imidazole ring with a nitrogen atom that can accept a proton,making it a basic amino acid.
Therefore,the correct option is $B$.

(C) The general structure of an amino acid is $R-CH(NH_{2})COOH$.
Given that the side chain group $R$ is $-CH_{2}OH$,the amino acid is $HOCH_{2}-CH(NH_{2})COOH$.
This structure corresponds to the amino acid Serine.
Therefore,the correct option is $C$.

(D) The molar masses of the given amino acids are as follows:
$1$. $Alanine$ $(C_3H_7NO_2)$: $89 \ g/mol$
$2$. $Aspartic \ acid$ $(C_4H_7NO_4)$: $133 \ g/mol$
$3$. $Arginine$ $(C_6H_{14}N_4O_2)$: $174 \ g/mol$
$4$. $Glycine$ $(C_2H_5NO_2)$: $75 \ g/mol$
Comparing these values,$Glycine$ has the lowest molar mass. Therefore,the correct option is $D$.

(D) The general formula for an amino acid is $NH_2-CH(R)-COOH$.
The molecular mass depends on the side chain $R$.
For $Glycine$,$R = H$ (molecular formula $C_2H_5NO_2$,molar mass $\approx 75 \ g/mol$).
For $Serine$,$R = CH_2OH$.
For $Aspartic \ acid$,$R = CH_2COOH$.
For $Proline$,it is a cyclic amino acid with a larger structure.
Since $Glycine$ has the simplest side chain $(H)$,it has the lowest molecular mass among the given options.
Therefore,the correct option is $D$.

(A) Hint: Amino acids are classified as acidic,basic,or neutral depending upon the relative number of amino $(-NH_2)$ and carboxyl $(-COOH)$ groups in their molecule.
Step $1$: Definition of basic amino acids. Basic amino acids are those that contain more amino groups than carboxyl groups,which imparts basic character to the molecule.
Step $2$: Analysis of given options.
$A$) Lysine: Contains two amino groups and one carboxyl group,making it basic.
$B$) Glycine: Contains one amino group and one carboxyl group,making it neutral.
$C$) Cystine: Contains two amino groups and two carboxyl groups,making it neutral.
$D$) Cysteine: Contains one amino group,one carboxyl group,and a thiol $(-SH)$ group,making it neutral.
Final Answer: The correct option is $(A)$ Lysine.

(B) Zwitter ion is a dipolar molecule that contains both a positively charged group and a negatively charged group within the same molecule,resulting in a net charge of zero.
Amino acids,such as glycine $(H_2NCH_2COOH)$,contain both a basic amino group $(-NH_2)$ and an acidic carboxylic group $(-COOH)$.
In an aqueous solution,the proton from the $-COOH$ group is transferred to the $-NH_2$ group,forming a dipolar ion known as a Zwitter ion: $H_2NCH_2COOH \rightleftharpoons ^+H_3NCH_2COO^-$.
Among the given options,only $H_2NCH_2COOH$ is an amino acid capable of forming a Zwitter ion.

(A) An amino acid is basic if it contains more amino groups than carboxyl groups in its side chain.
Histidine contains an imidazole ring which acts as a base.
Therefore,Histidine is basic in nature.

(C) Amino acids are classified as acidic,basic,or neutral based on the relative number of amino and carboxyl groups in their structure.
Arginine contains a guanidino group in its side chain,which makes it basic in nature.
Therefore,the correct option is $C$.

(D) zwitter ion is a dipolar ion that contains both a positive and a negative charge,typically formed by amino acids.
Glycine $(NH_2CH_2COOH)$ is the simplest amino acid.
In an aqueous solution,the acidic carboxyl group $(-COOH)$ loses a proton to the basic amino group $(-NH_2)$,resulting in the formation of a zwitter ion:
$NH_3^+CH_2COO^-$

(C) Proteins are classified into two types based on their molecular shape: fibrous proteins and globular proteins.
$1$. Fibrous proteins: These have a thread-like structure and are generally insoluble in water. Examples include keratin (hair,wool) and myosin (muscles).
$2$. Globular proteins: These result from the folding of the polypeptide chain around itself to give a spherical shape. They are usually soluble in water. Examples include insulin,egg albumin,and legumelin.
Since myosin is a fibrous protein,it is not a globular protein.

(A) Myosin is a fibrous protein,not a globular protein. It is primarily involved in muscle contraction and has a long,elongated structure.
The other options are globular proteins:
- $(B)$ Insulin: $A$ hormone involved in regulating blood sugar levels. It is a small globular protein.
- $(C)$ Egg albumin: Also known as albumin,it is a globular protein found in egg whites.
- $(D)$ Legumelin: $A$ globular protein found in legumes,such as peas and beans.
Globular proteins generally have a spherical shape and are water-soluble,whereas fibrous proteins,like myosin,have elongated shapes and are typically insoluble in water.

(B) Fibrous proteins consist of polypeptide chains held together by hydrogen and disulfide bonds to form fiber-like structures. They are generally insoluble in water.
$Myosin$ is a fibrous protein found in muscle tissue.
In contrast,$Legumelin$,$Insulin$,and $Serum \ albumin$ are globular proteins,which are generally soluble in water and have a spherical shape.

(C) The $\alpha$-helix is a common secondary structure of proteins.
In an $\alpha$-helix,the polypeptide chain is coiled into a right-handed screw.
Each turn of the helix contains approximately $3.6$ amino acid residues.
This structure is stabilized by hydrogen bonds between the $C=O$ group of one amino acid and the $N-H$ group of the fourth amino acid along the chain.

(A) Proteins are classified into two types based on their molecular shape: fibrous proteins and globular proteins.
Fibrous proteins have a thread-like structure and are generally insoluble in water,such as keratin and myosin.
Globular proteins have a spherical shape and are generally soluble in water,such as insulin,legumelin,and serum albumin.
Therefore,$Myosin$ is a fibrous protein,not a globular protein.

(D) $Insulin$,$egg \ albumin$ and $serum \ albumin$ are globular proteins,whereas $keratin$ is a fibrous protein.

(A) Glycylalanine is formed by the condensation of two amino acids,glycine and alanine,through a peptide bond.
Since it consists of two amino acid residues,it is classified as a dipeptide.

(D) By convention,the polypeptide chain is written with the $N$-terminal amino acid on the left and the $C$-terminal amino acid on the right.
In the given polypeptide fragment $Ala-Gly-Ser-Tyr-Gly$:
$1$. The amino acid at the $N$-terminal (left end) is $Ala$ (Alanine).
$2$. The amino acid at the $C$-terminal (right end) is $Gly$ (Glycine).
Therefore,the $N$-terminal and $C$-terminal amino acids are $Ala$ and $Gly$ respectively.

(B) $Keratin$ is a type of protein that forms the cells that make up the tissue in nails and other parts of the human body.
$Keratin$ plays an important role in nail health.
It protects nails from damage by making them strong and resilient.

(C) Wool is a natural protein fiber obtained primarily from sheep. While some other animals like goats (e.g.,Cashmere,Mohair) also produce wool,sheep are the primary and most common source.

(A) Globular proteins are spherical in shape and are generally soluble in water. $Insulin$ is a classic example of a globular protein.
Fibrous proteins are thread-like and insoluble in water. $Collagen$,$Fibroin$,and $Myosin$ are examples of fibrous proteins.

(C) Haemoglobin is a conjugated protein consisting of a protein part called globin and a non-protein prosthetic group called haem. The haem group contains an $Fe^{2+}$ ion coordinated to a porphyrin ring.

(A) In Millon's test,the protein is first treated with $HNO_3$ to form nitrated derivatives.
Upon addition of Millon's reagent (a solution of $Hg(NO_3)_2$ and $Hg(NO_3)_2$ in $HNO_3$),a white precipitate is initially formed due to the coagulation of proteins.
On heating,this precipitate turns red,which is a characteristic test for the presence of the phenolic group in the amino acid tyrosine.

(C) When proteins or amino acids are heated with a dilute solution of $Ninhydrin$ ($triketo-hydrindin$ hydrate),a characteristic blue or violet colour is produced.
This reaction is commonly used for the detection of proteins and amino acids.
$\text{Protein} + \text{Ninhydrin} \rightarrow \text{Blue colour}$

(D) The general structure of an $\alpha$-amino acid is $R-CH(NH_2)-COOH$.
In this structure,the $\alpha$-carbon is attached to four different groups ($H$,$NH_2$,$COOH$,and $R$) if $R$ is not a hydrogen atom.
For glycine,the $R$ group is a hydrogen atom $(H)$.
Thus,the structure of glycine is $H_2N-CH_2-COOH$.
Since the $\alpha$-carbon in glycine is attached to two identical hydrogen atoms,it is achiral.
Therefore,glycine is the only $\alpha$-amino acid that does not have a chiral carbon atom.

(A) The secondary structure of proteins refers to the shape in which a long polypeptide chain can exist.
There are two types of secondary structures: the $\alpha-$helix and the $\beta-$pleated sheet.
In the $\beta-$pleated sheet structure,all polypeptide chains are stretched out to nearly maximum extension and then laid side-by-side,which are held together by intermolecular hydrogen bonds.

(B) Zwitter ion is a dipolar ion that contains both a positive and a negative charge within the same molecule,resulting in a net charge of zero.
Amino acids,such as glycine $(NH_2-CH_2-COOH)$,contain both a basic amino group $(-NH_2)$ and an acidic carboxylic group $(-COOH)$.
In an aqueous solution,the proton from the $-COOH$ group is transferred to the $-NH_2$ group,forming the Zwitter ion: $^+NH_3-CH_2-COO^-$.
Therefore,the correct option is $B$.

(D) Proteins are classified into two types based on their molecular shape: fibrous and globular.
Fibrous proteins are long,thread-like structures that are generally insoluble in water.
$Myosin$ is a fibrous protein found in muscles,which makes it insoluble in water.
In contrast,$Albumin$ and $Insulin$ are globular proteins,which are generally soluble in water.
Therefore,the correct option is $D$.

(B)
Lysine is a basic amino acid because it contains two amino groups and one carboxyl group,which results in an isoelectric point $(pI)$ greater than $7$.

(B) Silk is a natural fiber produced by silkworms. The primary protein present in silk is $Fibroin$.
$Fibroin$ is a fibrous protein characterized by a high content of glycine and alanine amino acids,which allows the protein chains to form a stable $\beta$-pleated sheet structure.
Therefore,the correct option is $B$.

(D) The amino acid $Proline$ has a unique structure where the nitrogen atom is part of a five-membered pyrrolidine ring.
In this structure,the nitrogen is bonded to two carbon atoms within the ring,making it a secondary amine (or imino group).
Therefore,$Proline$ is the only proteinogenic amino acid that contains a secondary amino group.

(C) An amino acid is optically active if it contains a chiral carbon atom (a carbon atom bonded to four different groups).
Glycine has the structure $NH_2-CH_2-COOH$.
In glycine,the central carbon atom is bonded to two identical hydrogen atoms,meaning it is not a chiral center.
Therefore,glycine is the only amino acid among the options that is not optically active.

(A) Thyroxine $(T_4)$ is a hormone produced by the thyroid gland.
It is synthesized from the amino acid $Tyrosine$ by the addition of iodine atoms.
Therefore,it is an iodinated derivative of $Tyrosine$.

(B) Mass of $L$-amino acid $= 1.78 \ g$.
The reaction of an amino acid with $NaNO_{2} / HCl$ (Van Slyke method) releases $1 \ mol$ of $N_{2}$ gas per $1 \ mol$ of amino acid.
At $STP$,$1 \ mol$ of gas occupies $22400 \ cm^{3}$.
Moles of $N_{2}$ evolved $= \frac{448 \ cm^{3}}{22400 \ cm^{3} \ mol^{-1}} = 0.02 \ mol$.
Since $1 \ mol$ of amino acid produces $1 \ mol$ of $N_{2}$,moles of amino acid $= 0.02 \ mol$.
Molar mass of amino acid $= \frac{\text{Mass}}{\text{Moles}} = \frac{1.78 \ g}{0.02 \ mol} = 89 \ g \ mol^{-1}$.
The protein contains $0.25 \%$ of this amino acid by mass.
Let the molar mass of the protein be $M$.
$0.25 \% \text{ of } M = 89 \ g \ mol^{-1}$.
$\frac{0.25}{100} \times M = 89$.
$M = \frac{89 \times 100}{0.25} = 35600 \ g \ mol^{-1}$.

(C) Essential amino acids are those that the human body cannot synthesize on its own and must be obtained through the diet.
Out of the $20$ standard amino acids,$9$ are classified as essential.
Isoleucine is one of these $9$ essential amino acids.
Therefore,the correct option is $C$.

(B) In an alkaline medium (high $pH$),the amino acid glycine exists as a negatively charged particle because the carboxyl group $(-COOH)$ loses a proton to form a carboxylate ion $(-COO^-)$.
$H_2N-CH_2-COOH + OH^- \rightarrow H_2N-CH_2-COO^- + H_2O$
Thus,it exists as an $anion$.

(A) Alanine is an amino acid with the structure $CH_3-CH(NH_2)-COOH$.
In an alkaline medium $(pH > 7)$,the carboxylic acid group $(-COOH)$ loses a proton $(H^+)$ to form a carboxylate ion $(-COO^-)$.
Thus,the structure becomes $CH_3-CH(NH_2)-COO^-$,which carries a net negative charge.
Therefore,in an alkaline medium,alanine exists predominantly as an anion.