Amino Acids and Proteins Questions in English

(D) Amino acids are organic compounds that contain both an amino group $(-NH_2)$ and a carboxylic acid group $(-COOH)$.
Glycine,Alanine,and Histidine are standard $\alpha$-amino acids found in proteins.
Guanine is a nitrogenous base (purine) found in nucleic acids ($DNA$ and $RNA$),not an amino acid.

(A) zwitterion is a molecule that contains both a positive and a negative charge,resulting in a net charge of zero.
Amino acids,which contain both an acidic carboxyl group $(-COOH)$ and a basic amino group $(-NH_2)$,exist as zwitterions in aqueous solution.
Among the given options,$C_6H_5CH_2CH(NH_2)COOH$ (Phenylalanine),$(CH_3)_2CHCH(NH_2)COOH$ (Valine),and $HOOCCH_2CH_2CH(NH_2)COOH$ (Glutamic acid) are amino acids.
However,in standard multiple-choice contexts for this specific question,the structure representing a simple $\alpha$-amino acid is typically the intended answer.
Given the options provided in the prompt were structurally incorrect or incomplete,the standard amino acid $C_6H_5CH_2CH(NH_2)COOH$ is the correct representation of a zwitterionic species.

(A) The peptide bond $(C-N)$ in proteins exhibits partial double bond character due to resonance.
This resonance leads to a shorter $C-N$ bond length compared to a standard $C-N$ single bond.
Therefore,the statement that the $C-N$ bond length in proteins is greater than the normal $C-N$ bond length is incorrect.

(D) Most naturally occurring amino acids are primary amines $(R-CH(NH_2)-COOH)$.
$Proline$ is unique because its amino group is part of a five-membered pyrrolidine ring.
This structure makes the nitrogen atom a secondary amine,as it is bonded to two carbon atoms within the ring structure.

(A) Glycine $(NH_2-CH_2-COOH)$ is the simplest amino acid.
In glycine,the $\alpha$-carbon atom is bonded to two identical hydrogen atoms.
Because it lacks a chiral center,it does not exhibit optical isomerism.
All other naturally occurring amino acids have at least one chiral center and are optically active.

(D) The isoelectric point $(pI)$ is the $pH$ at which an amino acid carries no net electrical charge.
At this $pH$,the amino acid exists as a dipolar ion,commonly known as a zwitterion.
In a zwitterion,the carboxyl group loses a proton to become a carboxylate ion $(COO^{-})$ and the amino group gains a proton to become an ammonium ion $(NH_3^{+})$.
Therefore,the structure is $H_3N^{+} - CHR - COO^{-}$.

(B) The isoelectric point $(pI)$ is the $pH$ at which an amino acid exists as a zwitterion,having no net charge.
At this point,the electrostatic repulsion between molecules is minimized,and the intermolecular forces are dominated by attractive forces.
Consequently,the solubility of amino acids is at its minimum at the isoelectric point because the molecules tend to aggregate and precipitate out of the solution.

(C) Proteins are polymers of $\alpha$-amino acids. These amino acids are connected to each other by a covalent bond known as a peptide bond or peptide linkage. This linkage is formed by the condensation reaction between the amino group $(-NH_2)$ of one amino acid and the carboxyl group $(-COOH)$ of another amino acid.

(D) Proteins are primarily structural components of the body,such as in nails,skin,and muscles. While they can be broken down to provide energy during extreme starvation,their primary biological function is not to serve as a source of energy for metabolism. Carbohydrates and fats are the primary sources of energy for metabolism.

(C) Insulin is a peptide hormone produced by the beta cells of the pancreatic islets. It is stored in the pancreas in the form of a hexameric complex with $Zn^{2+}$ ions. Therefore,zinc is the metal that forms a complex with insulin.

(D) The $\alpha$-helix is one of the most common secondary structures of proteins.
It is formed by the regular coiling of the polypeptide chain,which is stabilized by intramolecular hydrogen bonding between the $C=O$ group of one amino acid residue and the $N-H$ group of the fourth amino acid residue along the chain.
Therefore,the $\alpha$-helix structure is characteristic of proteins.

(B) Insulin is a peptide hormone produced by the beta cells of the pancreatic islets. It consists of $51$ amino acids arranged in two polypeptide chains,referred to as chain $A$ and chain $B$,which are linked together by disulfide bridges. Therefore,it is classified as a peptide hormone.

(C) The process of protein synthesis based on genetic information stored in $mRNA$ is known as $Translation$.
In this process,the sequence of nucleotides in $mRNA$ is translated into a sequence of amino acids to form a polypeptide chain.

(C) Protein fibers are natural fibers that consist largely of proteins. Silk is a natural protein fiber produced by silkworms. Cotton is a cellulose fiber,Rayon is a regenerated cellulose fiber,and Polyester is a synthetic polymer fiber. Therefore,the correct answer is $Silk$.

(A) $Nylon-2-nylon-6$ is a polyamide copolymer formed by the condensation polymerization of two monomers:
$1$. Glycine $(H_2N-CH_2-COOH)$
$2$. Aminocaproic acid $(H_2N-(CH_2)_5-COOH)$
Since glycine is the only amino acid among the options that acts as a monomer for this polymer,the correct answer is $Glycine$.

(A) In $L$-amino acids,the $-NH_2$ group is located on the left side of the chiral carbon in the Fischer projection when the $-COOH$ group is placed at the top.
Serine is $HOCH_2-CH(NH_2)-COOH$.
In option $A$,the $-NH_2$ group is on the left side,which corresponds to the $L$-configuration of serine.

(D) Haemoglobin contains iron as the central metal ion in the heme group. Cytochromes are iron-containing hemoproteins that act as electron carriers. Therefore,both $Haemoglobin$ and $cytochromes$ contain iron.

(C) Lysine is a basic amino acid with two amino groups and one carboxyl group.
Its isoelectric point $(pI)$ is approximately $9.74$.
An amino acid is least soluble in water at its isoelectric point because the net charge on the molecule is zero,leading to minimum electrostatic repulsion between molecules.
However,in the context of standard textbook questions regarding lysine,the range $9-10$ is often cited as the region where it approaches its isoelectric state.

(B) In aqueous solution,$\alpha$-amino acids exist as zwitterions $(R-CH(NH_3^+)-COO^-)$.
The $-NH_3^+$ group acts as an acid (proton donor) and the $-COO^-$ group acts as a base (proton acceptor).

(D) . Loss of both secondary and tertiary structures
Denaturation of proteins involves the disruption and possible destruction of both the secondary and tertiary structures.
Since denaturation reactions are not strong enough to break the peptide bonds,the primary structure remains intact after a denaturation process.

(C) Denaturation is a process where proteins or nucleic acids lose their quaternary,tertiary,and secondary structures due to external stress such as heat,pH changes,or chemicals.
Statement $A$ is correct: Denaturation disrupts the hydrogen bonds and other interactions,leading to the loss of secondary and tertiary structures.
Statement $B$ is correct: In $DNA$,denaturation refers to the separation of the double-stranded helix into two single strands.
Statement $C$ is incorrect: The primary structure of proteins (the sequence of amino acids) remains intact during denaturation.
Therefore,statements $A$ and $B$ are correct.

(C) Except for glycine,all other naturally occurring amino acids contain a chiral center at the $\alpha$-carbon atom.
Glycine has the structure $H_2N-CH_2-COOH$,where the $\alpha$-carbon is bonded to two identical hydrogen atoms,making it achiral.
Therefore,all amino acids except glycine are optically active.

(B) Proteins are polymers of $\alpha$-amino acids linked by peptide bonds. On hydrolysis,they break down into their constituent amino acids.
Amino acids are bifunctional molecules containing both an amino group $(-NH_2)$ and a carboxylic acid group $(-COOH)$.
While both statements are scientifically correct,the presence of these functional groups in amino acids is not the direct reason why proteins produce amino acids upon hydrolysis; the hydrolysis is due to the cleavage of peptide bonds. Therefore,the Reason is not the correct explanation of the Assertion.

(C) The assertion is correct because haemoglobin acts as a respiratory pigment that transports oxygen in the blood.
The reason is incorrect because oxygen binds to the $Fe^{2+}$ ion of the haem group in its molecular form,$O_2$,not as the superoxide ion,$O_2^-$.

(C) Carboxypeptidase is an exopeptidase that specifically cleaves the peptide bond at the $C$-terminal end of a polypeptide chain.
The Reason states that it cleaves the $N$-terminal bond,which is incorrect; aminopeptidases cleave the $N$-terminal bond.
Therefore,the Assertion is correct,but the Reason is incorrect.

(A) $\alpha$-amino acids contain both an acidic carboxyl group $(-COOH)$ and a basic amino group $(-NH_2)$.
In aqueous solution,the carboxyl group loses a proton $(H^{+})$ and the amino group accepts it,forming a dipolar ion known as a zwitterion or internal salt: $NH_2-CHR-COOH \rightleftharpoons NH_3^+-CHR-COO^-$.
This occurs because the acidic and basic groups are in close proximity.
Therefore,both Assertion and Reason are correct,and the Reason is the correct explanation.

(B) Denaturation refers to the process where the natural structure of a protein (secondary,tertiary,or quaternary) is disrupted due to physical or chemical changes,such as temperature or $pH$ changes. This leads to the loss of biological activity.
Cooking an egg involves the coagulation of albumin protein,which is a classic example of denaturation.
Both the Assertion and the Reason are correct statements. However,the Reason describes a specific instance of denaturation and does not explain the general definition provided in the Assertion. Therefore,the Reason is not the correct explanation of the Assertion.

(B) The Assertion is correct because proteins are polymers of $\alpha-$ amino acids linked by peptide bonds.
The Reason is also correct because denaturation involves the unfolding of the protein's secondary and tertiary structures due to physical or chemical changes,while the primary structure remains intact.
However,the Reason does not explain why proteins are made of $\alpha-$ amino acids.
Therefore,both statements are correct,but the Reason is not the correct explanation of the Assertion.

(A) At the isoelectric point $(pH = pI)$,an amino acid exists as a zwitterion,which carries both a positive and a negative charge,resulting in a net charge of zero. Because the molecule is electrically neutral,it does not migrate towards either the cathode or the anode when an electric field is applied. Therefore,the Reason correctly explains the Assertion.

(C) Non-essential amino acids are those that can be synthesized by the human body.
Among the given options,$alanine$ is a non-essential amino acid.
$Valine$,$leucine$,and $lysine$ are essential amino acids that must be obtained through the diet.

(B) During the process of denaturation,the hydrogen bonds and other interactions that stabilize the $2^o$ and $3^o$ structures are disrupted,leading to their loss.
However,the peptide bonds that hold the amino acids together in the $1^o$ structure remain intact.

(C) Aspartame is a dipeptide methyl ester. Its structure contains:
$1$. One carboxylic acid group carbon $(C=O)$: $1$ $sp^2$ carbon.
$2$. One amide group carbon $(C=O)$: $1$ $sp^2$ carbon.
$3$. One ester group carbon $(C=O)$: $1$ $sp^2$ carbon.
$4$. One phenyl ring $(C_6H_5)$: $6$ $sp^2$ carbons.
Total number of $sp^2$ hybridised carbons = $1 + 1 + 1 + 6 = 9$.

(N/A) Both acidic (carboxyl) as well as basic (amino) groups are present in the same molecule of amino acids.
In aqueous solutions,the carboxyl group can lose a proton and the amino group can accept a proton,thus giving rise to a dipolar ion known as a $zwitter$ ion.
$R-CH(NH_2)-COOH \leftrightarrow R-CH(NH_3^+)-COO^-$ ($Zwitter$ ion)
Due to this dipolar behavior,they have strong electrostatic interactions within them and with water.
However,halo-acids do not exhibit such dipolar behavior.
For this reason,the melting points and the solubility of amino acids in water are higher than those of the corresponding halo-acids.

(N/A) When an egg is boiled,the proteins present inside the egg undergo denaturation and coagulation. After boiling,the water present in the egg is absorbed by the coagulated protein network through $H$-bonding.

(N/A) Essential amino acids are those that cannot be synthesized by the human body and must be obtained through the diet. Examples include $Valine$ and $Leucine$.
Non-essential amino acids are those that can be synthesized by the human body and do not necessarily need to be obtained through the diet. Examples include $Glycine$ and $Alanine$.

(N/A) $(i)$ Peptide linkage: The amide bond formed between the $-COOH$ group of one amino acid molecule and the $-NH_2$ group of another amino acid molecule by the elimination of a water molecule is called a peptide linkage. For example,the reaction between Valine and Alanine forms a dipeptide with a peptide linkage.
$(ii)$ Primary structure: The primary structure of a protein refers to the specific sequence in which various amino acids are linked together in a polypeptide chain. Each protein has a unique sequence of amino acids,and any change in this sequence results in a different protein.
$(iii)$ Denaturation: In a biological system,a protein exists in a unique $3-D$ structure with specific biological activity,known as a native protein. When this native protein is subjected to physical changes (like temperature change) or chemical changes (like $pH$ change),its hydrogen bonds are disturbed. This causes the globules to unfold and the helix to uncoil,leading to the loss of biological activity. This process is called denaturation. During denaturation,the secondary and tertiary structures are destroyed,but the primary structure remains intact. An example is the coagulation of egg white upon boiling.

(N/A) There are two common types of secondary structure of proteins:
$(i)$ $\alpha$-helix structure
$(ii)$ $\beta$-pleated sheet structure
$\alpha$-Helix structure:
In this structure,the $-NH$ group of an amino acid residue forms an $H$-bond with the $>C=O$ group of the adjacent turn of the right-handed screw ($\alpha$-helix).
$\beta$-Pleated sheet structure:
This structure is called so because it looks like the pleated folds of drapery. In this structure,all the peptide chains are stretched out to nearly the maximum extension and then laid side by side. These peptide chains are held together by intermolecular hydrogen bonds.

(A) The $H$-bonds formed between the $-NH$ group of each amino acid residue and the $>C=O$ group of the adjacent turns of the $\alpha$-helix help in stabilising the helix.

(N/A)

Fibrous protein	Globular protein
$1.$ It is a fibre-like structure formed by the polypeptide chain. These proteins are held together by strong hydrogen and disulphide bonds.	$1.$ The polypeptide chain in this protein is folded around itself,giving rise to a spherical structure.
$2.$ It is usually insoluble in water.	$2.$ It is usually soluble in water.
$3.$ Fibrous proteins are usually used for structural purposes. For example,keratin is present in nails and hair; collagen in tendons; and myosin in muscles.	$3.$ All enzymes are globular proteins. Some hormones such as insulin are also globular proteins.

In aqueous solution,the carboxyl group of an amino acid can lose a proton and the amino group can accept a proton to give a dipolar ion known as a zwitter ion.
$R-CH(NH_2)-COOH \leftrightarrow R-CH(NH_3^+)-COO^-$ (Zwitter ion)
Therefore,in zwitter ionic form,the amino acid can act both as an acid and as a base.
$R-CH(NH_2)-COOH \xrightarrow{H^+} R-CH(NH_3^+)-COOH$ (Acidic medium)
$R-CH(NH_3^+)-COO^- \xrightarrow{OH^-} R-CH(NH_2)-COO^-$ (Basic medium)
Thus,amino acids show amphoteric behaviour.

(B) As a result of denaturation,globules get unfolded and helixes get uncoiled.
Secondary and tertiary structures of protein are destroyed,but the primary structures remain unaltered.
It can be said that during denaturation,secondary and tertiary-structured proteins get converted into primary-structured proteins.
Also,as the secondary and tertiary structures of a protein are destroyed,the enzyme loses its activity.

(N/A) Proteins are the most abundant biomolecules in the living system. The chief sources of proteins are milk,cheese,pulses,peanuts,fish,meat,etc.
They occur in every part of the body and form the basis of the structure and functions of life. They are an important component for the growth and maintenance of the body.
The word protein is derived from the Greek word "proteios",which means primary or of prime importance. Proteins are polymers of $\alpha$-amino acids.
Amino acids contain amino $(-NH_{2})$ and carboxyl $(-COOH)$ functional groups. Depending upon the relative position of the amino group with respect to the carboxyl group,amino acids can be classified as $\alpha, \beta, \gamma, \delta$,and so on. Only $\alpha$-amino acids are obtained upon the hydrolysis of proteins. They may contain other functional groups as well.
All $\alpha$-amino acids have trivial names,which usually reflect the property of that compound or its source. For example,Glycine is so named since it has a sweet taste (in Greek,glykos means sweet),and Tyrosine was first obtained from cheese (in Greek,tyros means cheese). Amino acids are generally represented by three-letter symbols; sometimes,one-letter symbols are also used.
The general structure of an $\alpha$-amino acid is:
$R-CH(NH_{2})-COOH$
(where $R$ = side chain)

(N/A) Amino acids are organic compounds containing both an amino group $(-NH_2)$ and a carboxyl group $(-COOH)$ attached to the same carbon atom,known as the $\alpha$-carbon.
They are the building blocks of proteins.
The general structure of an $\alpha$-amino acid is $R-CH(NH_2)-COOH$.
In aqueous solution,the carboxyl group can lose a proton and the amino group can accept a proton,giving rise to a dipolar ion known as a zwitterion.
This is represented as: $R-CH(NH_2)-COOH \rightleftharpoons R-CH(NH_3^+)-COO^-$.
Except for glycine,all naturally occurring $\alpha$-amino acids are optically active.

(N/A) $(i)$ Acidic amino acids :
The amino acids with a greater number of carboxyl groups $(-COOH)$ compared to amino groups $(-NH_2)$ are called acidic amino acids. For example,aspartic acid,glutamic acid,etc.
$(ii)$ Basic amino acids :
The amino acids with a greater number of amino groups $(-NH_2)$ than carboxyl groups $(-COOH)$ are called basic amino acids. For example,arginine,lysine,etc.
$(iii)$ Neutral amino acids :
The amino acids with an equal number of carboxyl and amino groups are called neutral amino acids. For example,alanine,phenylalanine,leucine,etc.

(N/A) Proteins are complex polymers of $\alpha$-amino acids. Their structure is described at four levels:
$1$. Primary structure: The specific sequence of amino acids in a polypeptide chain.
$2$. Secondary structure: The conformation of the polypeptide chain,which includes $\alpha$-helix and $\beta$-pleated sheet structures,stabilized by hydrogen bonding.
$3$. Tertiary structure: The overall three-dimensional folding of the polypeptide chain,resulting from interactions like hydrophobic interactions,ionic bonds,hydrogen bonds,and disulfide linkages.
$4$. Quaternary structure: The spatial arrangement of two or more polypeptide chains (subunits) in a multi-subunit protein.
Based on shape,proteins are classified as:
$(i)$ Fibrous proteins: Long,thread-like molecules held together by hydrogen and disulfide bonds (e.g.,keratin,myosin).
(ii) Globular proteins: Spherical shapes resulting from the folding of polypeptide chains (e.g.,insulin,albumin).

(C) Proteins are classified into two types based on their molecular shape:
$1$. $Fibrous \ proteins$: These are long,thread-like molecules held together by hydrogen and disulfide bonds. They are generally insoluble in water. Examples include keratin (in hair,wool,silk) and myosin (in muscles).
$2$. $Globular \ proteins$: These result from the folding of polypeptide chains into a spherical shape. They are usually soluble in water. Examples include insulin and albumins.

(N/A) The differences between fibrous and globular proteins are as follows:
| No. | Fibrous Proteins | Globular Proteins |
| :--- | :--- | :--- |
| $1$ | These proteins are linear,having a thread-like structure. | These proteins have polypeptide chains folded to give a spheroidal shape. |
| $2$ | These proteins are water-insoluble. | These proteins are soluble in water because,during the folding of peptide chains,the hydrophilic groups are pushed outwards,which interact strongly with water. |
| $3$ | The polypeptide chains are held by strong hydrogen bonding. | The hydrogen bonding is comparatively weaker. |
| $4$ | They are stable to changes in $pH$. | They are sensitive to $pH$ changes. |

(N/A) The image illustrates the $\alpha$-helix structure,which is a type of secondary protein structure. Proteins have four levels of structural organization:
$1$. Primary Structure: The specific sequence of amino acids in a polypeptide chain.
$2$. Secondary Structure: Refers to the shape in which a long polypeptide chain can exist. It includes $\alpha$-helix and $\beta$-pleated sheet structures,stabilized by hydrogen bonding between the $C=O$ and $N-H$ groups of the peptide backbone.
$3$. Tertiary Structure: The overall three-dimensional folding of the entire polypeptide chain,stabilized by various interactions like hydrogen bonds,ionic bonds,disulfide linkages,and van der Waals forces.
$4$. Quaternary Structure: The spatial arrangement of two or more polypeptide chains (subunits) in a multi-subunit protein.