Amino Acids and Proteins Questions in English

(C) The bond that determines the secondary structure of proteins is the $Hydrogen \ bond$.
Depending on hydrogen bonding,two main types of secondary structure are the $\alpha$-helix and the $\beta$-sheet.

(C) Glycine is an amphoteric amino acid. The $pH$ of an amphoteric amino acid solution is given by the formula: $pH = \frac{pK_{a1} + pK_{a2}}{2}$.
$pK_{a1} = -\log(4.5 \times 10^{-3}) = 3 - 0.653 = 2.347$.
$pK_{a2} = -\log(1.7 \times 10^{-10}) = 10 - 0.23 = 9.77$.
$pH = \frac{2.347 + 9.77}{2} = \frac{12.117}{2} \approx 6.06 \approx 6.1$.

(A) Nitrogen is an essential constituent of all $Proteins$ because they are polymers of amino acids,which contain an amino group $(-NH_2)$.

(A) Biological nitrogen fixation is the process by which atmospheric molecular nitrogen $(N_2)$ is converted into ammonia $(NH_3)$ by nitrogen-fixing bacteria such as $Rhizobium$ and $Azotobacter$.
This ammonia is then assimilated into amino acids and other organic compounds.

(C) Insulin is a peptide hormone produced by beta cells of the pancreatic islets. It contains $Zinc$ ions $(Zn^{2+})$ which are essential for the crystallization and storage of the hormone in the pancreas.

(D) Glycine $(H_2N-CH_2-COOH)$ contains a central carbon atom bonded to two identical hydrogen atoms.
Since it lacks a chiral center,it is achiral and therefore not optically active.
Glycine acts as an acid due to the $-COOH$ group,a base due to the $-NH_2$ group,and exists as a zwitterion in aqueous solution.

(D) $\alpha$-amino acetic acid (Glycine) exists as a zwitterion in aqueous solution.
In this state,the molecule contains both a positive charge $(NH_3^+)$ and a negative charge $(COO^-)$,which gives it a salt-like character with high melting points and solubility in water.

(A) When urea $(NH_2CONH_2)$ is heated,two molecules of urea condense to form biuret $(NH_2CONHCONH_2)$ with the elimination of an ammonia molecule $(NH_3)$.
In the presence of an alkaline solution (usually $NaOH$),biuret reacts with copper$(II)$ sulfate $(CuSO_4)$ to form a complex compound.
This reaction is known as the biuret test,which produces a characteristic $Violet$ colour,indicating the presence of peptide bonds.

(A) . Glycine is an amphoteric acid because it contains both an acidic carboxyl group $(-COOH)$ and a basic amino group $(-NH_2)$ in the same molecule,allowing it to exist as a zwitterion.

(C) An amino acid contains both an amino group,$-NH_2$,and a carboxylic acid group,$-COOH$,in the same molecule.
As with all acids,the carbon chain is numbered so that the carbon in the $-COOH$ group is counted as number $1$.

(B) The correct answer is $(B)$ $CH_3-CH(NH_2)-COOH$.
An amino acid is a compound that contains both an amino group $(-NH_2)$ and a carboxylic acid group $(-COOH)$ in the same molecule.
Option $(B)$ represents $2$-aminopropanoic acid (alanine),which satisfies this definition.

(D) Glycine $(H_2N-CH_2-COOH)$ contains both an acidic carboxyl group $(-COOH)$ and a basic amino group $(-NH_2)$.
In aqueous solution,the proton from the carboxyl group is transferred to the amino group,forming a dipolar ion known as a zwitter ion: $H_3N^{+}-CH_2-COO^{-}$.

(D) Polypeptides are polymers formed by the condensation of $Amino \text{ } acid$ units.
These units are linked covalently by peptide bonds ($-CO-NH-$ linkage).
Therefore,$Amino \text{ } acid$ is the monomer of a polypeptide.

(D) Proteins are naturally occurring polyamides,where amino acids are linked by peptide bonds (amide bonds).
Nylon is a synthetic polyamide containing repeating amide groups $(-CONH-)$ in its backbone.
$A$ peptide is a short chain of amino acids connected by peptide bonds,which are essentially amide bonds.
Therefore,all of the given options contain amide links.

(D) Proteins contain the amide group,which is formed by the linkage of amino acids through a peptide bond.
The peptide bond is represented as $-CO-NH-$,which is an amide linkage.
The general structure of a polypeptide chain is: $(-NH-CH(R)-CO-NH-CH(R)-CO-)_n$.

(B) Insulin is a protein consisting of $51$ amino acids arranged in two chains,$\alpha$ and $\beta$.
The $\alpha$-chain contains $21$ amino acids,and the $\beta$-chain contains $30$ amino acids.
It is secreted by the pancreas to regulate the blood sugar level.

(C) peptide is a short chain of amino acids linked by peptide bonds.
These peptide bonds are formed by the reaction between the carboxyl group $(-COOH)$ of one amino acid and the amino group $(-NH_2)$ of another,resulting in the elimination of a water molecule.
Chemically,the peptide bond $(-CONH-)$ is an amide linkage.
Therefore,peptides are classified as amides.

(A) Fibrous proteins consist of polypeptide chains held together by strong hydrogen and disulfide bonds,which makes them stable and insoluble in water.
Globular proteins,on the other hand,are generally soluble in water as their chains fold into a spherical shape.

(A) The process of irreversible precipitation or loss of biological activity of proteins due to physical or chemical changes (such as heating or change in $pH$) is known as $Denaturation$.
$Protein \xrightarrow{\text{Heated or change in pH}} \text{Denatured protein}$

(C) Proteins are classified based on their composition.
$Simple \ protein + \text{Non-protein part (Prosthetic group)} \to \text{Conjugated protein}$.
Therefore,proteins that contain a non-protein prosthetic group attached to the polypeptide chain are known as conjugated proteins.

(B) The correct answer is $(B)$.
Haemoglobin is a conjugated protein consisting of a protein part called globin and a non-protein prosthetic group called heme.
Heme consists of a $Fe^{+2}$ ion coordinated to a porphyrin ring system.

(C) . When collagen,a fibrous protein,is boiled with water,it undergoes hydrolysis to form $Gelatin$. $Gelatin$ is a water-soluble protein mixture used in food products,capsules,and photographic emulsions.

(D) Essential amino acids cannot be synthesized by the human body and must be obtained through the diet.
The $9$ essential amino acids are: histidine,isoleucine,leucine,lysine,methionine,phenylalanine,threonine,tryptophan,and valine.
Glycine is a non-essential amino acid because the body can synthesize it.
Therefore,the correct answer is $D$.

(C) Amino acids are organic compounds containing both amino and carboxyl groups. Due to the presence of both acidic and basic groups,they exist as zwitterions in the solid state. This strong electrostatic interaction between zwitterions results in high melting points and makes them non-volatile crystalline compounds.

(C) The isoelectric point is the $pH$ at which the amino acid exists as a zwitterion and does not migrate towards any of the electrodes in an electric field.

(C) Proteins are complex polymers of amino acids linked by peptide bonds.
Enzymatic hydrolysis of proteins breaks these peptide bonds to yield their constituent monomers.
Therefore,the hydrolysis of proteins by enzymes results in the formation of $Amino \ acids$.
Reaction: $Protein \xrightarrow{\text{Enzyme}} Amino \ acids$.

(B) Protein + conc. $HNO_3$ $\to$ Yellow colour.
This test is known as the Xanthoproteic test. It is given by proteins containing $\alpha$-amino acids with an aromatic ring (such as tyrosine or phenylalanine). The yellow colour is produced due to the nitration of the benzene ring in these amino acids.

(B) . Proteins are polymers of amino acids.
Amino acid $\to$ Dipeptide $\to$ Polypeptide $\to$ Protein.

(C) Proteins are linear polymers formed by linking the $-\alpha$-carboxyl group of one amino acid to the $-\alpha$-amino group of another amino acid with a peptide bond ($-CONH-$ linkage).

(A) Pepsin is a digestive enzyme that acts as a protease.
It specifically hydrolyses peptide bonds in proteins to break them down into smaller peptides and amino acids.
Proteins are linear polymers formed by linking the $\alpha$-carboxyl group of one amino acid to the $\alpha$-amino group of another amino acid with a peptide bond.

(D) Haemoglobin is a protein pigment of red blood cells,which is remarkably suited for the transport of oxygen. It binds to oxygen under higher alveolar oxygen concentration and a little higher $pH$ and cooler temperature. The product is oxyhaemoglobin. Likewise,under lower concentration of oxygen in the tissues and a comparatively lower $pH$ and warmer temperature,the tissues facilitate the release of oxygen from haemoglobin and the formation of deoxyhaemoglobin $(Hb)$. This makes option $D$ correct. Myoglobin is the pigment of muscles that serves to store oxygen,but owing to its higher affinity for oxygen than haemoglobin,it does not release oxygen easily,which makes option $A$ incorrect. Insulin is secreted by islets of Langerhans of the pancreas and serves to lower the blood glucose level by stimulating the uptake of glucose by cells. Albumins,a group of proteins produced by the liver,are present in human plasma and serve to maintain the osmotic concentration of blood.

(D) Protein is considered an essential constituent of the diet.
Proteins are known as body-building foods.
They perform most of the work in cells and are required for the structure,function,and regulation of the body's tissues and organs.

(D) Proteins serve multiple functions in biological and industrial systems.
$1$. They are essential components of our diet (As food).
$2$. They are used in the textile industry (e.g.,silk and wool are protein fibers).
$3$. They act as biological catalysts (As enzymes) to speed up chemical reactions in living organisms.
Therefore,all the given options are correct.

(C) The correct answer is $(C)$.
Proteins are complex organic molecules that are polymers of amino acids.
Each amino acid contains an amino group $(-NH_2)$ and a carboxyl group $(-COOH)$,making proteins nitrogenous compounds.

(C) The $\alpha$-helix is a common motif in the secondary structure of proteins.
It is a right-handed helix conformation in which every backbone $N-H$ group forms a hydrogen bond with the backbone $C=O$ group of the amino acid located three or four residues earlier along the protein sequence.
Thus,the helical structure is stabilized by hydrogen bonds.

(B) Natural silk is composed of fibroin,which is an insoluble protein.
Proteins are polymers of amino acids linked by peptide bonds $(-CO-NH-)$,which are characteristic of polyamides.
Therefore,natural silk is a polyamide.

(A) Proteins are polymers of $\alpha$-amino acids. The general structure of an $\alpha$-amino acid is $R-CH(NH_2)COOH$.
In this formula,$C$ represents carbon,$H$ represents hydrogen,$N$ represents nitrogen,and $O$ represents oxygen.
Therefore,proteins invariably contain $C, H, O$ and $N$ as their constituent elements.

(A) The end product of protein digestion is amino acids.
Once consumed,proteins are digested and broken down into amino acids by enzymes.

(A) The protein content in the given options is as follows:
$1$. An average-sized egg contains about $6-7 \ g$ of protein.
$2$. Cow's milk contains approximately $3.5 \ \%$ protein.
$3$. Wheat contains protein ranging from $9$ to $15 \ \%$.
$4$. Groundnut contains protein ranging from $22-30 \ \%$ of its total weight.
Therefore,groundnut has the highest percentage of protein among the given options.

(D) An amino acid is a compound that contains both an amino group $(-NH_2)$ and a carboxylic acid group $(-COOH)$.
In the given options,$NH_2CH_2-COOH$ (Glycine) contains both these functional groups.
Therefore,the correct option is $(D)$.

(C) The Biuret test is a chemical test used for detecting the presence of peptide bonds,which indirectly indicates the presence of proteins,as proteins are composed of amino acids linked by peptide bonds.
In the presence of peptides,a copper$(II)$ ion forms violet-colored coordination complexes in an alkaline solution.
The Biuret reaction is used to assess the concentration of proteins because peptide bonds occur with a consistent frequency per amino acid residue in the peptide chain.

(A) Proteins are polymers of $\alpha$-amino acids linked by peptide bonds.
Since the peptide bond $(-CO-NH-)$ is an amide linkage,proteins are chemically classified as polyamides.

(B) Proteins are complex macromolecules composed of one or more polypeptide chains.
Polypeptides are polymers of amino acids.
Polypeptides with more than $100$ amino acid residues and a molecular mass typically greater than $10,000 \text{ u}$ are classified as proteins.

(C) Antigens are typically polysaccharides or glycoproteins present on the surface of $RBCs$. While some antigens are proteins,the term 'antigen' itself refers to a functional property (triggering an immune response) rather than a structural classification of proteins like enzymes,antibodies,or hormones.

(D) Keratin is the major structural fibrous protein that forms hair,wool,feathers,nails,and horns of many kinds of animals.
It has a high concentration of cysteine,$7$ to $20 \%$ of the total amino acid residues,which form inter- and intramolecular disulfide bonds.

(B) The $pH$ at which the concentration of anions and cations is equal is the isoelectric point $(pI)$. At this $pH$,the amino acid exists as a zwitterion,which has the minimum solubility in water,not maximum.
$(b)$ All $\alpha$-amino acids react with ninhydrin to produce a characteristic purple-blue colored complex.
$(c)$ $\alpha$-amino acids react with nitrous acid $(HNO_2)$ to form a diazonium salt,which is unstable and decomposes to release $N_2$ gas.
Therefore,statements $(b)$ and $(c)$ are true.

(D) Secondary structure refers to the regular folding patterns of continuous portions of the polypeptide chain,such as $\alpha$-helices and $\beta$-pleated sheets.
These structures are stabilized by hydrogen bonds formed between the carbonyl oxygen $(C=O)$ of one amino acid residue and the amino hydrogen $(N-H)$ of another residue in the polypeptide backbone.

(A) . Proteins are polymers of $\alpha$-amino acids linked by peptide bonds.
These amino acids undergo condensation reactions to form long polypeptide chains.
$H_2N-CH(R)-COOH + H_2N-CH(R')-COOH \xrightarrow{-H_2O} H_2N-CH(R)-CONH-CH(R')-COOH$.

(D) Statement $(A)$ is correct: Albumin is a simple protein.
Statement $(B)$ is incorrect: Alanine has a methyl group $(-CH_3)$ as its side chain,which is non-polar and neutral,not acidic.
Statement $(C)$ is correct: Insulin is a peptide hormone.
Statement $(D)$ is incorrect: Muscles contain contractile proteins like myosin and actin,whereas keratin is a structural protein found in hair,skin,and nails.
Therefore,statements $(B)$ and $(D)$ are wrong.

(B) The statement in option $(B)$ is incorrect because $(C_6H_{10}O_5)_n$ is the general formula for polysaccharides like starch and cellulose,not proteins. Proteins are polymers of amino acids.