The \alpha -amino acid which contains the aromatic side chain is:

An aromatic amino acid is an amino acid that has an aromatic ring. Among 20 standard amino acids, some aromatic amino acids are phenylalanine and tryptophan.

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Biomolecules

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Amino Acids Amino acids can get a little challenging to learn as there are many different structures of different amino acids. Then they have 3-letter and 1-letter symbols that you need to remember. What's even more confusing is that they are classified in many different types of ways! Let's make things a little simpler: What they are? Amino acids are the building blocks of proteins. They are molecules that possess both amino group

(N H_{2})

and carboxylic group

(C O O H)

. Their Many Classifications: Based on position of amino group: Amino acids can be classified into

α

β

γ

δ

and so on... depending upon the position of amino group.

Based on acidic or basic nature: Amino acids are classified into acidic, basic and neutral depending upon the relative number of carboxyl and amino groups.

Neutral amino acid: number of amino groups = number of carboxyl groups

Acidic amino acid: number of amino groups < number of carboxyl groups

Basic amino acid: number of amino groups > number of carboxyl groups

Based on synthesis in the body:

Amino acids that can be synthesised in the body are known as nonessential amino acids.

Those that cannot be synthesised in the body and must be obtained through diet, are known as essential amino acids.

Naming Amino Acids:

Generally,

α -

amino acids have trivial names that are based on their properties. Glycine is one example. It is named Glycine from the Greek term Glykos which means sweet, as it has a sweet taste (Glykos means sweet in Greek). Amino acids are also represented using 3-letter and 1-letter symbols.

3-letter symbols: The three 3-letter symbols are obtained from the first three letters of the name of the amino acids. Just remember these four exceptions: Asparagine (Asn), Glutamine (Gln), Isoleucine (Ile), Tryptophan (Trp)

1-letter symbols: The 1-letter symbols are the first letters of the name of the amino acids. Just remember these nine exceptions: Arginine (R), Asparagine (N), Aspartic acid (D), Glutamic acid (E), Glutamine (Q), Lysine (K), Phenylalanine (F), Tryptophan (W), and Tyrosine (Y).

Let's practice!

Which of the following is not an amino acid?

Glycine

Alanine

Histidine

Benzidine

View solution

The

α

-amino acid which contains the aromatic side chain is:

proline

tyrosine

valine

tryptophan

View solution

Structure of Proteins

In simple words, proteins are a chain of amino acids. But their structure is not that simple. When learning about protein structure, students can find it tough to understand protein folding at different levels (secondary, tertiary, etc.). Visualising these structures can make it easy to understand how amino acid chains fold and combine with each other to form a protein.

Primary Structure:

This is the basic chain of amino acids. More than one amino acids join together through peptide bonds to form a chain. Peptide bond is formed when amino group of one amino acid bonds with carboxyl group of another amino acid. The resulting chain of amino acids is called a polypeptide chain.

Secondary Structure:

The polypeptide chain folds into sheets and helices to form secondary structures. There are two types of secondary structures:

α -

helix and

β -

pleated structure.

Tertiary Structure:

The polypeptide chains having

α -

helix and

β -

pleated structures further fold within themselves through hydrogen bonds, van der Waals forces, disulphide linkages, and electrostatic forces of attraction to give the tertiary structure.

Tertiary structures lead to two major molecular shapes of proteins: Fibrous and globular.

Here is a story to help you understand better:

Structure of Proteins-I

2 mins

Quaternary Structure:

More than one amino acid chains (polypeptide chains) combine to give a quaternary structure. Quaternary structure of a protein is in fact several polypeptide chains with tertiary folding that combine to form one functional protein.

Time to practice

Assertion

β - p l e a t e d

sheet structure of protein shows maximum extension.

Reason

Intermolecular hydrogen bonding is present in them.

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

Assertion is correct but Reason is incorrect

Both Assertion and Reason are incorrect

View solution

Carbohydrates

Q: Which of the following is not an amino acid?

Benzidine is not an amino acid but it is a aromatic amine. Its molecular formula is C_{12}H_{12}N_2 . Its IUPAC name is 4,4'-diaminobiphenyl, which is used as part of a test for cyanide and also in the synthesis of dyes.Option D is correct.

Q: Glycogen, on hydrolysis, gives:

Glycogen: It is a multibranched polysaccharide of glucose that serves as a source of energy in animals. The carbohydrates are stored in an animal body as glycogen. It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched.

There are so many different types of carbohydrates. While the structures of monosaccharides like glucose and fructose are easy to learn, learning structures gets difficult when we move to di and polysaccharides. Let's break it down to make learning easy:

Glycosidic Linkage: The glue that binds them all

Two or more monosaccharides link through oxide linkage to form di and polysaccharides. This is called glycosidic linkage.

Sucrose = Glucose + Fructose

Sucrose is a disaccharide formed by glycosidic linkage between C1 of

α

-D-glucose and C2 of

β

-D-fructose.

Maltose = Glucose + Glucose

Maltose is formed from two

α

-D-glucose units. C1 of one glucose is linked to C4 of the other glucose unit.

Lactose = Galactose + Glucose

Lactose is formed by glycosidic linkage between C1 of

β

-D-galactose and C4 of

β

-D-glucose.

Starch = Amylose + Amylopectin

Starch is a polysaccharide made of

α

-glucose units.

The

α

-glucose units can be unbranched (amylose) or branched (amylopectin).

Cellulose (unbranched $β$ -D-glucose units)

Cellulose is a straight chain of

β

-D-glucose units joined by glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit.

Glycogen (branched $α$ -D-glucose units)

Glycogen is a highly branched structure of repeating units of

α

-D-glucose.

Now, try solving these questions:

Pyranose ring structure of glucose is due to hemiacetal formation between:

_{1}

and C

_{5}

_{1}

and C

_{4}

_{1}

and C

_{3}

_{2}

and C

_{4}

View solution

Glycogen, on hydrolysis, gives:

lactose and glucose

only glucose

glucose and fructose

glucose and maltose

View solution

Biomolecules

Secondary Structure:

Quaternary Structure:

Carbohydrates

Glycosidic Linkage: The glue that binds them all

Sucrose = Glucose + Fructose

Maltose = Glucose + Glucose

Lactose = Galactose + Glucose

Starch = Amylose + Amylopectin

Cellulose (unbranched β-D-glucose units)

Glycogen (branched α-D-glucose units)

Cellulose (unbranched $β$ -D-glucose units)

Glycogen (branched $α$ -D-glucose units)