Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 401: At what wavelength in the spectrum of light does the Soret band in porphyrins absorb light?

A. 300 nm
B. 400 nm (Correct Answer)
C. 500 nm
D. 200 nm

Explanation: ***400 nm*** - The **Soret band**, also known as the B-band, is a strong absorption band characteristic of porphyrins, located in the **near-ultraviolet or blue region** of the electromagnetic spectrum. - This high-intensity absorption is due to **π-π* electronic transitions** within the porphyrin macrocycle, typically peaking around **400-420 nm**. *200 nm* - This wavelength is in the **far-ultraviolet region** and is generally associated with the absorption of **peptide bonds** (amide bonds) in proteins, but not the primary Soret band of porphyrins. - While some porphyrins might show minor absorption in this region, it is not the characteristic and strong Soret band. *300 nm* - This wavelength is in the **mid-ultraviolet range**; while some organic molecules absorb here, it is not the primary absorption maximum for the Soret band of porphyrins. - The Soret band is distinctly shifted towards higher wavelengths (bluer light) compared to 300 nm. *500 nm* - This wavelength is in the **visible green region** of the spectrum, where porphyrins typically exhibit weaker absorption bands known as **Q-bands (or α/β bands)**, not the Soret band. - The Q-bands are observed at longer wavelengths and are generally much less intense than the Soret band.

Question 402: Actin can exist in which forms?

A. Fibrous form
B. Globular form
C. Both forms (Correct Answer)
D. Neither form

Explanation: *Fibrous form* - While actin does exist in a **fibrous (F-actin)** form, this option is incomplete as it misses the globular form. - **F-actin** is a polymer formed from individual globular actin monomers and is crucial for cellular structures. *Globular form* - Actin does exist as a **globular (G-actin)** monomer, but this option is incomplete as G-actin polymerizes into filamentous actin. - **G-actin** is the monomeric form of actin with ATP-binding capability. *Neither form* - This option is incorrect because actin is well-known to exist in both **globular (G-actin)** and **fibrous (F-actin)** states. - Actin's ability to transition between these forms is fundamental to its biological functions. ***Both forms*** - Actin monomers (G-actin) exist in the **globular form** and can polymerize to form **filamentous actin** (F-actin). - The dynamic interconversion between these two forms is crucial for various cellular processes like **cell motility**, **cytoskeletal organization**, and **muscle contraction**. - This is the most complete and accurate answer as actin physiologically exists in both states.

Question 403: Which of the following is a biologically important tripeptide?

A. Thyrotropin releasing hormone (Correct Answer)
B. Thyroid stimulating hormone
C. Gonadotropin releasing hormone
D. Follicle Stimulating hormone

Explanation: ***Thyrotropin releasing hormone*** - **Thyrotropin-releasing hormone (TRH)** is a neurohormone consisting of only **three amino acids (glutamate-histidine-prolineamide)**, making it a tripeptide. - It stimulates the release of **TSH** from the anterior pituitary gland, playing a crucial role in thyroid function. *Thyroid stimulating hormone* - **Thyroid stimulating hormone (TSH)** is a **glycoprotein hormone** composed of two subunits (alpha and beta chains), making it significantly larger than a tripeptide. - TSH acts on the thyroid gland to stimulate the production and release of **thyroid hormones**. *Gonadotropin releasing hormone* - **Gonadotropin-releasing hormone (GnRH)** is a **decapeptide**, meaning it is composed of **ten amino acids**, not three. - GnRH stimulates the anterior pituitary to release **luteinizing hormone (LH)** and **follicle-stimulating hormone (FSH)**. *Follicle Stimulating hormone* - **Follicle-stimulating hormone (FSH)** is a **glycoprotein hormone**, similar to TSH, composed of two polypeptide subunits (alpha and beta). - FSH is essential for **gamete production** in both males and females.

Question 404: All of the following are true about collagen structure except which of the following?

A. Hydroxylysine undergoes glycosylation
B. Collagen is secreted by fibroblasts as procollagen
C. Lysyl oxidase is dependent on Vitamin C (Correct Answer)
D. Glycine is the most abundant amino acid of collagen

Explanation: ***Lysyl oxidase is dependent on Vitamin C*** - **Lysyl oxidase** is an enzyme crucial for the cross-linking of collagen fibers, which provides tensile strength to collagen. - However, **lysyl oxidase** is a **copper-dependent enzyme**, not vitamin C-dependent. **Vitamin C (ascorbic acid)** is essential for the hydroxylation of proline and lysine residues by **prolyl and lysyl hydroxylases**. *Collagen is secreted by fibroblasts as procollagen* - **Fibroblasts** are the primary cells responsible for synthesizing and secreting collagen in connective tissues. - They secrete collagen in a precursor form called **procollagen**, which has globular extensions at its ends to prevent premature fibril formation inside the cell. *Hydroxylysine undergoes glycosylation* - Following hydroxylation of lysine residues to **hydroxylysine** by **lysyl hydroxylase** (a vitamin C–dependent enzyme), these hydroxylysine residues can be further modified by **glycosylation**. - **Glycosylation** involves the addition of glucose or galactose residues, which may play a role in collagen fibril assembly and stability. *Glycine is the most abundant amino acid of collagen* - The characteristic triple-helical structure of collagen is largely due to its unique amino acid composition, with **glycine** being the most abundant. - The repeating sequence **Gly-X-Y** (where X is often proline and Y is often hydroxyproline) is found throughout the collagen molecule, allowing for tight packing of the helical chains.

Question 405: Ubiquitin Proteasome pathway is used for degradation of?

A. Extracellular long lived proteins
B. Intracellular long lived proteins
C. Intracellular short lived proteins (Correct Answer)
D. Extracellular short lived proteins

Explanation: ***Intracellular short lived proteins*** - The **ubiquitin-proteasome pathway** is the primary mechanism for degrading **misfolded**, damaged, or **short-lived regulatory proteins** within the cell. - This pathway ensures proper protein turnover and cellular homeostasis by targeting specific **intracellular proteins** for destruction. *Extracellular long lived proteins* - **Extracellular proteins** are typically degraded by **lysosomal pathways** or specific proteases in the extracellular matrix, not the ubiquitin-proteasome system. - The ubiquitin-proteasome system acts exclusively on proteins located within the cytoplasm and nucleus. *Intracellular long lived proteins* - While some **long-lived intracellular proteins** can eventually be degraded by the ubiquitin-proteasome system, the primary target of this pathway is often **short-lived** or rapidly turned over proteins. - **Autophagy** is another major pathway for degrading long-lived intracellular proteins and organelles. *Extracellular short lived proteins* - Similar to other **extracellular proteins**, short-lived extracellular proteins are degraded by mechanisms outside the cell, such as **extracellular proteases** or endocytosis followed by late endosome/lysosome processing. - The ubiquitin-proteasome system is confined to the intracellular environment.

Question 406: Edman's reagent is used for

A. Protein sequencing (Correct Answer)
B. Nucleic acid sequencing
C. Protein unfolding
D. Nucleic acid denaturation

Explanation: ***Protein sequencing*** - **Edman degradation**, using Edman's reagent (phenyl isothiocyanate), is a chemical method to determine the **amino acid sequence** of a protein from its N-terminus. - It sequentially removes one amino acid at a time, allowing for its identification and thus determining the protein's primary structure. *Nucleic acid sequencing* - **Nucleic acid sequencing** methods, like Sanger sequencing or next-generation sequencing, are used to determine the order of **nucleotides** in DNA or RNA, not amino acids. - Edman's reagent is not involved in these processes, which typically involve **DNA polymerases** or ligases. *Protein unfolding* - **Protein unfolding** refers to the disruption of a protein's tertiary or secondary structure, often caused by agents like **heat**, **acid**, or **denaturants** (e.g., urea, guanidinium chloride). - While protein sequencing requires the protein to be in a disaggregated state, Edman's reagent itself is not primarily used as an unfolding agent. *Nucleic acid denaturation* - **Nucleic acid denaturation** is the process of separating double-stranded DNA into single strands, typically induced by **heat** or **alkaline conditions**. - This process is distinct from sequencing and does not involve Edman's reagent, which is specific to peptide bond cleavage and amino acid detection.

Question 407: Which type of isomerism does D and L isomerism represent?

A. Optical isomerism (Correct Answer)
B. Functional isomerism
C. Epimerism
D. Enantiomerism

Explanation: ***Optical isomerism*** - D and L isomerism is a classic form of **optical isomerism**, which refers to stereoisomers that can rotate **plane-polarized light**. - The D (dextrorotatory) and L (levorotatory) nomenclature is based on the direction of optical rotation and the configuration relative to **glyceraldehyde**. - D and L isomers are **enantiomers** (non-superimposable mirror images), making this the most direct and accurate answer. - This nomenclature system is fundamental in biochemistry, especially for **amino acids** and **carbohydrates**. *Enantiomerism* - While D and L isomers are indeed **enantiomers**, this term refers to the relationship between the isomers rather than the classification system itself. - The modern R/S system (Cahn-Ingold-Prelog) is more commonly associated with the term "enantiomerism." - D/L nomenclature specifically emphasizes the **optical activity** aspect, making "optical isomerism" the more precise answer. *Functional isomerism* - This involves compounds with the same **molecular formula** but different **functional groups** (e.g., ethanol vs. dimethyl ether). - Does not involve chirality or stereochemistry. *Epimerism* - **Epimers** are diastereomers differing at only **one chiral center** (e.g., glucose and galactose differ at C-4). - D and L forms are complete mirror images differing at **all chiral centers**, not just one.

Question 408: How many molecules of cysteine are combined to form one molecule of cystine?

A. 1
B. 2 (Correct Answer)
C. 3
D. 4

Explanation: ***2*** - **Cystine** is formed by the **oxidation** of **two molecules of cysteine**, leading to the formation of a **disulfide bond** between their sulfhydryl groups. - This **disulfide bond** is a crucial covalent linkage that plays a significant role in stabilizing the tertiary and quaternary structures of proteins. *1* - One molecule of cysteine cannot form cystine, as cystine is a **dimer** requiring the linkage of two cysteine residues. - A single cysteine molecule contains a **thiol group (-SH)**, which needs to react with another thiol group to form a disulfide bond. *3* - While cystine involves a disulfide bond, it is specifically formed from **two cysteine molecules**, not three. - The formation of a disulfide bond connects two cysteine residues, creating a stable dimer. *4* - The chemical structure of **cystine** consists of two linked cysteine units; it does not involve four cysteine molecules. - The reaction is a **dimerization**, involving precisely two molecules of the starting material, cysteine.

Question 409: Which amino acid is primarily responsible for the unique folding of collagen?

A. Glycine (Correct Answer)
B. Proline
C. Hydroxyproline
D. Lysine

Explanation: ***Glycine*** - **Glycine** is the smallest amino acid, lacking a side chain, which allows the collagen triple helix to pack tightly. - Its presence at every third position in the collagen primary sequence is crucial for forming the characteristic **triple helical structure**. *Proline* - **Proline** introduces kinks in the polypeptide chain due to its rigid ring structure, which is important for the stability of the collagen helix. - While essential for collagen structure, it does not enable the tight packing required for the triple helix in the same way as glycine. *Hydroxyproline* - **Hydroxyproline** is a post-translational modification of proline, critical for the stability of the collagen triple helix through hydrogen bonding. - While vital for collagen's strength, its role is structural stabilization rather than facilitating the initial tight coiling and folding. *Lysine* - **Lysine** is involved in forming **cross-links** between collagen molecules, contributing to the tensile strength of collagen fibrils. - It does not primarily dictate the initial unique folding or tight packing of the individual collagen polypeptide chains.

Question 410: Which amino acid is optically inert?

A. Valine
B. Alanine
C. Glycine (Correct Answer)
D. Threonine

Explanation: ***Glycine*** - **Glycine** is the only common amino acid that lacks a **chiral center**, as its side chain is a hydrogen atom. - Due to the absence of a chiral carbon, glycine does not rotate plane-polarized light and is thus **optically inert**. *Valine* - **Valine** has a side chain with a branched methyl group (CH(CH₃)₂), creating a **chiral center**. - This chiral center allows valine to exist as ᴅ- and ʟ-stereoisomers, making it **optically active**. *Alanine* - **Alanine** has a methyl group (–CH₃) as its side chain, making its alpha carbon **chiral**. - As a result, alanine exhibits **optical activity** and can rotate plane-polarized light. *Threonine* - **Threonine** is one of two amino acids (the other being isoleucine) with **two chiral centers**. - Its complex structure ensures it is highly **optically active**.

Practice by Chapter

Amino Acids: Structure and Properties

Practice Questions

Peptide Bond Formation

Practice Questions

Primary Structure of Proteins

Practice Questions

Secondary Structure of Proteins

Practice Questions

Tertiary and Quaternary Structures

Practice Questions

Protein Folding and Chaperones

Practice Questions

Protein Domains and Motifs

Practice Questions

Structure-Function Relationships

Practice Questions

Hemoglobin and Myoglobin

Practice Questions

Collagen and Elastin

Practice Questions

Albumin and Plasma Proteins

Practice Questions

Post-Translational Modifications

Practice Questions

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