Protein Structure and Function Practice Questions

Q: Edman's reagent is used for

Protein sequencing. ***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.

Q: Which of the following is a biologically important tripeptide?

Thyrotropin releasing hormone. ***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.

Q: Which of the following is an acidic amino acid?

Glutamic acid. ***Glutamic acid*** - **Glutamic acid** is an **acidic amino acid** because its side chain (R-group) contains a carboxyl group (-COOH) that can donate a proton, lowering the pH. - At physiological pH, the carboxyl group is deprotonated, giving it a **negative charge**. - The two acidic amino acids are **glutamic acid** and **aspartic acid**. *Alanine* - **Alanine** is a **non-polar, aliphatic amino acid** with a methyl group (-CH₃) as its side chain. - It is **neutral** at physiological pH and does not have acidic or basic properties. *Lysine* - **Lysine** is a **basic amino acid** because its side chain contains an amine group (-NH₂) that can accept a proton. - At physiological pH, the amine group is protonated, giving it a **positive charge**. *None of the options* - This option is incorrect because **glutamic acid** is indeed an acidic amino acid. - Therefore, there is a correct option provided in the list.

Q: Which type of isomerism does D and L isomerism represent?

Optical isomerism. ***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.

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

Glycine. ***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.

Q: Which amino acid is optically inert?

Glycine. ***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**.

Q: Which of the following statements regarding sulfhydryl groups is false?

They are not involved in reduction of peroxides. ***They are not involved in reduction of peroxides*** - This statement is **false** because **sulfhydryl groups**, particularly in **glutathione**, play a crucial role in the **reduction of hydrogen peroxide** and organic peroxides, protecting cells from oxidative damage. - **Glutathione peroxidase**, an enzyme that contains selenium, uses glutathione (with its sulfhydryl group) to catalyze this reaction, converting peroxides into water or less harmful alcohols. *They are present in coenzyme A and lipoic acid* - This statement is **true**; **Coenzyme A** contains a terminal **sulfhydryl group (-SH)** that is essential for its role in transferring acyl groups in metabolic reactions, such as the **Krebs cycle** and fatty acid metabolism. - **Lipoic acid** also contains a **disulfide bond** that can be reduced to two sulfhydryl groups, enabling its function as a coenzyme in **pyruvate dehydrogenase** and alpha-ketoglutarate dehydrogenase complexes. *They are present in Captopril and penicillamine* - This statement is **true**; **Captopril** is an **ACE inhibitor** that contains a **sulfhydryl group**, which is critical for its inhibitory activity against **angiotensin-converting enzyme**. - **Penicillamine** is a chelating agent and immunosuppressant, also containing a **sulfhydryl group**, used in conditions like **Wilson's disease** and **rheumatoid arthritis**. *They are present in cysteine* - This statement is **true**; **Cysteine** is an **amino acid** uniquely characterized by its **sulfhydryl group (-SH)**, making it a key component in **protein structure** (forming disulfide bonds with other cysteine residues) and in the synthesis of **glutathione**. - The sulfhydryl group in cysteine is highly reactive and contributes to its **redox properties** and metal-binding capabilities.

Q: Which amino acid contains an extra NH2 (amino) group in its structure?

Lysine. ***Lysine*** - Lysine is a basic amino acid that contains an **additional amino group (-NH2)** at the epsilon (ε) position of its side chain, in addition to the standard α-amino group. - Its structure: H2N-CH(COOH)-(CH2)4-NH2 clearly shows **two amino groups**. - This ε-amino group is positively charged at physiological pH, making lysine a **dibasic amino acid**. - The presence of this extra amino group makes lysine essential for **protein cross-linking** (e.g., in collagen) and various post-translational modifications. *Histidine* - Histidine contains an **imidazole ring** in its side chain with two nitrogen atoms, but this is NOT an amino group (-NH2). - While the imidazole ring makes histidine basic and can accept protons, it structurally does not contain an "extra NH2 group." - The imidazole side chain acts as a **buffer at physiological pH** due to its pKa (~6.0). *Aspartate* - Aspartate contains an **extra carboxyl group (-COOH)**, not an amino group. - This makes it an **acidic amino acid** with a negatively charged side chain at physiological pH. *Glutamate* - Glutamate also contains an **extra carboxyl group (-COOH)**, not an amino group. - Like aspartate, it is an **acidic amino acid** with a longer side chain by one methylene group.

Q: Which of the following is present in cornea ?

Chondroitin sulfate. ***Chondroitin sulfate*** - **Chondroitin sulfate** is one of the two major **glycosaminoglycans (GAGs)** present in the **corneal stroma**, comprising approximately 35-45% of corneal GAGs. - Along with keratan sulfate, it maintains **collagen fibril spacing and organization**, which is critical for **corneal transparency**. - The regular arrangement of these GAGs between collagen fibrils prevents light scattering and maintains corneal clarity. *Hyaluronic acid* - **Hyaluronic acid** is abundant in the **vitreous humor** and **synovial fluid** but is present only in minimal amounts in the cornea. - Its primary functions are lubrication and hydration in other tissues, not structural support in the cornea. *Heparan sulfate* - **Heparan sulfate** is found in **basement membranes** throughout the body, including Descemet's membrane of the cornea. - However, it is not a major component of the **corneal stroma** where structural transparency is maintained. *Dermatan sulfate* - **Dermatan sulfate** is predominantly found in the **sclera**, **skin**, and **blood vessels**, not in significant amounts in the cornea. - The cornea specifically requires **keratan sulfate** and **chondroitin sulfate** for its unique optical properties.

Q: Fibrin is degraded by which enzyme?

Plasmin. ***Plasmin*** - Fibrin is cleaved and degraded by **plasmin**, an enzyme that plays a crucial role in the fibrinolytic pathway [1]. - Plasminogen is activated to plasmin by tissue plasminogen activator (tPA), leading to the dissolution of fibrin clots in the process [1]. *Fibrin* - Fibrin is the **end product of coagulation**, not an enzyme; it serves as a scaffold for blood clot formation. - It does not have the ability to degrade itself; rather, it is broken down by **plasmin**. *Thrombin* - Thrombin is primarily involved in the **conversion of fibrinogen to fibrin** but does not degrade fibrin. - It promotes clot formation and stabilization rather than their dissolution. *None* - This option suggests that fibrin is not degraded by any substance, which is incorrect. - Fibrin degradation is specifically mediated by **plasmin**, contradicting the assertion that no agents are involved. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Hemodynamic Disorders, Thromboembolic Disease, and Shock, pp. 130-132.

Question 1

Edman's reagent is used for

Accepted Answer

Protein sequencing

Answer

Nucleic acid sequencing

Answer

Protein unfolding

Answer

Nucleic acid denaturation

Question 2

Which of the following is a biologically important tripeptide?

Accepted Answer

Thyrotropin releasing hormone

Answer

Thyroid stimulating hormone

Answer

Gonadotropin releasing hormone

Answer

Follicle Stimulating hormone

Question 3

Which of the following is an acidic amino acid?

Accepted Answer

Glutamic acid

Answer

Alanine

Answer

Lysine

Answer

None of the options

Question 4

Which type of isomerism does D and L isomerism represent?

Accepted Answer

Optical isomerism

Answer

Functional isomerism

Answer

Epimerism

Answer

Enantiomerism

Question 5

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

Accepted Answer

Glycine

Answer

Proline

Answer

Hydroxyproline

Answer

Lysine

Question 6

Which amino acid is optically inert?

Accepted Answer

Glycine

Answer

Valine

Answer

Alanine

Answer

Threonine

Question 7

Which of the following statements regarding sulfhydryl groups is false?

Accepted Answer

They are not involved in reduction of peroxides

Answer

They are present in coenzyme A and lipoic acid

Answer

They are present in cysteine

Answer

They are present in Captopril and penicillamine

Question 8

Which amino acid contains an extra NH2 (amino) group in its structure?

Accepted Answer

Lysine

Answer

Aspartate

Answer

Glutamate

Answer

Histidine

Question 9

Which of the following is present in cornea ?

Accepted Answer

Chondroitin sulfate

Answer

Hyaluronic acid

Answer

Heparan sulfate

Answer

Dermatan sulfate

Question 10

Fibrin is degraded by which enzyme?

Accepted Answer

Plasmin

Answer

Fibrinogen

Answer

Thrombin

Answer

None of the options

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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