Protein Structure and Function Practice Questions

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

Lysyl oxidase is dependent on Vitamin C. ***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.

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

2. ***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.

Q: Ubiquitin Proteasome pathway is used for degradation of?

Intracellular short lived proteins. ***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.

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 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: Which of the following is a component of intermediate filaments?

Keratin. ***Keratin*** - **Keratin** is the primary protein component of **intermediate filaments** found in epithelial cells, providing structural integrity and mechanical support. - It forms a diverse family of fibrous proteins essential for cell and tissue toughness, particularly prominent in skin, hair, and nails. *Actin* - **Actin** is the main component of **microfilaments** (also known as actin filaments), which are involved in cell motility, muscle contraction, and maintaining cell shape. - It does not form intermediate filaments but rather forms dynamic, thin filaments that work with myosin for various cellular functions. *Tubulin* - **Tubulin** is the protein subunit that polymerizes to form **microtubules**, which are part of the cytoskeleton involved in intracellular transport, cell division, and ciliary/flagellar movement. - Microtubules are distinct from intermediate filaments in their structure, function, and protein composition. *Myosin* - **Myosin** is a motor protein that interacts with actin to facilitate muscle contraction and other forms of cell motility. - It is not a component of intermediate filaments but rather a key player in the assembly and function of **contractile fibers** within cells.

Question 1

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

Accepted Answer

Glycine

Answer

Proline

Answer

Hydroxyproline

Answer

Lysine

Question 2

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

Accepted Answer

Lysyl oxidase is dependent on Vitamin C

Answer

Hydroxylysine undergoes glycosylation

Answer

Collagen is secreted by fibroblasts as procollagen

Answer

Glycine is the most abundant amino acid of collagen

Question 3

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

Accepted Answer

2

Answer

1

Answer

3

Answer

4

Question 4

Ubiquitin Proteasome pathway is used for degradation of?

Accepted Answer

Intracellular short lived proteins

Answer

Extracellular long lived proteins

Answer

Intracellular long lived proteins

Answer

Extracellular short lived proteins

Question 5

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 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

Which of the following is a component of intermediate filaments?

Accepted Answer

Keratin

Answer

Tubulin

Answer

Actin

Answer

Myosin

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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