#protein degradation

Animal Tissue Culture 1

Protease inhibitors are added to prevent:

Innate immunity balances protease mediated tissue remodeling with protective antiproteases preventing autodamage. Neutrophils recruited to injury sites degranulate releasing azurophilic granules containing serine protease neutrophil elastase capable of degrading elastin collagen, cathepsin G, proteinase 3, and specific granules releasing matrix metalloproteases MMP8 collagenase and MMP9 gelatinase facilitating migration through basement membrane. Uncontrolled release would destroy cartilage and lung parenchyma causing emphysema in alpha-1 antitrypsin deficiency. Plasma contains defensive inhib

Ref: Lodish Molecular Cell Biology serpin α1 antitrypsin α2 macroglobulin; Alberts MBoC protease inhibitors serum protection lysis cocktail.

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Protein Sorting and Trafficking

Which enzyme trims mannose residues to signal degradation of misfolded proteins?

Distinguishing proteins still capable of folding from terminally misfolded clients destined for destruction relies on slow enzymatic removal of mannose residues acting as molecular clock. ER mannosidase I and ER-degradation-enhancing α-mannosidase-like proteins EDEM1, EDEM2 and EDEM3, which are catalytically active despite name suggesting lectin, cleave specific α1-2 linked mannoses from outer branches of high-mannose N-glycan. Initial Man9 processed to Man8B isoform, then after prolonged ER residence to Man7, Man6 species lacking terminal mannose on C-branch, disfavoring interaction with regl

Ref: Mast & Ng, Crit Rev Biochem Mol Biol 47: 2012, Mannose Trimming Signals ERAD.

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Protein Sorting and Trafficking

What happens when a protein fails to fold correctly in the ER?

Inability to achieve native conformation triggers layered proteostasis network aimed at restoring balance and if impossible eliminating dangerous species. Initially, chaperone BiP binds exposed hydrophobic stretches, UGGT reglucosylates non-native N-glycoproteins returning them to calnexin-calreticulin cycle for additional attempts at oxidative folding assisted by PDI family, ERp57, Ero1 and peptidyl-prolyl isomerases. Kinetic competition with slow mannosidases EDEM1-3 acts as timer: prolonged residence leads to progressive removal of terminal mannoses preventing reglucosylation and generating

Ref: Walter & Ron, Science 334: 2011, Handling of Misfolded Proteins and UPR.

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Protein Sorting and Trafficking

Misfolded proteins in the ER are eventually:

When attempts at native conformation repeatedly fail despite chaperone assistance including BiP binding and calnexin-mediated retention plus UGGT reglucosylation cycles, terminally misfolded proteins must be removed to prevent proteotoxic aggregation, ER stress and activation of apoptotic pathways. Lumenal quality control uses mannose timer: ER mannosidase I and EDEM1-3 remove specific α1-2 mannose residues from core glycan converting Man9GlcNAc2 to Man7-6 isoforms with reduced affinity for folding lectins and increased affinity for degradation lectins. Trimmed glycans recognized by MRH domain

Ref: Ruggiano et al., Nature Rev Mol Cell Biol 15: 2014, ERAD of Misfolded Proteins.

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Cell Cycle Regulation

The SCF complex plays a crucial role in:

SCF complex, acronym for SKP1-CUL1-F-box protein, represents prototypical cullin-RING ubiquitin ligase family controlling G1/S transition through regulated proteolysis of CDK inhibitors. Core scaffold comprises elongated Cullin1 linking adaptor SKP1 and RING domain protein Rbx1 that recruits E2 ubiquitin-conjugating enzyme. Substrate specificity arises from interchangeable F-box proteins containing leucine-rich repeats or WD40 domains that bind phosphorylated degrons. In budding yeast, F-box Cdc4 recognizes Sic1 inhibitor after phosphorylation by Cln-CDK1 at multiple CDK consensus sites, while

Ref: Morgan, The Cell Cycle: Principles of Control, 2nd ed., Chapter 5: SCF Ligases and CDK Inhibitors.

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Cell Cycle Regulation

The SCF complex is primarily responsible for:

SCF complex forms prototypical cullin-RING ubiquitin ligase governing G1/S and S-phase progression, distinct from mitotic APC/C. Architecture comprises scaffold Cullin1, adaptor Skp1, RING finger Rbx1 recruiting E2 ubiquitin conjugating enzyme, and interchangeable F-box protein providing substrate specificity through WD40 or leucine-rich repeats. Activity regulated by substrate phosphorylation creating phosphodegron that interacts with F-box. During late G1, Cdk2-Cyclin E phosphorylates cyclin-dependent kinase inhibitor p27 Kip1 at Thr187, Sic1 ortholog in yeast at multiple CDK sites and p21 a

Ref: Cardozo & Pagano, SCF Ubiquitin Ligases Regulate Cell Cycle, Nat Rev Mol Cell Biol 2004; Frescas & Pagano, SCF Functions in G1/S Control.

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Cell Cycle Regulation

What is the function of the SCF complex?

Ordered progression through interphase depends on timely degradation of regulatory proteins via ubiquitination. The SCF complex, meaning Skp1-Cullin-F-box, is a modular cullin-RING E3 ligase active from late G1 to early M. Core components include scaffold Cullin1, adaptor Skp1, RING finger Rbx1 and interchangeable F-box protein conferring substrate specificity. Canonical function involves CDK inhibitor p27 Kip1. When Cyclin E-CDK2 phosphorylates p27 at Thr187, phosphodegron is recognized by F-box protein Skp2 together with accessory Cks1, leading to Lys48-linked polyubiquitination and proteaso

Ref: Cardozo & Pagano, SCF Ubiquitin Ligases and Cell Cycle Control, Nat Rev Mol Cell Biol 2004; NCBI, SCF Functions.

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DNA & RNA isolation

Which agent is used to degrade proteins during DNA extraction?

Proteinase K is a broad-spectrum serine protease widely used during DNA extraction to digest proteins, including nucleases, histones, and cellular structural proteins. It exhibits high activity in presence of SDS and EDTA and at elevated temperatures around 50-65°C, conditions that simultaneously lyse cells and denature substrates. By hydrolyzing peptide bonds, it deproteinizes DNA, inactivates DNases and RNases, and improves yield and purity. RNase specifically degrades RNA, DNase degrades DNA, and Triton X-100 is a non-ionic detergent for membrane permeabilization, not for proteolysis.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Cell Signaling ELR Pathways - I

SOCS box functions to

recruit E3 ubiquitin ligase, is consistent with established principles of cell signaling, receptor pharmacology and cellular regulation. Experimental measurements of binding parameters, genetic loss-of-function studies and pharmacological interventions all converge on the same interpretation. Related options address neighboring concepts but do not satisfy the precise criterion stated in the question.

Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)

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Protein

In a misfolded protein, what happens if refolding attempts fail?

The protein is degraded by the proteasome correctly describes the effect or change asked about in this question. In Protein Folding, understanding cause-and-effect relationships is essential for predicting biological outcomes. The protein is degraded by the proteasome occurs because of specific molecular interactions, thermodynamic principles, or regulatory mechanisms that govern this biological process. The other options (The protein refolds by spontaneous diffusion, The protein aggregates and remains inside the cell, and The protein is stored for later refolding) describe either opposite eff

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4

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Protein

Which amino acid sequence is most resistant to degradation?

A sequence rich in Proline, Glutamate, Serine, and Threonine is the scientifically accurate answer to this question. Within the study of Protein Degradation, this concept is well-established through extensive research and is documented in standard scientific literature. The specific properties, mechanisms, or characteristics of A sequence rich in Proline, Glutamate, Serine, and Threonine directly address what is being asked. Among the other options, A sequence rich in Arginine and Lysine, A sequence rich in Methionine and Cysteine, and A sequence rich in Phenylalanine and Tyrosine do not corre

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4

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Protein

Which of the following describes the role of proteases in protein degradation?

They break peptide bonds to produce smaller peptides or amino acids is the accurate answer because it correctly identifies the biological function or role described in this question. In Protein Degradation, understanding the specific functions of molecules, enzymes, or structures is fundamental. They break peptide bonds to produce smaller peptides or amino acids fulfills this particular biological role through its specific structural properties, biochemical activity, or physiological mechanism. The other options (They synthesize proteins from amino acids, They add phosphate groups to proteins,

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4

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