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Questions and Answers
What is the function of a chaperone in the protein cycle after translation?
What is the function of a chaperone in the protein cycle after translation?
What process involves attaching several ubiquitin molecules to a protein?
What process involves attaching several ubiquitin molecules to a protein?
What can trigger an enzyme to adopt a catalytically active conformation?
What can trigger an enzyme to adopt a catalytically active conformation?
Which event is most likely to occur during the processing phase of the protein cycle?
Which event is most likely to occur during the processing phase of the protein cycle?
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What happens during the degradation phase of the protein cycle?
What happens during the degradation phase of the protein cycle?
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What determines the tertiary structure of a protein?
What determines the tertiary structure of a protein?
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What is the primary factor in favoring the formation of alpha helices?
What is the primary factor in favoring the formation of alpha helices?
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Which type of protein has a structure that is nearly entirely a-helical?
Which type of protein has a structure that is nearly entirely a-helical?
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How is a beta-pleated sheet stabilized?
How is a beta-pleated sheet stabilized?
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What defines the secondary structure of a protein?
What defines the secondary structure of a protein?
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Study Notes
Protein Cycle
- Translation: synthesis of a polypeptide chain on a ribosome (RER) with its primary structure determined by an mRNA template
- Folding/Refolding: nascent polypeptide folds into its native conformation with the assistance of chaperones to prevent misfolding
Processing
- Accompanied by processing events such as:
- Proteolytic cleavage of an N-terminal leader sequence
- Formation of disulfide bonds (S-S)
Covalent Modification
- Removing unneeded polypeptides
- Attaching a fatty acid molecule, for example
Translocation
- Modified peptide is translocated to a membrane
- Examples: hemoglobin for the circulatory system
Activation/Catalysis
- Binding an allosteric effector triggers a catalytically active conformation
- Enzymes are involved in this step
Aging
- Proteins become damaged by chemical attacks, deamidation, or denaturation
Ubiquitination
- Proteins may be labeled by the covalent attachment of several ubiquitin molecules (Ub)
Degradation
- Ubiquitinated protein is degraded to its component amino acids (reused for new protein synthesis)
- Proteasome is involved in this step
Four Levels of Protein Structure
- Primary Structure: sequence of amino acids and disulfide links
- Secondary Structure: formed by hydrogen bonding, examples: alpha helix and beta-pleated sheet
- Tertiary Structure: complete 3-D conformation of a protein
- Quaternary Structure: association of two or more polypeptide chains
Protein Structure Examples
- Alpha Helix:
- Produces a helical coiling of the peptide backbone
- R-groups lie on the exterior of the helix and perpendicular to its axis
- Approximately 11 amino acids long (3.6 per complete turn)
- DELIMA (amino acids) favor alpha helix formation
- GP favors disruption of the helix
- Beta-Pleated Sheet (B-Sheet):
- Composed of 2 or more regions of stretches of at least 5-10 amino acids
- Stabilized by H-bonding between amide N's and carbonyl C's
- Found in both fibrous and globular proteins
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Description
This quiz covers the protein cycle involving translation on a ribosome, folding with the help of chaperones, and processing events like proteolytic cleavage. Test your knowledge on how polypeptides are synthesized, folded, and processed during protein synthesis.
