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Questions and Answers
What is the primary function of proteins in living organisms?
Which part of the amino acid structure gives each amino acid its unique properties?
How are amino acids in a protein chain linked together?
What distinguishes the 20 different amino acids from one another?
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What is the simplest amino acid, and what is its R group?
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Which of the following amino acids has the one-letter code 'W'?
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What defines the primary structure of a protein?
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Which secondary structure involves a right-handed spiral arrangement of amino acids?
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What is the role of the -C=O and -NH groups in the α-helix structure?
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Which structure is NOT considered a common secondary structure in proteins?
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Study Notes
Introduction to Proteins
- Proteins are essential to life and are involved in many vital functions
- Proteins are composed of amino acids linked by peptide bonds
- Amino acids are composed of an amino group (-NH2), a carboxyl group (-COOH), and a central carbon atom.
- The side chain, known as the "R group," determines the identity of each amino acid.
Amino Acid Structure
- There are 20 different amino acids that make up proteins.
- Glycine is the simplest and smallest amino acid, with a hydrogen (H) as its R-group.
Structural Organization of Proteins
- Proteins have four levels of structural organization: primary, secondary, tertiary, and quaternary.
Primary Structure
- The primary structure of a protein is the linear sequence of amino acids.
- The primary structure is determined by the genetic code.
Secondary Structure
- Secondary structure refers to the local folding of the polypeptide chain.
- The secondary structure is stabilized by hydrogen bonds between the backbone atoms.
- There are three types of secondary structures: α-helix, β-sheet, and β-turns.
α-helix
- The α-helix is a right-handed spiral structure.
- The -C=O group of each amino acid is hydrogen bonded to the –NH group of the amino acid four residues ahead.
- The R-groups of amino acids are projected outside of the spiral.
β-sheet
- The β-sheet is a more stable form of secondary structure.
- It is formed between two or more polypeptide chains that are placed parallel or antiparallel to each other.
- The polypeptide backbones are linked by hydrogen bonds between the -CO and -NH groups.
- The R-groups are alternately projected above and below the sheet.
β-turn
- A β-turn is composed of four amino acids.
- The C=O group of the first amino acid forms a hydrogen bond with the N–H group of the fourth amino acid.
- Glycine and proline are frequently found in β-turns.
Tertiary Structure
- The tertiary structure is the three-dimensional shape of a protein.
- It is formed by interactions between different secondary structures.
- Interactions include:
- Hydrogen bonds
- Hydrophobic interactions
- Ionic bonds
- Disulfide bonds
Quaternary Structure
- The quaternary structure is the three-dimensional arrangement of two or more polypeptide chains.
- These polypeptide chains are called subunits.
- The subunits can be identical (homodimers, homotrimers, homopolymers) or different (heteropolymers).
From Gene to Protein
- Genes contain the instructions for making proteins.
DNA
- DNA is a double-stranded nucleic acid that carries the genetic instructions.
- DNA is organized into a superhelical structure called the nucleoid.
- In eukaryotes, DNA is contained within the nucleus and organized into chromosomes.
Chromosomes
- Chromosomes contain both DNA and protein.
- The proteins in eukaryotes are divided into two classes: histones and nonhistone chromosomal proteins.
RNA
- RNA is a single-stranded nucleic acid.
- It is synthesized from DNA by RNA polymerases.
Ribosomes
- Ribosomes are the sites of protein synthesis.
- Ribosomes are made up of ribosomal RNA and proteins.
- Ribosomes consist of two subunits: a larger subunit and a smaller subunit.
Transfer RNA (tRNA)
- tRNA carries amino acids to the ribosome.
- Each tRNA has an anticodon that is complementary to a codon on mRNA.
Translation
- The process of translating mRNA into protein.
- Translation occurs in three stages: initiation, elongation, and termination.
Initiation
- The initiator tRNA carrying methionine binds to the AUG start codon on the mRNA at the ribosome’s P site.
- Initiation factors (IFs) facilitate the binding of the small ribosomal subunit to mRNA.
- The large ribosomal subunit joins the complex.
Elongation
- The ribosome moves along the mRNA, adding amino acids to the growing polypeptide chain.
- Each amino acid is added to the chain by a specific tRNA molecule that is complementary to the codon on the mRNA.
- The amino acids are linked together by peptide bonds.
Termination
- The ribosome encounters a stop codon on the mRNA, signaling the end of translation.
- The completed polypeptide chain is released from the ribosome.
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Description
Explore the fundamental aspects of proteins, including their essential functions, amino acid composition, and structural organization. This quiz covers primary, secondary, tertiary, and quaternary protein structures, highlighting the role of amino acids in their formation.
