PROTEIN FOLDING

TRANSLATION OF mRNA TO PROTEINS

  • Process Overview: The synthesis of proteins from mRNA involves several steps, known as translation.

    • 1. mRNA binds to the ribosome.

    • 2. The ribosome assists a tRNA (transfer RNA) carrying a specific amino acid to bind to the mRNA strand.

    • Codon-Anti-codon Interaction: Codons on mRNA bind with complementary anti-codons on tRNA via complementary base pairing.

    • 3. A peptide bond is formed between the amino acids during this assembly process, following which the ribosome moves over by one codon on the mRNA.

    • 4. The above steps (2 & 3) are repeated until a STOP codon is read, which leads to separation among the mRNA, ribosome, and the polypeptide chain.

    • Reusability: It is crucial to note that the mRNA, ribosome, and tRNA can be reused multiple times in different translation cycles.

PROTEIN STRUCTURE

  • The folding of a polypeptide to form a protein with a unique three-dimensional shape is determined primarily by its sequence of amino acids.

  • Factors Influencing Structure:

    • Primary Structure: The order of amino acids in the polypeptide chain determines the potential for folding into secondary and tertiary structures.

    • Secondary Structure: Involves the local folded structures that form within the polypeptide chain, such as alpha helices and beta-pleated sheets, held together by hydrogen bonds.

    • Tertiary Structure: The overall three-dimensional shape of a polypeptide, determined by interactions such as disulphide bridges, hydrogen bonds, and ionic interactions between side chains (R-groups) of the amino acids.

    • Quaternary Structure: Refers to the structure formed when two or more polypeptide chains (tertiary structures) bond together to form a functional protein. Common examples include hemoglobin, which is crucial for oxygen transport in blood.

  • Significance of 3D Shape:

    • The specific three-dimensional shape of a protein is critical for its function.

    • Proteins often have shapes complementary to their target, which can be antibodies and antigens, enzymes and substrates, or receptor proteins and hormones.

TYPES OF PROTEINS

  • Hormones:

    • They act as chemical messages traveling throughout an organism and are typically protein-based.

    • Example: Estrogen, produced in the ovaries and pituitary gland, binds to its receptor protein in the endometrium causing it to thicken and plays a role in libido and reproductive functions.

    • Plants produce gibberellin, a growth hormone that can be applied to enhance plant growth.

    • Note: Not all hormones are proteins—this distinction is important in the study of homeostasis.

  • Cell Receptors:

    • Embedded in the cell membrane, these proteins facilitate communication between cells. Hormones or chemicals with complementary shapes bind to these receptors to stimulate cellular responses.

    • Example: Different cells have specific receptors designed for particular functions, highlighting the principle of gene expression where only relevant genes are expressed in a given cell type.

  • Antibodies:

    • Specialized proteins that identify and mark foreign bodies, including microbes and toxins, for action by the immune system.

  • Enzymes:

    • Serve as catalysts in biochemical reactions, significantly accelerating the rate of reactions crucial for various cellular processes.

AMINO ACIDS

  • List of standard amino acids involved in protein synthesis:

    • Pro: Proline

    • Ala: Alanine

    • Arg: Arginine

    • Asn: Asparagine

    • Asp: Aspartic acid

    • Cys: Cysteine

    • Gln: Glutamine

    • Glu: Glutamic acid

    • Gly: Glycine

    • His: Histidine

    • Ile: Isoleucine

    • Leu: Leucine

    • Lys: Lysine

    • Met: Methionine

    • Phe: Phenylalanine

    • Ser: Serine

    • Thr: Threonine

    • Trp: Tryptophan

    • Tyr: Tyrosine

    • Val: Valine

STRUCTURAL FORMS OF PROTEINS

  • Primary Protein:

    • Defined as a simple polypeptide chain, containing necessary amino acids in the specific order but lacks the correct functional shape.

  • Secondary Protein:

    • Involves initial folding patterns such as beta-pleated sheets and alpha-helices, driven by attractions (like hydrogen bonding) among specific amino acids.

  • Tertiary Protein:

    • Achieves a further degree of folding through interactions (like disulfide bridges and ionic bonding) leading to a three-dimensional shape crucial for functionality.

  • Quaternary Protein:

    • Refers to the assembly of multiple tertiary protein subunits into a single functional unit, exemplified by hemoglobin, a protein that efficiently transports oxygen in the blood.

OVERVIEW OF PROTEIN STRUCTURES

  • Summary:

    • Primary Protein: Amino acids held together by peptide bonds.

    • Secondary Protein: Structural folding due to attractive forces and hydrogen bonding.

    • Tertiary Protein: Extensive folding owing to various interactions (disulfide bridges, ionic interactions).

    • Quaternary Protein: Binding between two or more tertiary proteins (no specific bonding name necessary).

EXAMPLES AND APPLICATIONS

  • Irbesartan: A medication used to lower high blood pressure by blocking angiotensin II receptors in blood vessels, suggesting that irbesartan's shape is complementary to angiotensin II molecules, allowing it to bind effectively and inhibit vasoconstriction.

  • T-cell Function: T-cells, a type of white blood cell, bind to proteins on diseased cells but do not target healthy body cells, as T-cell receptors are specifically shaped to interact with disease-causing cell proteins, underscoring the importance of protein shapes in immune response and specificity.