قناة سنة ثانية بشري (دفعة 53)

Tertiary Structure • Describes the three-dimensional folding of a single polypeptide chain. • Determined by the amino acid sequence (primary structure). • Stabilized by interactions between side chains (R-groups): • Hydrogen bonds • Disulfide bonds • Ionic bonds • Hydrophobic interactions • Composed mainly of α-helices, β-sheets, or both. • These structures maximize internal hydrogen bonding and minimize interaction with water. • Proteins may contain domains, which are compact, stable regions with a hydrophobic core and hydrophilic surface. • Each domain usually has a specific function, such as an active site. Quaternary Structure • Present only in proteins composed of two or more polypeptide chains. • Formed by association of multiple tertiary subunits. • Subunits are held together by non-covalent interactions (hydrogen bonds, ionic bonds, hydrophobic interactions). • Subunits may be identical or non-identical. • Examples: • Dimer: 2 subunits • Tetramer: 4 subunits (e.g. hemoglobin) • Proteins with a single polypeptide chain are called monomers or protomers. • The primary structure, encoded by DNA, determines all higher levels of protein structure.

β-Sheet • Another form of secondary structure where the polypeptide chain is fully extended, not coiled. • Composed of two or more polypeptide chains or segments of the same chain. • Can be parallel or antiparallel. • Stabilized by hydrogen bonds perpendicular to the polypeptide backbone. • Hydrogen bonds may be interchain or intrachain. • Common in fibrous proteins such as keratin and collagen, and also found in some globular proteins like lysozyme. • Alzheimer’s disease is associated with abnormal deposition of β-pleated amyloid proteins in the brain. β-Turn (Reverse Turn) • A short secondary structure that connects adjacent strands of antiparallel β-sheets. • Usually consists of four amino acids. • Commonly contains proline (causes bending) and glycine (small size). • Stabilized by hydrogen bonds.

Secondary Protein Structure Secondary structure describes the local folding of the polypeptide chain stabilized by hydrogen bonds between peptide bonds. The main types are α-helix, β-sheet, and β-turn. α-Helix • A spiral (helical) structure. • Peptide bonds form the backbone, while R-groups extend outward. • Stabilized by intrachain hydrogen bonds between the carbonyl oxygen of one amino acid and the –NH of the 4th amino acid below it. • Each turn contains 3.6 amino acids. • The α-helix in proteins is right-handed and more stable. • Found abundantly in proteins such as hemoglobin and myoglobin. Amino acids that disrupt α-helix formation: • Proline (imino group causes a kink). • Charged amino acids (Asp, Glu, Lys, His) due to electrostatic repulsion. • Bulky amino acids (e.g. tryptophan) if present in large amounts. • β-branched amino acids (e.g. valine) when abundant.

Protein Structure and Bonds – Summary Proteins are made of amino acids linked together by peptide bonds forming a polypeptide chain. The structure and function of proteins are maintained by different types of bonds and interactions. 1. Bonds that Maintain Protein Structure • Peptide bond: • Covalent bond between the carboxyl group (-COOH) of one amino acid and the amino group (-NH₂) of the next. • It is an amide bond, planar, rigid, partially double, strong, and polar but uncharged. • Typically in trans configuration and can participate in hydrogen bonding. • Disulfide bond (-S-S-): • Covalent bond formed between the sulfur atoms of cysteine residues. • Stabilizes tertiary and quaternary structures by linking different parts of the same polypeptide or two separate polypeptides. • Hydrogen bond: • Weak interaction between hydrogen and electronegative atoms (O or N) in side chains or backbone. • Stabilizes secondary structures like α-helices and β-sheets and increases protein solubility. • Hydrophobic interaction: • Nonpolar side chains cluster inside the protein in aqueous environments. • Polar and charged side chains are usually on the surface. • Helps stabilize the tertiary structure. • Ionic bond (electrostatic interaction): • Interaction between negatively charged side chains (Asp, Glu) and positively charged side chains (Lys, Arg). • Contributes to tertiary and quaternary structure stability.

كملنا جزئيه ال amino acid كلهن شباب ♥️.

Amino Acid Analysis: 1. Acid hydrolysis: 6N HCl, 110°C, 24 h (destroy tryptophan; Asn → Asp, Gln → Glu) 2. Chromatography: Ion-exchange separation, detection by ninhydrin Peptide Sequencing: • N-terminal sequencing: Edman degradation, 2,4-dinitrofluorobenzene (Sanger reagent) • C-terminal sequencing: Hydrazine cleavage, carboxypeptidase • Polypeptide cleavage: • Enzymes: Trypsin (Arg/Lys), chymotrypsin (aromatic amino acids) • Chemical: Cyanogen bromide (cleaves after Met → homoserine lactone)

Physical Properties: 1. Melting point: >200°C 2. Optical activity: Chiral except glycine 3. UV absorption: Aromatic amino acids (Phe, Tyr, Trp, His) 4. Solubility: Water-soluble; cystine and tyrosine less soluble Chemical Properties: • Color reactions: Ninhydrin (purple, yellow for proline/hydroxyproline), fluorescamine • Chelation: Amino acids bind metals (e.g., glycine + Ca²⁺ → calcium diglycinate) • Acid-base properties: • Amphoteric (act as acid or base) • Can act as buffers, resist pH changes • Isoelectric point (pI): pH where net charge = 0 (zwitterion) Example: Alanine pI = (pKa₁ + pKa₂)/2 = (2.3 + 9.1)/2 = 5.7