Protein

• Proteins


  Naturally occurring, unbranched polymers in which the monomer units are amino acids
• Proteins are the most abundant molecules in the cells after water – account for about 15% of a cell's overall mass
• Characteristics of proteins


  • Elemental composition contains Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), and Sulfur (S)
  • Average nitrogen content of proteins is 15.4% by mass
  • Also present are Iron (Fe), phosphorus (P), and some other metals in some specialized proteins

  
• Polypeptide


  A protein in which at least 40 amino acid residues are present

  
• Several proteins with >10,000 amino acid residues are known

  
• Common proteins contain 400–500 amino acid residues
• Small proteins contain 40–100 amino acid residues
• Monomeric proteins


  • Contains one polypeptide chain
• Multimeric proteins


  • Contains 2 or more polypeptide chains
• Amino acid


  The building block of proteins. An organic compound that contains both an amino (−𝑁𝐻2) group and a carboxyl (−𝐶𝑂𝑂𝐻) group, and a side chain (−𝑅) attached to the same carbon

  
• Properties of amino acid side chains

  • Size
  • Shape
  • Charge
  • Acidity
  • Functional groups present
  • Hydrogen-bonding ability
  • Chemical reactivity

  
• Groups of standard amino acids

  • Non-polar amino acids
  • Polar neutral amino acids
  • Polar acidic amino acids
  • Polar basic amino acids

  
• Derived amino acids


  Nonstandard amino acids usually formed by an enzyme-facilitated reaction on a common amino acid after that amino acid has been incorporated into a protein structure
• Examples of derived amino acids


  • Cystine
  • Desmosine
  • Isodesmosine
  • Hydroxyproline
  • Hydroxylysine

  
• Nomenclature of amino acids


  Common names assigned, three letter abbreviations widely used
• Essential amino acids


  Amino acids needed for protein synthesis that must be obtained from dietary sources – adequate amounts cannot be synthesized in the human body
• Nine of the 20 standard amino acids are considered essential
• Complete protein


  Contains all the essential amino acids in the proper amounts
• Incomplete protein

  Low in one or more of the essential amino acids, usually lysine, tryptophan, or methionine

  
• Peptide

  An unbranched chain of amino acids, each joined to the next by a peptide bond
• Peptide bond


  The covalent bonds between amino acids
• Peptide nomenclature

  1. C-terminal amino acid residue keeps its full name

  2. All other amino acid residues have names that end in −𝑦𝑙

  3. Amino acid naming sequence begins at the N-terminal amino acid residue

  
• Peptide nomenclature examples


  • Val-Ser-Ala
  • Gly-Tyr-Leu-Val

  
• Biochemically important small peptides

  • Oxytocin and Vasopressin (small peptide hormones)
  • Enkephalins (small peptide neurotransmitters)
  • Glutathione (small peptide antioxidant)
  • Aspartame (small peptide artificial sweetener)
• Simple proteins


  Proteins in which ONLY amino acid residues are present
• Conjugated (complex) proteins

  Proteins that have one or more non-amino acid entities (prosthetic groups) present in their structure
• Aspartame


  It is a dipeptide (Asp-Phe) sold under trade names Equal and Nutrasweet
• Aspartame


  180x as sweet as sucrose
• Methods of classifying lipids

  • Based on Chemical Composition
  • Based on Shape and Structure
  • Based on Biochemical Function
• Types of simple proteins


  • Albuminoids (keratin in hair)
  • Albumins (egg white)
  • Globulin (antibodies)
  • Histones (chromatin in chromosomes)

  
• Types of conjugated proteins


  • Hemoglobin
  • Myoglobin
  • LDL
  • HDL
• Simple Proteins

  • A protein in which ONLY amino acid residues are present
• Conjugated (Complex) Proteins

  • A protein that has one or more non-amino acid entities (prosthetic groups) present in its structure
• Prosthetic group

  A non-amino acid entity present in the structure of conjugated proteins
• Fibrous Proteins

  • The polypeptide chains are arranged in long strands or sheets
  • Have long, rod-shaped or string-like molecules that can intertwine with one another and form strong fibers
  • Water-insoluble
  • Structural functions
• Globular Proteins

  • The polypeptide chains are folded into spherical or globular shapes
  • Water-soluble which allows them to travel through the blood and other body fluids to sites where their activity is needed
  • Dynamic functions
• Functions of proteins


  • Catalytic
  • Defense
  • Transport
  • Messenger
  • Contractile
  • Structural
  • Transmembrane
  • Storage
  • Regulatory
  • Nutrient
• Catalytic Proteins


  Proteins that act as biochemical catalysts, driving almost every chemical reaction in the body
• Enzymes


  Proteins that act as biochemical catalysts
• Defense Proteins


  Immunoglobulins (antibodies) that bind to foreign substances to help combat invasion of the body