BIOCHEM MIDTERMS

• Proteins


  Compounds that are the third of the bioorganic classes of molecules, with a wide variety of functions in the human body
• A typical human cell contains about 9000 different kinds of proteins, and the human body contains about 100,000 different proteins
• Proteins are needed for the synthesis of enzymes, certain hormones, and some blood components; for the maintenance and repair of existing tissues; for the synthesis of new tissue; and sometimes for energy
• Next to water, proteins are the most abundant substances in nearly all cells — they account for about 15% of a cell's overall mass and for almost half of a cell's dry mass
• All proteins contain the elements carbon, hydrogen, oxygen, and nitrogen; most also contain sulfur
• The average nitrogen content of proteins is 15.4% by mass

  
• Protein


  A naturally occurring, unbranched polymer in which the monomer units are amino acids
• Amino acid

  An organic compound that contains both an amino (-NH2) group and a carboxyl (-COOH) group
• The amino acids found in proteins are always α-amino acids
• Categories of standard amino acids

  • Nonpolar
  • Polar neutral
  • Polar acidic
  • Polar basic

  
• Nonpolar amino acid


  An amino acid that contains one amino group, one carboxyl group, and a nonpolar side chain
• Polar neutral amino acid


  An amino acid that contains one amino group, one carboxyl group, and a side chain that is polar but neutral
• Polar acidic amino acid


  An amino acid that contains one amino group and two carboxyl groups, the second carboxyl group being part of the side chain

  
• Polar basic amino acid


  An amino acid that contains two amino groups and one carboxyl group, the second amino group being part of the side chain

  
• In pure form, amino acids are white crystalline solids with relatively high decomposition points
• Most amino acids are not very soluble in water because of strong intermolecular forces within their crystal structures
• Zwitterion


  A molecule that has a positive charge on one atom and a negative charge on another atom, but which has no net charge
• In solution and also in the solid state, α-amino acids exist as zwitterions

  
• Cysteine


  The only standard amino acid that has a side chain that contains a sulfhydryl group
• Cysteine, in the presence of mild oxidizing agents, readily dimerizes to form a cystine molecule

  
• Peptide


  An unbranched chain of amino acids, each joined to the next by a peptide bond
• A compound containing two amino acids is specifically called a dipeptide; three amino acids joined together in a chain constitute a tripeptide; and so on

  
• The name oligopeptide is loosely used to refer to peptides with 10 to 20 amino acid residues, and the name polypeptide is used to refer to longer peptides
• Peptide bond

  The covalent bond between the carboxyl group of one amino acid and the amino group of another amino acid
• Peptides that contain the same amino acids but in different order are different molecules (constitutional isomers) with different properties
• The number of isomeric peptides possible increases rapidly as the length of the peptide chain increases
• Amino acid residue


  The portion of an amino acid structure that remains, after the release of H2O, when an amino acid participates in peptide bond formation as it becomes part of a peptide chain
• Oxytocin and vasopressin


  The two best-known peptide hormones, both produced by the pituitary gland
• Enkephalins


  Pentapeptide neurotransmitters produced by the brain itself that bind at receptor sites in the brain to reduce pain
• The action of the prescription painkillers morphine and codeine is based on their binding at the same receptor sites in the brain as the naturally occurring enkephalins
• Glutathione


  The tripeptide (Glu–Cys–Gly) that is present in significant concentrations in most cells and functions as an antioxidant, protecting cellular contents from oxidizing agents
• Protein


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


  A protein in which only one peptide chain is present
• Simple protein


  A protein in which only amino acid residues are present
• Conjugated protein


  A protein that has one or more non-amino acid entities present in its structure in addition to one or more peptide chains
• Prosthetic group


  A non-amino acid group present in a conjugated protein
• Primary protein structure


  The order in which amino acids are linked together in a protein
• Insulin, the hormone that regulates blood-glucose levels, was the first protein for which primary structure was determined in 1953
• Secondary protein structure


  The arrangement in space adopted by the backbone portion of a protein, including alpha helix and beta pleated sheet

  
• Alpha helix


  A protein secondary structure in which a single protein chain adopts a shape that resembles a coiled spring, with the coil configuration maintained by hydrogen bonds