Topic 15: Protein Catabolism

• Protein: large organic molecules that include cellular enzymes and many strutures
  • Amino acids: subunits that make up a protein; consist of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur
  • amino acids bond together by peptide bonds forming a small chain, peptide, or a larger molecule, polypeptide.
• Bacteria can hydrolyze peptides or polypeptides to release amino acids
  • Used as carbon or energy souce when carbohydrates are not available
  • Amino acids are primarily used in anabolic reactions
• Gelatin, large protein, are hydrolyzed by exoenzymes, and the smaller products of hydrolysis are transported into the cell
  • Hydrolysis of gelatin can be demonstrated by growing bacteria in nutrient gelatin
  • Nutrient gelatin must be incubated at room temperature; will liquify in incubator and give a false positive.
• Urea is a waste product of protein digestion and is excreted in the urine.
  • Urease liberates ammonia from urea
  • Presence of urease used to ID bacteria
• Urea agar contains peptone, glucose, urea, and phenol red.
  • Bacteria will urease will produce ammonia; increases pH turning indicator fuchsia
• Before amino acid can be used as carbon or energy source, the amino group must be removed.
  • Deamination: removal of an amino group
  • Amino group is converted to ammonia which is excreted fromm the cell
  • Deamination results in formation of organic acid
  • Deamination of amino acid phenylalanine can be detected by forming a colored ferric ion complex with the resulting acid.
  • Deamination can also ascertained by testing for the presence of ammonia using Nessler’s reagent, which turns deep yellow in presence of ammonia.
• Amino acids can be decarboxylated yielding products that cen be used for synthesis of other cellular components.
  • Decarboxylation: removal of carbon dioxide from an amino acid.
  • Presence of a specific decarboxylase enzyme results in the breakdown of the amino acid with the formation of the corresponding amine, liberation of carbon dioxide, and a shift in pH to alkaline.
  • Media for decarboxylase reactions consist of glucose, nutrient broth, a pH indicator, and the desired amino acid
• Some bacteria liberate hydrogen sufide (H2S) from the sulfur-containing amino acids: cystine, cystein, add methionine
  • H2S can be produced from reduction of inorganic compounds such as thiosulfate.
  • To detect H2S production - a heavy metal salt containing ferrous ion (Fe 2+) is added to nurtient culture medium
  • When H2S produced - black precipitate forms
• The ability of some bacteria to convert the amino acid tryptophan to indole or a blue compound indigo is a useful diagnostic tool.
  • Indole test is performing by inoculating a bacterium into tryptone broth and detecting indole by the addition of Kovac’s reagent.
  • Motility indole ornithine (MIO) agar used - detects motility, indole production, and ornithine decarboxylase activity

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