Unit 9: Organic Composition of Food Part 3 - CH.11 Proteins: Amino Acids and Peptides CH. 12 Enzymes: The Protein Catalyst

Structure of Protein Composed of the elements carbon, hydrogen, oxygen & nitrogen Monomer of protein – amino acid Monomer: a molecule that will bind to other identical molecules to form a polymer

Amino Acids Have 3 parts to their structure Amine group, carboxyl group, and variable side chain 20 different amino acids in the human body

Amino Acids Continued… The carboxyl group acts as an acid, while the amine group acts as a base, making the amino acids readily combine. 2 amino acids combine and water is released (dehydration synthesis). The bond between amino acids is called a peptide bond.

Amino Acids Continued… 2 amino acids = dipeptide 3 or more amino acids = polypeptide Shortest known protein is 2o amino acids long. Most commonly proteins have 100-500 amino acids. Amine end Carboxyl end

Amino Acids & Nutritional Use Classified by their nutritional use & nature of their side chains 9 essential (not made by the body) amino acids. These must come from the diet. Body cannot grow new tissue or maintain health without these amino acids.

Amino Acids & Nutritional Use Continued… Foods that contain all 9 essential amino acids – complete proteins. Mainly come from animal sources Eggs, milk, fish, poultry, meats, soybeans (only plant source) Incomplete Proteins – lack 1 or more essential amino acids needed for human growth. Usually plant sources of protein (grains and vegetables).

Amino Acids & Nutritional Use Continued… Vegetarians must coordinate their choices of incomplete proteins to ensure they are getting all the nutrients needed for growth and development. Ex) Whole-wheat bread & peanut butter, Rice and red beans

Amino Acids & Side Chain Classification Side chains can be uncharged polar, nonpolar, positively or negatively charged. Affects how the protein is shaped and its function. Nonpolar = non-soluble in water Neutral Polar = form hydrogen bonds easily, and water soluble Positively or Negatively Charged = enables some proteins to act as buffers Buffer= a solution that resists changes in pH when acid or alkali is added to it. Buffers typically involve a weak acid or alkali together with one of its salts.

Protein Structures Primary Structure – order of amino acids in the chain

Protein Structures Continued… Secondary Structure – the bonding between amino acid side chains Helix: repeating coil (slinky). Very stable structure. Random coil: no specific shape (tangled slinky). Pleated Sheet: paper fan shape.

Protein Structures Continued… Tertiary Structure – 3D structure of the amino acid chain Globular: do NOT form links between other proteins Examples) Caseins in milk, albumin in egg white Fibrous: strong & create protein network Examples) Collagen (skin), Keratin (hair)

Molecular Interactions of Proteins More hydrogen bonds = more water soluble Example) Albumin (in egg whites) readily dissolves in water More Disulfide cross-links (covalent bonds that form between side chains containing sulfur) = more stable protein Ex) Breakdown of disulfide bond causes “rotten” odor in spoiled eggs

Molecular Interactions of Proteins Continued… Hydrophobic proteins: nonpolar side chains repel water Example) Caseins found in milk. Important for forming curds in cheese making. A by- product of cheese making in Whey - protein that is used as an additive in commercially processed foods.

Color Changes of Protein Pigments Helps food scientists control color changes in meat during storage and processing. Important because consumers believe that bright red color means meat is fresh.

Color Changes of Protein Pigments Continued… Myoglobin is the iron-binding pigment that provides the color. When oxygen is bound to myoglobin, it is a bright red color. When oxygen is not present, the protein is purplish. After prolonged exposure to oxygen, meat turns brown.

Color Changes of Protein Pigments Continued… Bright red does not mean the meat is fresh. Green or yellow discolorations are a sign of bacterial growth. Nitrites are used to preserve meats during curing, helping maintain the pink/red color. Consumers commonly mistake red juice from meat as blood. This is really water with dissolved myoglobin.

Denaturation of Proteins Denaturation – any change in shape of a protein molecule without breaking the peptide bond. Denaturation is SOMETIMES reversible. Ex) beating egg whites until foamy. Letting the foam sit will allow it to return back to a liquid.

Denaturation of Proteins Continued… USUALLY NOT reversible. By coagulation. Ex) Milk curdling to form cheese Ex) Cooking eggs causes them to solidify