User:Andy Maloney/Kinesin & Microtubule Page/Surface passivation/Dairy Chemistry and Biochemistry Chapter 4

Disclaimer

These are my notes. Please read the book before reading my notes.

Book

Dairy Chemistry and Biochemistry

Notes

• "The high heat treatments to which many milk products are subjected are possible only because of the exceptionally high heat stability of the principal milk proteins, the caseins."
• In the late 1890's, Hammarsten was able to separate milk into caseins and whey by acidification at 4.6 and 30 ˚C which is the isoelectric pH of milk.
• Whey does not precipitate out of milk when the pH is adjusted to 4.6.
• Milk can be heated at 100˚C for 24 hours without coagulation.
• Caseins contain on average 0.85% phosphorus.
  • The phosphate groups on casein are responsible for binding calcium by esterification of calcium phosphate via the hydroxyl group of serine.
  • Casein is produced in the mammary gland and is found nowhere else in nature.
• New Zealand in 1998 was the principle producer of casein. The book wrote "principal".
• Ha! Inorganic colloidal calcium phosphate dissolves when milk is brought to a pH of 4.6. This means that isoelectric casein is essentially free of calcium phosphate! But, it still has to have some otherwise casein micelles won't form.
  • There is a recipe to do this.
    • Acidify milk at pH 4.6 and 2˚C. Usually 1N HCl.
    • Let it stand for about 30 minutes.
    • Warm to 30 - 35˚C.
    • Filter or centrifuge and wash with water to get rid of lactose and salts.
• The s on the alpha caseins means sensitive.
• alpha-s1, alpha-s2, beta, kappa caseins are approximately 37, 10, 35, 12% of whole casein, respectively.
• Each casein is phosphorylate to a characteristic level (which is variable)
  • «s1
    • 8, occasionally 9
  • «s2
    • 10, 11, 12 or 13
  • ß
    • 5, occasionally 4
  • K
    • 1, occasionally 2 or perhaps 3
• "All the caseins have a high content (35-45%) of apolar amino acids (Val, Leu, He, Phe, Tyr, Pro) and would be expected to be poorly soluble in aqueous systems, but the high content of phosphate groups, low level of sulphur containing amino acids and high carbohydrate content in the case of kappa-casein offset the influence of apolar amino acids. The caseins are, in fact, quite soluble: solutions containing up to 20% protein can be prepared in water at 80-90°C. High temperatures are necessary to offset high viscosity, which is the limiting factor in preparing casein solutions. The high viscosity is a reflection of the high water binding capacity (WBC) of casein, i.e. about 2.5 g H2Og^-1 protein. Such high WBC gives casein very desirable functional properties for incorporation into various foods, e.g. sausage and other comminuted meat products, instant desserts, synthetic whipping creams, etc., and large quantities of casein are used commercially for these purposes."
  • Yeah!
• "All the caseins have a very high proline content: 17, 10, 35 and 20 Pro residues per mole of asl-, as2-, beta- and kapp-caseins, respectively (out of a total

of 199, 207, 209 and 169 residues, respectively). Such high levels of proline result in a very low content of alpha-helix or beta-sheet structures in the caseins. The caseins are, therefore, readily susceptible to proteolysis without prior denaturation by, for example, acid or heat. Perhaps this is an important characteristic in neonatal nutrition."

• "As a group, the caseins are deficient in sulphur amino acids which limits their biological value (80; egg albumen = 100). alpha-s1- and beta-caseins contain no cysteine or cystine while alphs-s2- and kappa-caseins have two cysteine residues per mole, which normally exist as intermolecular disulphides.
• The caseins, especially as2-casein, are rich in lysine, an essential amino acid in which many plant proteins are deficient. Consequently, casein and skim-milk powder are very good nutritional supplements for cereal proteins which are deficient in lysine. Owing to the high lysine content, casein and products containing it may undergo extensive non-enzymatic Maillard browning on heating in the presence of reducing sugars (Chapter 2).
• The organic phosphates on caseins are attached to serines and occur in clusters.
• They say that a phosphorylated serine group can serve as a recognition site.
• There is relatively little secondary or tertiary structure to the caseins.
• Ha! Small hydrophobic peptides tend to be bitter.
• alpha caseins precipitate at concentrations higher than 4 mM calcium.
• At 400 mM calcium, beta casein will precipitate above 18˚C.
• If you remove more than 70% of the colloidal calcium phosphate in milk, casein micelles will disintegrate.
• Removal of the colloidal calcium phosphate results in the caseins having different properties. I think this means different properties in milk.
• Huh, it also says that colloidal calcium phosphate if chelated by EDTA or EGTA will cause the micelles to disintegrate. This is curious because we use EGTA in our PEM.
• The potential on the surface of the casein micelle is about -20 mV.
• Caseins form rather viscous solutions.

Take home

I'm curious about getting rid of the EGTA in our PEM just to see if I can get a gliding motility assay working. It could be that when using whole casein as your surface blocker, the EGTA denatures the casein by chelating the calcium phosphate in it.