MILK BIOSYNTHESIS Part 4: Protein

IMPORTANCE OF PROTEINS

Protein helps make milk nature’s most nearly perfect food

In dairy cows, there is more than a 5-fold increase in energy and protein requirement from late gestation to lactation

Protein synthesis is a highly active and energetically costly process, with only a minor part of the synthetic machinery apparently being used for production of milk proteins

Amount and composition of proteins in milk is largely determined by the genetics of the animal

Difficult to change through nutrition

WHAT ARE MILK PROTEINS?

Milk Protein

Non-casein (Whey) (20%)

β-lactoglobulinSerum albumin,

immunoglobulimsα-lactalbumin

Casein (80%)

CASEIN MICELLES

Whey proteins exist as individual units dissolved in water

Casein exists as a micelle

Casein phosphate groups covalently bound to casein molecules are involved in binding Ca This [casein-PO4- Ca++ - PO4-casein] structure is key to micelle formation.

The casein micelle functions as a source of nutrients for the neonate: supplying amino acids, calcium and phosphate.

Casein micelles are 0.04 to 0.3 µm in diameter Fat globules are ~ 1 µm in homogenized milk

Porous

Stable but dynamic structures that do not settle out of solution

HOW ARE MILK PROTEINS MADE?

Under direction of gene transcription (DNA to mRNA) and translation (mRNA to protein)

Transcription = generation of mRNA

Translation = assembly of amino acids by ribosomes; tRNA brings amino acids to build protein

HOW ARE PROTEINS MADE?

Videos: https://www.youtube.com/watch?v=gG7uCskUOrA

https://www.youtube.com/watch?v=mbwgM_wUZxo

https://www.youtube.com/watch?v=KWQyURdNlDk&nohtml5=False

HOW ARE MILK PROTEINS MADE?

All major milk proteins (except serum albumin and immunoglobulins) synthesized by epithelial cells in the mammary gland from amino acids extracted from blood

Mammary-specific genes turn on when milk synthesis is active

α-lactalbumin, caseins

Need amino acids and energy (5 ATP per peptide bond)

AMINO ACIDS TO PROTEINS

Absorbed through basal membrane of cell

Inside the cell, covalently bound together to form proteins at the polysomes (poly-ribosomes) on the rough endoplasmic reticulum

Includes proteins to be secreted (e.g. casein, ß-lactoglobulin, and a-lactalbumin)

Includes membrane bound proteins (e.g. proteins involved in cell-cell contacts and membrane bound enzymes)

Newly synthesized proteins transferred from the RER to the Golgi apparatus to be processed for transport out of the cell

SECRETORY PROCESS

Undergo post-translational processing in the Golgi

Casein micelle formed in Golgi from the casein molecules, calcium, and phosphorous

Proteins that remain in the cell are synthesized by the ribosomes in the cytoplasm (all cellular enzymes and structural proteins like keratin)

Milk proteins and lactose are transported to apical cell membrane via secretory vesicles that bud off of the Golgi

Secretory vesicles make their way to the apical membrane by microtubules (think: cellular scaffolding)

Secretory vesicles fuse with inner surface of apical membrane, making an opening through which vesicle contents are released into the lumen

https://www.youtube.com/watch?v=mbwgM_wUZxo

PROTEIN DEGRADATION/PROTEOLYSIS

Proteins can be degraded by enzyme action or light exposure

Protease action can be desirable

In yogurt and cheese processing, bacteria with desirable proteolytic properties are added to the milk

Undesirable degradation results in milk with off-flavors and poor quality

Methionine and cystine are sensitive to light and may be degraded with exposure to light (results in off-flavor and loss of nutritional quality for these 2 amino acids)

Perception that protein degradation occurs during pasteurization is a driving factor for consumers drinking milk

PASTEURIZATION

Caseins are stable to heat treatment

High temperature treatments can cause interactions between casein and whey proteins that affect the functional but not the nutritional properties (e.g. ß-lactoglobulin can form a layer over the casein micelle that prevents curd formation in cheese

Whey proteins are more sensitive to heat than the caseins

Higher heat treatments may cause denaturation of ß-lactoglobulin, which is an advantage in the production of some foods (yogurt) because of the ability of the proteins to bind more water

Nutritional content of proteins is not changed during normal pasteurization process

RAW MILK

California profile: younger than 40, male, Hispanic and to have less than a high school education

Michigan profile: well-educated adult in his/her late 20s who typically lives in a rural area

Ages of raw-milk drinkers, which included family members, ranged from less than one year to 75

Nationally, 50 percent of the dairy’s raw-milk customers are well-educated moms between 20 and 45 years old

Average number of miles a respondent traveled out of his/her way to buy raw milk was 24.2 miles (4.1times per month)

WHY DO PEOPLE DRINK RAW MILK?

86% said supporting local farms

84% said taste

77% said “holistic health benefits”

57% said they don’t feel processed milk is safe

A majority of the study’s raw-milk drinkers shared their beliefs that raw milk was beneficial for relieving digestive problems, intestinal diseases, allergies, heart disease, neurologic disease, acne, and cancer

Others claimed that when they drink pasteurized milk, they experience lactose intolerance, which doesn’t happen when they drink unpasteurized milk

http://www.marlerblog.com/uploads/image/120229%20Motivation%20Michigan%20Raw%20Milk%20article%5B1%5D.pdf

WHY DO PEOPLE DRINK RAW MILK?

Study out of Stanford Medical School (financed by raw milk advocates) raised questions about how widespread lactose intolerance really is, but found that raw milk did not confer any benefit over pasteurized milk in relieving symptoms of lactose intolerance

https://www.youtube.com/watch?v=rXndQT4v90A

http://www.realrawmilkfacts.com/ (Mari Tardiff)

More about lactose to come in the next lecture…

http://grist.org/food/2010-11-01-raw-milk-mystery-new-stanford-study-indicates-it-doesnt-reduce/

QUESTIONS?