written by Rebecca Green, MSc PDt, and David LaPierre, MD 2010
Proteins are extremely important biomolecules responsible for almost all cellular activity. The over 20,000 genes in the human genome mean there are over 20,000 proteins as well, but changes in protein isoforms and post-translational modifications results in over 100,000 functionally distinct proteins.
RDA |
AMDR |
|
infants |
1.5g/kg/day |
Not determinable |
1-3 years |
1.1g/kg/day |
5-20% |
4-13 years |
0.95g/kg/day |
10-30% |
4-18 years |
0.85g/kg/day |
10-30% |
19+ years |
0.8g/kg/day |
10-35% |
pregnant and breastfeeding |
1.1g/kg/day (pre-preg. weight) |
10-35% |
Dietary protein supplies 4kcal/gram. Approximately 10% of the day's calories should come fom protein (check this).
Too little protein results in malnutrition, but too much is also dangerous. Athletes may require more protein, but The National Academy of Sciences recommends no more than 2 x RDA for protein. A high-protein diet can pose many health risks.
For each 10g/day increase in protein consumption, there was an adjusted decreased GFR in women with mild renal insufficiency.
Dietary protein can be classified into two main groups - complete and incomplete.
High-quality (complete) protein contains adequate amounts of all 9 essential aa:
Low-quality (incomplete) protein is deficient or low in one or more essential amino acids:
Complementary Proteins represent the combination of two or more incomplete dietary proteins to compensate for deficiencies in each protein. For example:
Protein digestion and absorption occurs in the GI tract. Protein degradation provides energy during fasting, primarily through gluconeogenesis. Protein synthesis is ongoing throughout all cells and tissues.
There are many different classes of proteins and protein products
Protein function depends heavily on its structure, which is determined by various forces.
Proteins are linear polymers made up of amino acids. The primary structure of a protein is its sequence of amino acid residues.
Amino acid side chains interact with each other to form three-dimensional structures stabilized by hydrogen bonding. these structures are called alpha helices or beta sheets.
Alpha helices have 3.6 residues per turn and are stabilized by multiple hydrogen bonds.
Beta sheets
The arrangement of alpha helices and beta sheets determine the net three-dimensional arrangement of a polypeptide, which is called tertiary structure. Hydrophobic residues are often buried inside a protein, while hydrophilic side chains point outwards. Covalent disulfide bonds can form between cystein residues, affecting protein shape.
Changes in a protein's 3-D conformation is involved in diseases such as Alzheimier's disease or CJD.
Many proteins become fully functional when different polypeptide chains combine to form multi-subunit structures, such as antibodies.
A protein's native structure can be altered in the presence of heat, changes in pH, or chemicals such as urea to denature the protein, rendering it nonfunctional and insoluble.
Following protein folding, many changes can occur to affect shape, function and location.
Phosphorlylation is the transfer of inorganic phosphate (PO4) from ATP to serine, threonine, or tyrosine residues. Phosphorylation is critical in changing the activity, eiother positively or negatively, of many proteins. Signal trnasduction pathways usually invlove multiple phosphorylation steps.
Phosophorylation occurs via action of kinases, while phoshates are removed by phosphatates.
Glycosylation is the addition of carbohydrate chains to asparagine or serine residues. Many proteins, especially cell surface or extracellular proteins, are glycosylated in extremely complex and varied branching patterns. Glycosylation is involved in protein stability, signaling, and targeting.
Proteolyis is the enzymatic cleavage of a polypeptide. Many proteins, especially those with signaling functions, become active only following proteolysis by regulated enzymes.
At physiological pH (7.2-7.4), amino and carboxyl groups are ionized.