Proteins as a Source of Nitrogen

The term "proteins" is derived from the Greek "proteno," which means "I take first place." Berzelius and Mulder, who coined this term during the first half of the nineteenth century, meant proteins to be nitrogen-containing compounds in foods, without which life is impossible. In fact, proteins are the sole source of usable nitrogen (N) for humans. Plants obtain N from nitrogenous organic or inorganic soil compounds (A). When plants are harvested, that N is lost to the soil. It therefore needs to be replenished regularly from exogenous sources such as fertilizer, composted plants, and animal waste. Bacteria associated with legumes that are able to fix atmospheric N and release it into the soil (when they are decomposed) import N into this otherwise closed circuit. Plants use N to synthesize amino acids from which they make their species-specific proteins.

All living things use the same mechanism to bond amino acids together: during protein biosynthesis, the carb-oxyl group of one amino acid forms a bond with the amino group of the next one. Water is removed in this process (dehydration synthesis). The resulting peptide bond (B) can be rehydrolyzed, during digestion, for example. During hydrolysis, the water that was removed during dehydration synthesis is reinserted between two molecules. Unlike the synthesis of carbohydrates and lipids, protein biosynthesis is not a tightly regulated, enzymatically catalyzed process. The bonding of amino acids occurs exclusively with the help of RNA (translation). The exact sequence of the amino acids in each protein is predetermined by DNA, with its species- or cell-specific sequence. There fore, as opposed to carbohydrates and lipids, the proteins of different species may be similar but are rarely identical.

Endogenously synthesized proteins (several thousand) can be divided into several large groups according to function. The mechanical stability of organs and tissues is due to structural proteins. Most of them consist of the long amino acid chains of so-called fibrous (structural) proteins. Approximately a third of the total human protein mass is made up of a single structural protein, collagen.

The transport of substances through plasma, inside cells, and across cell membranes is another function of proteins. The transported substances may be gases, reversibly bound to a protein (O2 to hemoglobin in erythrocytes), substances with poor water-solubility (e. g., fat-soluble vitamin A bound to RBP), or polar molecules carried across nonpolar barriers (e. g., ion channels in lipid bilayers).

The body uses the immune system (e. g., immunoglobulins) and the blood-clotting system (e. g., fibrinogen) as defense and protective mechanisms. Hormones and their cellular receptors (e. g., insulin and insulin receptor) are essential for control and regulation. Other metabolic processes would be unthinkable without DNA-encoded enzymes. It is the controlled catalysis of chemical reactions that enables the body to react to changing conditions in a highly specific manner.

- A. Nitrogen Cycle

Organic N-compounds

N fixing bacteria

Organic N-compounds

N fixing bacteria

Plant amino acids-

Plant proteins

Nitrogenous

- excretions

- products of

decomposition

Human proteins

I- B. Peptide Bonds

Hydrolysis

Amino terminal end

Hydrolysis

Synthesis

Peptide bond

Synthesis

Peptide bond

C OH

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Good Carb Diet

Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

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