Human Anatomy and Physiology - N. I. Fedyukovich 2003

Metabolism and Energy
Protein Metabolism

It is well known that Proteins are composed of Amino Acids. Amino acids serve not only as building blocks for the synthesis of new structural proteins, Enzymes, Hormones (protein and peptide in nature), and other BIOLOGICALLY ACTIVE SUBSTANCES, but also as a source of energy. The Nutritional Value of dietary proteins depends both on their energy content and on their amino acid profile.

The average half-life of proteins varies across different living organisms; in humans, for example, it is approximately 80 days. Furthermore, individual proteins within the same Organism turn over at different rates. Muscle Proteins renew at a much slower pace, whereas human Blood Plasma Proteins have a half-life of about 10 days, and protein-Peptide Hormones survive for only a few minutes. In humans, roughly 400 g of protein are degraded and resynthesized daily. Of the resulting free amino acids, about 70% are reused for de novo Protein Synthesis, while about 30% are converted into energy and must be replenished through exogenous dietary amino acids.

Numerous protein structures are built from unique combinations of just 20 amino acids. Some of these can be synthesized within the body (such as Glycine, Alanine, and Cysteine), whereas others (such as Arginine, leucine, Lysine, and Tryptophan) cannot be synthesized endogenously and must be obtained from the diet. The latter are referred to as Essential Amino Acids. Both types are vital for the organism. Proteins containing a full Complement of essential Amino acids are termed biologically complete. An adult human requires a daily Dietary Protein Intake of approximately 70–90 g (1 g per 1 kg of body weight), with about 30 g of this protein being of plant origin. The optimal protein intake also depends on physical activity levels: under moderate exertion, an individual should consume 100–120 g of protein per day, whereas heavy physical labor increases the requirement to 150 g. The amount of protein broken down in the body is estimated by measuring the nitrogen excreted (via urine and sweat). This principle relies on the fact that nitrogen is exclusively a component of proteins (amino acids). The state in which the amount of nitrogen ingested equals the amount excreted is known as nitrogen balance. Notably, 1 g of nitrogen corresponds to 6.25 g of protein.

Accordingly, nitrogen balance calculations are based on the premise that proteins contain approximately 16% nitrogen. A condition in which dietary nitrogen intake is lower than nitrogen excretion is termed a negative nitrogen balance. In this state, Protein Catabolism outweighs synthesis, which occurs during protein starvation, febrile states, and Disorders of the NEUROENDOCRINE REGULATION OF Protein METABOLISM. A positive nitrogen balance is a condition where the amount of excreted nitrogen is significantly less than that ingested with food, resulting in net nitrogen retention. This state is characteristic of Pregnancy, periods of growth in children, and convalescence following severe illnesses, among others.

Proteins primarily perform a plastic (structural) function in the body. They constitute enzymes and hormones, regulate diverse physiological processes, provide immune defense, and determine the species and individual CHARACTERISTICS OF THE organism. In addition, proteins serve as an energy source; an inadequate supply leads to the depletion of endogenous proteins. The primary sources of free amino acids are plasma proteins, enzymes, and proteins of the Liver, intestinal mucosa, and Muscles, which allows the body to sustain the turnover of vital Heart and Brain proteins without loss for extended periods.

Protein metabolism is regulated by The Nervous system, as well as hormones secreted by the Pituitary Gland (somatotropin), The Thyroid Gland (thyroxine), and the Adrenal Glands (glucocorticoids).