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Amino acid metabolism
Amino acids assimilated by your body cells face two possible fates. One of them is protein synthesis, either directly, in the form in which they have been assimilated into the cell, or after being restructured by transamination to specific other (non-essential) amino acids, needed by the cell to assemble particular proteins.
The other fate is amino acid degradation, i.e. splitting amino group from the carbon skeleton, with the amino group either disposed of through the urea cycle, or used for nucleotide synthesis, and carbon skeleton converted to metabolites feeding catabolic energy producing pathways - glycolysis and Krebs cycle.
Most proteins are synthesized in millions of tiny structures made around ribosomal RNA (rRNA) molecules, called ribosomes. Free ribosomes are scattered throughout cytosol (intercellular fluid filling the space between outer cell membrane and cellular organelles), while membrane-bound ribosomes are attached to the cytosolic side of the endoplasmic reticulum (ER) membrane.
Ribosomes bind to the messenger RNA (mRNA), carrying protein codes taken from DNA strands in the nucleus, and synthesize corresponding protein molecules of amino acids present in the cytosol (synthesis of all amino acids is initiated in the cytosol, with significant portion of them - mainly glycoproteins - being completed in the ER) and brought to them by the transfer RNA (tRNA) molecules.
These amino acids feed anabolic pathways, that are creating unimaginable volume of protein molecules. An average cell uses some 10,000 to 20,000 different proteins, and contains about
10 billion protein molecules!
Where each protein molecule goes during and after synthesis is determined by its specific chemical "tag", making it subject to some form of cellular transport. Completed proteins are usually transferred to the special cellular organelle called Golgi apparatus, from which they are sent to their final destination. Untagged protein molecules remain at the location of their synthesis.
Excess amino acids, not needed for protein synthesis, are converted to one of the transient-form amino acids - usually glutamate - by aminotransferase enzymes, and then degraded in the liver by removing its amino (NH2) group (deamination); this creates ammonia ion (NH4+), which is then metabolized by the liver to urea - as a way of detoxifying free ammonia (NH3) - and excreted by the kidneys.
Amino acids also supply some of constituents needed for the synthesis of nucleic acid molecules - DNA and RNA. These large, complex molecules are built from a variety of molecular sources, including amino acids glutamine, glycine and aspartic acid (as its salt, aspartate), as well as those coming from the key metabolite of the glucose-originating pentose phosphate pathway, ribose 5-phosphate and one of its intermediates, PRPP (5-phospho-a-D-ribosyl 1-pyrophosphate).
Carbon skeletons remaining from amino acid degradation are further degraded into compounds that feed energy-producing catabolic pathways. According to the metabolic destination of their carbon skeletons, amino acids can be either glucogenic - when their carbon skeletons are metabolized to pyruvate, or to one of Krebs' cycle intermediaries, either way available for the conversion to glucose - or ketogenic, metabolized to acetyl CoA, or acetoacetate (acetoacetyl CoA), either way available for inclusion in fatty acid metabolism, resulting in creation of ketone bodies.
Ketons are compounds containing carbon doubly bonded to oxygen, and to two other carbons, or carbon groups. Keton bodies produced by fat metabolism - acetone, acetoacetate and beta-hydroxybutyrate (BHB) - can be used as alternative energy source by body cells, but their accumulation is toxic.
Most amino acids are glucogenic - alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, histidine, methionine, proline, serine and valine - some are both, glucogenic and ketogenic (large-molecule amino acids isoleucine, phenylalanine, threonine, tryptophan and tyrosine), while only two - leucine and lysine - are purely ketogenic.
The key coenzyme in channeling amino groups to glutamate, or glutamine, either for the purpose of amino acid synthesis by transamination, or for elimination of nitrogenous waste (ammonia), is the vitamin-B6-dependant pyridoxal phosphate (also needed for breakdown of the short-term energy storage stock in the liver and muscles, glycogen).
What is happening with the excess of amino acid carbon skeletons, not needed for energy production? Since the skeletons of both, glicogenic and ketogenic amino acids are metabolized to either acetyl CoA or acetoacetyl CoA, they can be shifted off to fatty acid synthesis. Therefore, excessive intake of dietary protein can also contribute to body fat accumulation, although due to its lower caloric intake relative to that of carbohydrates and fats, as well as its relatively low catabolic usage, significantly less so than these other two macronutrients.
Normally, the body satisfies less than 10% of its caloric needs from the catabolism of amino acid carbon skeletons. Low glucose levels, due to short supply of carbohydrates, or insulin inefficiency (diabetes) prompt the cells - particularly brain cells - to burn more of so called glucogenic amino acids (those that the body can turn into glucose) for energy.
In low-glucose conditions, brain cells - who rely mainly on glucose, due to larger molecules of fatty acids having more difficulty to pass the blood-brain barrier - can switch to beta-hydroxybyturate, keton body produced by breakdown of fat in the liver, as glucose substitute. If fats are also in low supply, the body will cannibalize its muscle tissue for proteins to survive.
In normal conditions, most of amino acids extracted from dietary protein are used for protein synthesis.
With fatty acids obtained from dietary fats, the metabolic score is roughly 50/50 between the anabolic and catabolic role, while most of sugars (both, dietary sugars and those from complex carbohydrate breakdown) are burned for energy. It is the latter two - sugars and fatty acids - that provide most of the fuel molecules for the energy-producing cascade within the cell: glycolysis, Krebs cycle and, above all, electron transport chain.
Following page addresses fatty acid metabolism.