File Name: list of essential and nonessential amino acids .zip
- Essential and Nonessential Amino Acids
- Essential Amino Acids: Definition, Benefits and Food Sources
- Essential amino acid
Amino acids AAs are defined as organic compounds containing both amino and acid groups 1.
Essential and Nonessential Amino Acids
Peter J. Here, we compared the traditional nutritional definition of the dispensable and indispensable amino acids for humans with categorizations based on amino acid metabolism and function. The three views lead to somewhat different interpretations.
From a nutritional perspective, it is quite clear that some amino acids are absolute dietary necessities if normal growth is to be maintained. Even so, growth responses to deficiencies of dispensable amino acids can be found in the literature.
From a strictly metabolic perspective, there are only three indispensable amino acids lysine, threonine and tryptophan and two dispensable amino acids glutamate and serine. In addition, a consideration of in vivo amino acid metabolism leads to the definition of a third class of amino acids, termed conditionally essential, whose synthesis can be carried out by mammals but can be limited by a variety of factors.
These factors include the dietary supply of the appropriate precursors and the maturity and health of the individual. From a functional perspective, all amino acids are essential, and an argument in favor of the idea of the critical importance of nonessential and conditionally essential amino acids to physiological function is developed.
For at least 60 years, it has been the convention to divide amino acids into two categories: indispensable or essential and dispensable or nonessential. This categorization provides a convenient, and generally useful, way of viewing amino acid nutrition.
However, despite the longevity of the convention, as more information has become available, the distinctions between dispensable and indispensable amino acids, at least at the metabolic level, have become increasingly blurred. Indeed, W. Rose, who was responsible for the initial definition of the two terms, was not especially enamored with the way in which they were applied by others and wrote the following Womack and Rose, :.
In fact, as far as I can ascertain, the original nutritional definition of an indispensable amino acid Borman et al. Indeed, the rate of synthesis becomes of specific importance when we consider a group of amino acids, exemplified by arginine, cysteine, proline and perhaps glycine, that are frequently described as conditionally essential.
For example, Womack and Rose made the important point that the degree to which arginine could be regarded as indispensable was very much a function of the quantities of its natural precursors, proline and glutamate, in the diet. First, it serves to emphasize that the definitions were originally constructed in the context of growth. For example, it is possible to show Table 1 that the ingestion of diets completely devoid of glutamate, which in some ways can be regarded as the doyen of dispensable amino acids, leads to a small but statistically significant slower rate of growth.
Second, constraining the definition of essentiality to growth does not encompass the importance of some amino acids to pathways of disposal other than protein deposition, a subject that I discuss later. It is also possible to define amino acid essentiality and nonessentiality in chemical and metabolic terms.
An examination of the amino acids that are generally considered to be nutritionally essential indicates that each has a specific structural feature, the synthesis of which cannot be catalyzed by mammalian enzymes Table 2.
In this regard, it is very important to note that the loss of the ability to carry out these biosyntheses appeared early in evolution and is a common feature of the metabolism of eukaryotic organisms in general, and not just of mammals.
However, within this view, the important term is de novo synthesis. This is because some indispensable amino acids can be synthesized from precursors that are structurally very similar. For example, methionine can be synthesized both by transamination of its keto acid analogue and by remethylation of homocysteine. In this sense, then, the mammal is capable of synthesizing leucine, isoleucine, valine, phenylalanine and methionine. However, this is not new synthesis, because the branched-chain keto acids and homocysteine were originally derived from branched-chain amino acids and methionine, respectively.
According to this restricted metabolic definition of essentiality, threonine and lysine and perhaps tryptophan are the only truly essential amino acids. Structural features that render amino acids essential components of the diet of mammals. The reverse applies to dispensable amino acids. Strictly speaking, a truly nonessential amino acid is one that can be synthesized de novo from a non—amino acid source of nitrogen e.
According to this metabolic definition, the only truly metabolically nonessential amino acids are glutamic acid and serine. If this is so, then these two amino acids are the ultimate precursors of the other nonessential amino acids.
This conclusion leads to the prediction that the contribution of endogenous synthesis to the systemic fluxes of glutamate and serine should be higher than its contribution to the fluxes of other nonessential amino acids. This appears to be so Table 3.
Interestingly, there is a reciprocal relationship between the contribution of endogenous synthesis to the plasma flux of a given nonessential amino acid and the degree to which the intestine metabolizes the dietary amino acids in first pass see Reeds et al. Contribution of endogenous synthesis to the systemic flux of nonessential amino acids in humans.
From Berthold et al. Although the nitrogen of the amino acids that mammals can synthesize derives ultimately from either glutamate or serine, there are some amino acids that are synthesized by more complex pathways than the simple transamination of an appropriate keto acid. When this limit is attained, the amino acid in question becomes an essential component of the diet.
The limitations can result from a number of factors. First, the synthesis of these amino acids Table 4 requires the provision of another amino acid, either as the carbon donor or as a donor of an accessory group, such as the sulfur group of cysteine. Thus, the ability of the organism to synthesize a given conditionally essential amino acid is set by the availability of its amino acid precursor, a point that was emphasized by Rose in his studies of the interactions among glutamate, proline and arginine nutrition.
In some cases, e. Second, some amino acids may be synthesized in only a limited number of tissues. For example, the synthesis of both proline and arginine is crucially dependent on intestinal metabolism Wakabayashi et al.
Moreover, in the case of these two amino acids, the available evidence suggests that dietary, as opposed to systemic, amino acid precursors are obligatory Beaumier et al. It follows from this that alterations in either intestinal metabolism or the route of nutrition can have a critical bearing on the ability of the organism to synthesize these amino acids. This is shown strikingly by the problems of arginine and ammonia homeostasis that accompany total parenteral nutrition Brunton et al.
Third, most evidence suggests that even in the presence of abundant quantities of the appropriate precursors, the quantities of conditionally essential amino acids that can be synthesized may be quite limited Beaumier et al. Such appears to be the case with regard to the proline nutrition of burned individuals Jaksic et al. Moreover, in immature individuals, such as low-birth-weight infants, it is possible that conditionally essential amino acid synthesis may be limited by a frank lack of enzymic activity Gaull et al.
These comments do, however, have to be tempered with caution, because it seems likely that the metabolism of some conditionally essential amino acids is tightly compartmentalized and hence that isotopic measurements in the plasma pool may give a quantitatively misleading impression of the scale of biosynthesis.
This apparently applies to proline, arginine and cysteine metabolism, because estimates of their rate of synthesis from parallel measurements of intake and body proteolysis do not agree with estimates based on isotopic incorporation from labeled precursors cf Beaumier et al. Furthermore, there is also evidence to suggest that newly synthesized conditionally essential amino acids may be used within their cells of origin and hence do not equilibrate with the plasma pool Miller et al.
Nevertheless, even with these uncertainties, it appears that the synthesis of these amino acids can become limiting for growth and other physiological functions and that an absolute, as opposed to a relative, dietary requirement can be defined.
When the definitions are applied in this way, there is relatively little confusion, at least regarding the indispensable amino acids. Quantification of the minimum needs for indispensable amino acids to support growth is relatively easy because these are simply the product of the rate of protein deposition and the amino acid composition of the proteins that are deposited. In this regard, there is a good consensus that the relative needs of individual amino acids to support protein deposition are very similar among mammalian species Table 5.
In other words, the amino acid requirements for the support of protein deposition in the human infant differ from those of other mammals only to the degree to which their respective rates of protein deposition differ.
Essential amino acid composition of the mixed body protein of immature mammals. In humans, the obligatory amino acid needs for net protein deposition are a very minor portion of the total amino acid requirement Dewey et al. At least as important as the technical and experimental difficulties associated with the measurements of maintenance amino acid needs Fuller and Garlick, is the problem of identifying the processes that consume amino acids close to nitrogen equilibrium.
A portion of these needs is, of course, directly associated with protein metabolism and reflects two related factors: that amino acids released from tissue protein degradation are unlikely to be recycled with complete efficiency, and that the presence of finite concentrations of free amino acids inevitably leads to some degree of catabolism. This aspect of basal or maintenance amino acid needs is amenable to direct measurement, although some technical aspects of these measurements, notably those associated with intestinal protein metabolic function, pose difficulties see Fuller and Reeds, However, as more information has accrued, it has become increasingly clear that amino acids are involved and hence consumed in a number of physiological functions that are not directly related to protein metabolism itself.
Before passing to a discussion of these pathways, it is critical to emphasize two additional points. First, at protein intakes that are just sufficient to maintain body protein equilibrium, metabolic nitrogen itself, rather than any single amino acid, may be the limiting nutrient. In other words, because nitrogen is in short supply, the ability of the organism to synthesize amino acids may become compromised to the extent that nonessential amino acid intake could become limiting.
This might be particularly applicable to conditions associated with the consumption of low quantities of so-called high quality proteins i. Second, there is now evidence to show that the adult human is capable of lowering the catabolism of any single amino acid close to zero if that amino acid is strongly limiting Raguso et al.
However, the rate of catabolism of the amino acid observed under this circumstance is much lower than that found when protein as a whole is the limiting dietary nutrient. One explanation for this observation is that under protein-free feeding conditions, the free amino acid pool derives exclusively from tissue proteolysis so that all amino acids are equally limiting.
The consequence is that the utilization of any single amino acid in the support of a nonprotein process automatically limits the ability of the organism to recycle all others back into the protein stores of the body. The questions that arise are: What are these nonprotein pathways of consumption, and what is their quantitative impact on amino acid needs in general? The short answer to both questions is simple: There is not sufficient current information to provide accurate answers.
Nevertheless, it is possible to hypothesize which pathways could be the most important at the level of overall physiological function. To develop these hypotheses, it is useful to consider those functions that are necessary to maintain health.
This is not a new approach, as its usefulness was clearly appreciated by some of the founders of nutritional science. For example, Voit , as quoted by Lusk , wrote the following:. Within each system, critical metabolic roles for some specific amino acids can be identified Table 6. The involvement of amino acids in physiological and metabolic function.
Perhaps the most interesting point that emerges from a consideration of Table 6 is that with the exception of the involvement of phenylalanine and tryptophan in the maintenance of the adrenergic and serotonergic neurotransmitter systems and of methionine as a methyl group donor for the synthesis of creatine, the necessary precursors are nonessential or conditionally essential amino acids.
That being so, it is tempting to argue that the ability to maintain the synthesis of these amino acids is of sufficiently high functional priority that under conditions in which protein nitrogen is limiting, essential amino acids are used to maintain these pathways.
In Table 7 I attempted to compare estimates of the turnover or losses of some critical end products with the kinetics of their precursor amino acids.
This comparison suggests that some pathways, e. Thus, creatine synthesis as estimated from creatinine excretion and the turnover of glutathione estimated from measurements in the plasma and red blood cells have a substantial impact on the utilization of some precursors, especially if the rate of product synthesis is set against the intake or net synthesis of the precursor.
The continuing synthesis of these two end products, one involved in energy transduction in both the musculature and central nervous system and the other a critical factor in detoxification mechanisms, appears to have a substantial potential impact on the nutritional status of the individual. Indeed, data obtained in protein-depleted pigs Jahoor et al. Potential contribution of functionally important end product synthesis to amino acid needs in adult humans.
From Matthews and Campbell From Yu et al. From Fukagawa et al. From Castillo et al. Calculated from methionine transulfuration Fukagawa et al. Calculated from the transfer of citrulline to arginine Castillo et al. From taurine excretion Naismith et al. Refers only to erythrocyte and plasma glutathione Jahoor et al.
Essential Amino Acids: Definition, Benefits and Food Sources
All of the proteins on the face of the earth are made up of the same 20 amino acids. Linked together in long chains called polypeptides, amino acids are the building blocks for the vast assortment of proteins found in all living cells. All amino acids have the same basic structure, which is shown in Figure 2. With the exception of glycine, which has an R-group consisting of a hydrogen atom, all of the amino acids in proteins have four different groups attached to them and consequently can exist in two mirror image forms, L and D. With only very minor exceptions, every amino acid found in cells and in proteins is in the L configuration. There are 22 amino acids that are found in proteins and of these, only 20 are specified by the universal genetic code.
You might be familiar with the term amino acids, but have you ever stopped to think about what these powerful organic compounds do for you? Simply put, amino acids help your body produce energy and form tissues. Keep scrolling to learn more about the difference between essential and nonessential amino acids, and start reaching for the protein powder, chia seeds, nuts, chicken, fish and other great sources of amino acids. Essential and nonessential amino acids both produce energy and build proteins, and some form neurotransmitters and hormones. The unique chemical structure of each amino acid determines its function. Amino acids are primarily made of carbon, hydrogen, oxygen, and nitrogen. Every protein contains a specific sequence of a few to a thousand amino acids source.
Essential amino acid
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An essential amino acid , or indispensable amino acid , is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life forms, the nine amino acids humans cannot synthesize are phenylalanine , valine , threonine , tryptophan , methionine , leucine , isoleucine , lysine , and histidine. Six other amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress. Six amino acids are non-essential dispensable in humans, meaning they can be synthesized in sufficient quantities in the body. These six are alanine , aspartic acid , asparagine , glutamic acid , serine ,  and selenocysteine considered the 21st amino acid.
Amino acids, often referred to as the building blocks of proteins, are compounds that play many critical roles in your body. Some may also be taken in supplement form for a natural way to boost athletic performance or improve mood. This article tells you everything you need to know about essential amino acids, including how they function, possible food sources and the benefits of taking a supplement.
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