The general functions of amino acids include the involvement in protein synthesis, biosynthetic products, and metabolic energy. Essentially, there is a crucial difference between positive and negative nitrogen balance, which is critical for understanding the amino acid metabolism. In a positive balance, the nitrogen consumed is more considerable than nitrogen excreted, while in a negative balance, the nitrogen consumed is less than nitrogen excreted. A positive balance denotes net protein synthesis. It occurs when the organism is recovering from starvation, growth, and, even, pregnancy. In contrast, a negative balance entails mobilization of the amino acids, tissue necrosis, or a poor-quality condition of the human body as a consequence of 3rd-degree burns or significant surgical operations.

Glutamate is a non-essential amino acid, which can be synthesized from alpha-ketoglutaric acid in the Krebs or citric acid cycle. In the brain and spinal cord, glutamate is synthesized from glutamine as part of the glutamate-glutamine cycle by the enzyme glutaminase. Glutamate cannot cross the blood-brain barrier unaided, and itself serves as a metabolic precursor for the neurotransmitter gamma-aminobutyric acid (GABA), via the action of the enzyme glutamate decarboxylase.

Methionine is converted to S-adenosylmethionine (SAM) by methionine adenosyltransferase. Loss of methionine has correlated with an accumulation of hydrogen peroxide (H2O2) in hair follicles, a decrease in tyrosinase effectiveness, and a gradual loss of the natural hair color. Methionine is crucial for the increase in the intracellular concentration of glutathione (GSH). GSH is an antioxidant, found in animals, plants, fungi, and some bacteria, as well as archaea. The steps of promoting antioxidant mediated cell defense and redox regulation are critical in protecting cells against dopamine-induced nigral cell loss by oxidative binding metabolites. Methionine is an amino acid, which is an intermediate component for the biosynthesis of some amino acids. These amino acids are cysteine, carnitine, taurine, lecithin, and phosphatidylcholine. Also, methionine is medium in the biosynthesis of additional phospholipids. Improper transformation of methionine can lead to atherosclerosis due to the accumulation of homocysteine. Moreover, this amino acid is essential to reversing damaging methylation of glucocorticoid receptors caused by repeated stress exposures, with implications for depression.

Glycine is considered to be not essential to the human diet. The body can synthesize this amino acid from the amino acid serine. However, the metabolic capacity for glycine biosynthesis does not satisfy the need for collagen synthesis in several organisms. In the liver of some of them at vertebrate level, glycine synthesis is catalyzed by glycine synthase, which is also known as glycine cleavage enzyme. Glycine is integral to the creation of alpha-helices in secondary protein structure, and, mainly, it is the most copious amino acid in collagen harboring triple-helices. Glycine is also an inhibitory neurotransmitter. The interference of its release within the central nervous system (spinal cord) can induce spastic paralysis due to uninhibited muscle contraction.

The amino acids subdivide as essential and non-essential. There are amino acids that need to be obtained directly (diet) and amino acids that can be synthesized by the organism. The amino acids that the human body cannot produce are called essential amino acids [3], which contain His, Ile, Leu, Lys, Met, Val, Phe, Thr, and Trp. The human body gets these nine essential amino acids from food or nutritional supplement. In specific medical conditions or different ages, the other amino acids may be conditionally essential for the human body [3]. Cys is synthesized from Met, while Tyr synthesis can occur using Phe, taking into account that the amino acid precursors can be available in the body. The amino acid Arg, which arises from the urea cycle, is considered "semi-essential" because the synthetic capacity of the human body is limited. Non-essential amino acids need their precursors, which need to be available into the organism. Specifically, the amino acids Ala and Gly need pyruvate to be synthesized, while aspartic acid and Asn rely on oxaloacetic acid (OAA). Thus, six essential amino acids and three non-essential are integrated from OAA, or better oxaloacetate, and pyruvate.

The transamination from Glu is vital in forming Asp and Ala from OAA and pyruvate. Asp is crucial in the synthesis of Asn, Met, Lys, and Thr. OAA is critical because no Asp would form without it. The alpha-ketoglutaric acid or 2-oxoglutaric acid is one of two ketone derivatives of glutaric acid. Its anion, alpha-ketoglutarate (alpha-KG), which is also known as 2-oxoglutarate, is a biological compound of paramount importance. It is the keto acid produced by the deamination of Glu and is an intermediate compound in the urea cycle or Krebs cycle. The amino acids, glutamic acid, and Gln arise from alpha-KG.

Finally, the amino acid Pro derives from Glu, while Ser from 3-phosphoglyceric acid (3PG). The 3PG is the conjugate acid of glycerate 3-phosphate. It is a biochemically significant metabolic intermediate in both glycolysis and the Calvin cycle. In the Calvin cycle or photosynthetic carbon reduction (PCR) cycle of photosynthesis, 3PG is vital. It is the product of the spontaneous scission of an unstable 6-carbon intermediate formed upon CO fixation. Thus, glycerate 3-phosphate is a precursor for Ser, which, in turn, can create Cys and Gly through the homocysteine cycle. Therefore, Pro arises from Glu, while Ser from 3PG. In the transamination reaction, an amino acid (Ala or Asp) exchanges its amine group for the oxy group in alpha-KG. The products are Glu and pyruvate or OAA (from Ala or Asp accordingly).

Different kinds of proteases can degrade the proteins into many small peptides or amino acids by hydrolyzing their peptide bonds. The unused amino acids may degrade further to join several metabolic processes. At first, the amino acids deaminate to their metabolic intermediates. This process is helpful for the excretion of the excessive amount of nitrogen. Subsequently, they can transform into the remaining carbon skeleton. In particular, this deamination process contains two steps. The first part uses deamination, such as de-amine. In this step, the aminotransferase catalyzes the -NH2 group of the amino acid to alpha-KG. After that, it produces Glu and a novel alpha-keto acid of the specific amino acid. The Glu -NH2 group could then be transferred to OAA to form alpha-KG and Asp. This trans-amination series only degrade the primary amino acid, while the -NH2 group nitrogen does not exclude. There is an alternative pathway, using NADP or NAD+ as the oxidizing agent, and Glu dehydrogenase deaminates Glu. Then, it produces ammonia and alpha-KG. In the evaluation of the biochemistry of the amino acids, seven metabolic intermediates of the aminoacidic degradation platform are paramount. They include acetyl-CoA, pyruvate, alpha-KG, acetoacetate, fumarate, succinyl-CoA, and OAA. In the most updated classification, Leu, Ile, Thr, Lys degrade to acetyl-CoA, while Cys, Ala, Thr, Gly, Trp, Ser degrade to pyruvate. Glu, Arg, His, Pro, Gln degrade to alpha-KG, while Lys, Leu, Trp, Tyr, Phe break down to acetoacetate. Finally, Tyr, Phe, Asp degrade to fumarate, Val, Met, Ile break down to succinyl-CoA, and Asp and, of course, Asn degrade to OAA. Isoleucine is an essential nutrient because it is unsynthesized in the body. This amino acid is both a glucogenic and ketogenic amino acid. In microorganisms and plants, it is synthesized via several steps beginning from pyruvate and alpha-ketobutyrate. The enzymes involved in this biosynthesis include acetolactate synthase, acetohydroxy acid isomeroreductase, dihydroxy acid dehydratase, and valine aminotransferase.

In the clinic, plasma or urine amino acids undergo testing to evaluate the patients with possible inborn metabolism problems. They can also be used to assess many different diseases, such as liver diseases, endocrine disorders, muscular diseases, neurological disorders, neoplastic diseases, renal failure, burns, and nutritional disturbances. Both high-performance liquid chromatography (HPLC) and gas chromatography (GC) have been used to quantitative the plasma or urine amino acids in the clinic before.[4][5] Currently, capillary electrophoresis (CE), liquid chromatography with tandem mass spectrometry (LC-MS-MS), nuclear magnetic resonance (NMR) methods, and a chip-type miniaturization technology are also useful to detect the amino acids in the laboratory.[6][7][8][9]

Amino acid disorders are identifiable at any age; most of them become evident during infancy or early childhood. Many inborn amino metabolism diseases occur in infancy or childhood. These disorders may include cystinuria, histidinemia, phenylketonuria (PKU), methyl-malonyl CoA mutase deficiency (MCM deficiency), albinism, and tyrosinemia. Other amino acid disorders, including homocystinuria, alkaptonuria, maple syrup urine disease (MSUD), and cystathioninuria, may be encountered later in life. These disorders lead to clinical symptoms or signs of the specific amino acid disorder, which results in the deficiency or accumulation of one or more amino acids in the body’s biological fluids such as plasma or urine.

The deficiency of Phe hydroxylase causes PKU. Currently, there are more than 400 mutations were identified in the gene related to the cause of PKU.[10] This disease is usually heterozygous. Besides, the deficiency of enzymes such as dihydropteridine reductase (DHPR) or any tetrahydrobiopterin (BH4) synthesis enzymes also leads to hyperphenylalaninemia.[11][12] The BH4 or generated product replacement therapy is the treatment for these enzymes deficiency-related PKU. In the case of the classic PKU, the Phe, phenyl lactate, phenylpyruvate, and phenylacetate are increased in the plasma, urine as well as other tissue samples. The phenyl pyruvic acid excreted in urine produces a “mousy” odor. Central nervous system symptoms, such as mental retardation, seizures, failure to walk or speak, tremor, and hyperactivity, also show in these patients. Another characteristic of classic PKU is hypopigmentation, which is because of the deficiency in the formation of melanin leading to pigmentation deficiency. Usually, the patients show light skin, fair hair, and blue eyes. Temporally, low Phe content synthetic nutrient supplemented with Tyr is the treatment of the classic PKU.

Albinism is a congenital disorder that is the defect of Tyr metabolism leading to a deficiency in melanin production. The characteristics of albinism are hypopigmentation by the full or partial absence of pigment in the hair, skin, and eyes. There is no cure for albinism because it is a genetic disorder.[13] At the moment, getting proper eye care, such as using sunglasses to prevent the ultraviolet (UV) rays damage from the sun and monitoring for signs of abnormalities of the skin, is the treatment of albinism.

Alkaptonuria is a rare disease with the defect of homogentisic acid oxidase, which is an enzyme in the Tyr degradation pathway. The urine specimen of alkaptonuria patient shows some darkening on the surface after standing for fifteen minutes, which is due to the homogentisate acid oxidation. And after two hours of standing, the urine of the patients is entirely black. The characteristics of alkaptonuria include the accumulation of homogentisic aciduria, large joint arthritis, and the intervertebral disks of vertebrae deposit with dense black pigments.[14] Low protein diet with small in Phe and Tyr is the treatment of alkaptonuria, which helps reduce the homogentisic acid levels.

Tyrosinemia type 1 results from a deficiency in fumarylacetoacetate hydrolase, leading to the accumulation of fumarylacetoacetate and its metabolites (especially succinylacetone) in urine, which makes cabbage-like odor. The patients show renal tubular acidosis and liver failure.[15] The treatment is Phe and Tyr restriction dietary.

MCM deficiency is a disease due to the defect of methyl malonyl CoA mutase, which catalyzes isomerization between methyl malonyl-CoA and succinyl-CoA in the pathway.[16] It shows high levels of methyl malonyl CoA in the blood samples from the patients. Symptoms of MCM deficiency include vomiting, dehydration, fatigue, hypotonia, fever, breathing difficulty, and infections. Also, metabolic acidosis and developmental delay occur as long-term complications. The treatment of MCM deficiency includes a special diet with low proteins (low in Ile, Met, Thr, and Val amino acids) and certain fats, but high in calories.

Maple syrup urine disease (MSUD) is a rare autosomal recessive disease with a partial or full defect of branched-chain alpha-keto acid dehydrogenase. The enzyme can decarboxylate Leu, Ile, and Val. This deficiency leads to the accumulation of branched-chain alpha-keto acid substrates. These three amino acids cause functional abnormalities in the brain. The urine with classic maple syrup odor is a hallmark characteristic of MSUD. MSUD patients show symptoms such as vomiting, feeding difficulties, dehydration, and severe metabolic acidosis.[17] In the clinic, a synthetic formula containing a limited amount of Leu, Ile, and Val is the suggested therapy to give to the MSUD infants. MSUD (OMIM #248600) demonstrates a disturbance of the regular activity of the branched-chain α-ketoacid dehydrogenase (BCKAD) complex, the second step in the catabolic trail for the branched-chain amino acids (BCAAs) that include leucine, isoleucine, and valine. MSUD can occur early in life, but late-onset MSUD is also common and include neurologic symptoms. These symptoms may include inappropriate, extreme or erratic behavior and moods, hallucinations, oscillating hypertonia/hypotonia, ataxia, seizures, opisthotonos, and coma.

Cystathioninuria is a rare autosomal recessive metabolic disorder due to the deficiency in cystathionase. It links with the lower activity of the enzyme cystathionase. There are two types of primary cystathioninuria based on the inherited mutation of the CTH gene: vitamin B6 responsive and vitamin B6 unresponsive cystathioninuria.[18][19] It is characterized by the accumulation of cystathionine and its metabolites in plasma and urine, but no clinical symptoms are present. The treatment of cystathioninuria is varying according to the category in different cystathioninuria patients. Increased consumption of vitamin B6 is considered as the best treatment for the active form of vitamin B6.[20]

Homocystinuria is an inherited disorder due to the defect of the metabolism of Met amino acid. The most common cause is the enzyme cystathionine beta-synthetase deficiency, which results in the elevation of Met and homocysteine as well as low levels of Cys in plasma and urine.[21] The characteristics of homocystinuria, including the lens of the eye displacement, skeletal abnormalities, osteoporosis, premature arterial disease, and mental delay.[22][23] The treatment of homocystinuria involves the restriction of Met intake and supplementation with folate, vitamin B6, and B12.[24]

Histidinemia is a rare autosomal recessive inborn metabolic error due to the defect of the enzyme histidase.[25] It characteristically demonstrates high levels of His, histamine and imidazole in blood, urine as well as cerebrospinal fluid. A low in His intake diet is suggested for the treatment of histidinemia, though the restriction diet is unnecessary for most of the cases.[26]