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Amino acids are a class of important biomolecules that contain both amino groups and carboxylate groups. In most contexts, the term ‘amino acids’ refers to the α-amino acids, so-called because both the amino and carboxyl groups are attached to the α-carbon of the structure. However, other types of amino acids are encountered in nature, such as the β-amino acids, in which the amino and carboxyl groups are attached to different carbons in the backbone. The key elements of an amino acid are carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), although other elements are found in the side chains of certain amino acids. About 500 naturally occurring amino acids are known (though only 20 appear in the genetic code) and can be classified in many ways. They can be classified according to the core structural functional groups’ locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, pH level, and side-chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). In the form of proteins, amino acid residues form the second-largest component (water is the largest) of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis.
The first few amino acids were discovered in the early 19th century. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound in asparagus that was subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, cysteine, remained undiscovered until 1884. Glycine and leucine were discovered in 1820. The last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who also determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. The first use of the term “amino acid” in the English language dates from 1898, while the German term, Aminosäure, was used earlier. Proteins were found to yield amino acids after enzymatic digestion or acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed “peptide”.
The term amino acid usually refers to an α-amino carboxylic acid in which the α carbon atom adjacent to a carboxylic acid moiety (-COOH) carries three other substituents: an amino group (-NH2), a hydrogen atom (-H), and a variable side chain conventionally symbolized as “-R” . These four substituents are arranged around the α carbon in a tetrahedral fashion. Two nonoverlapping arrangements are possible. By convention, these optically active, mirror-image stereoisomers are designated the l and d forms. Except in the case of glycine, in which R is a second hydrogen atom, the four substituents are different, making the α carbon atom a center of chirality. Only the l-isomers of amino acids are commonly found in proteins. The biosynthetic pathways that produce amino acids are stereospecific, and most generate only l-isomers. Therefore, l-isomers are present at much higher concentrations in human tissues and bodily fluids. d-Amino acids, although less common than l-amino acids, do have some important roles in nature. For example, they have long been known to be components of the cell walls of certain bacteria. Recently, d-aspartate and d-serine have been found to exist in significant concentrations in mammalian brains. In fact, evidence suggests that d-serine acts as a genuine neurotransmitter.
When dissolved in aqueous solutions at neutral pH, the carboxylic acid groups of amino acids are deprotonated, while their amino groups are protonated. This makes each amino acid molecule a dipolar ion or zwitterion (from the German zwitter, meaning mongrel).
In aqueous solution, amino acids exist in two forms (as illustrated at the right), the molecular form and the zwitterion form in equilibrium with each other. The two forms co-exist over the pH range pK1 – 2 to pK2 + 2, which for glycine is pH 0-12. The ratio of the concentrations of the two isomers is independent of pH. The value of this ratio cannot be determined experimentally.
Because all amino acids contain amine and carboxylic acid functional groups, they are amphiprotic.At pH = pK1 (ca. 2.2) there will be equal concentration of the species NH3+CH(R)CO2H and NH3+CH(R)CO2– and at pH = pK2 (ca. 10) there will be equal concentration of the species NH3+CH(R)CO2– and NH2CH(R)CO2–. It follows that the neutral molecule and the zwitterion are effectively the only species present at biological pH.
It is generally assumed that the concentration of the zwitterion is much greater than the concentration of the neutral molecule on the basis of comparisons with the known pK values of amines and carboxylic acids.
4) Isoelectric point:
The variation in titration curves when the amino acids can be grouped by category.With the exception of tyrosine, using titration to distinguish among hydrophobic amino acids is problematic.
At pH values between the two pKa values, the zwitterion predominates but coexists in dynamic equilibrium with small amounts of net negative and net positive ions. At the exact midpoint between the two pKa values, the trace amount of net negative and trace of net positive ions exactly balance, so that average net charge of all forms present is zero. This pH is known as the isoelectric point pI, so pI = ½(pKa1 + pKa2). The individual amino acids all have slightly different pKa values and therefore have different isoelectric points. For amino acids with charged side chains, the pKa of the side chain is involved. Thus for Asp or Glu with negative side chains, pI = ½(pKa1 + pKaR), where pKaR is the side chain pKa. Cysteine also has a potentially negative side chain with pKaR = 8.14, so pI should be calculated as for Asp and Glu, even though the side chain is not significantly charged at physiological pH. For His, Lys, and Arg with positive side chains, pI = ½(pKaR + pKa2). Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behavior is more usually exploited for peptides and proteins than single amino acids. Zwitterions have minimum solubility at their isoelectric point, and some amino acids (in particular, with non-polar side chains) can be isolated by precipitation from water by adjusting the pH to the required isoelectric point.
5) Essential amino acids:
The body needs 20 different amino acids to maintain good health and normal functioning. People must obtain nine of these amino acids, called the essential amino acids, through food. Good dietary sources include meat, eggs, tofu, soy, buckwheat, quinoa, and dairy.
Amino acids are compounds that combine to make proteins. When a person eats a food that contains protein, their digestive system breaks the protein down into amino acids. The body then combines the amino acids in various ways to carry out bodily functions.
A healthy body can manufacture the other 11 amino acids, so these do not usually need to enter the body through the diet.
Amino acids build muscles, cause chemical reactions in the body, transport nutrients, prevent illness, and carry out other functions. Amino acid deficiency can result in decreased immunity, digestive problems, depression, fertility issues, lower mental alertness slowed growth in children and many other health issues.
Lysine plays a vital role in building muscle, maintaining bone strength, aiding recovery from injury or surgery, and regulating hormones, antibodies, and enzymes. It may also have antiviral effects. There is not a lot of research available on lysine deficiency, but a study on rats indicates that lysine deficiency can lead to stress-induced anxiety.
Histidine facilitates growth, the creation of blood cells, and tissue repair. It also helps maintain the special protective covering over nerve cells, which is called the myelin sheath. The body metabolizes histidine into histamine, which is crucial for immunity, reproductive health, and digestion. The results of a study that recruited women with obesity and metabolic syndrome suggest that histidine supplements may lower BMI and insulin resistance. Deficiency can cause anemia, and low blood levels appear to be more common among people with arthritis and kidney disease.
Threonine is necessary for healthy skin and teeth, as it is a component in tooth enamel, collagen, and elastin. It helps aid fat metabolism and may be beneficial for people with indigestion, anxiety, and mild depression.A 2018 study found that threonine deficiency in fish led to these animals having a lowered resistance to disease.
Methionine and the nonessential amino acid cysteine play a role in the health and flexibility of skin and hair. Methionine also helps keep nails strong. It aids the proper absorption of selenium and zinc and the removal of heavy metals, such as lead and mercury.
Valine is essential for mental focus, muscle coordination, and emotional calm. People may use valine supplements for muscle growth, tissue repair, and energy. Deficiency may cause insomnia and reduced mental function.
Isoleucine helps with wound healing, immunity, blood sugar regulation, and hormone production. It is primarily present in muscle tissue and regulates energy levels. Older adults may be more prone to isoleucine deficiency than younger people. This deficiency may cause muscle wasting and shaking.
Leucine helps regulate blood sugar levels and aids the growth and repair of muscle and bone. It is also necessary for wound healing and the production of growth hormone. Leucine deficiency can lead to skin rashes, hair loss, and fatigue.
Phenylalanine helps the body use other amino acids as well as proteins and enzymes. The body converts phenylalanine to tyrosine, which is necessary for specific brain functions. Phenylalanine deficiency, though rare, can lead to poor weight gain in infants. It may also cause eczema, fatigue, and memory problems in adults. Phenylalanine is often in the artificial sweetener aspartame, which manufacturers use to make diet sodas. Large doses of aspartame can increase the levels of phenylalanine in the brain and may cause anxiety and jitteriness and affect sleep. People with a rare genetic disorder called phenylketonuria (PKU) are unable to metabolize phenylalanine. As a result, they should avoid consuming foods that contain high levels of this amino acid.
Tryptophan is necessary for proper growth in infants and is a precursor of serotonin and melatonin. Serotonin is a neurotransmitter that regulates appetite, sleep, mood, and pain. Melatonin also regulates sleep. Tryptophan is a sedative, and it is an ingredient in some sleep aids. One study indicates that tryptophan supplementation can improve mental energy and emotional processing in healthy women. Tryptophan deficiency can cause a condition called pellagra, which can lead to dementia, skin rashes, and digestive issues.
The commercial production of amino acids usually relies on mutant bacteria that overproduce individual amino acids using glucose as a carbon source. Some amino acids are produced by enzymatic conversions of synthetic intermediates. 2-Aminothiazoline-4-carboxylic acid is an intermediate in one industrial synthesis of L-cysteine for example. Aspartic acid is produced by the addition of ammonia to fumarate using a lyase.
In plants, nitrogen is first assimilated into organic compounds in the form of glutamate, formed from alpha-ketoglutarate and ammonia in the mitochondrion. For other amino acids, plants use transaminases to move the amino group from glutamate to another alpha-keto acid. For example, aspartate aminotransferase converts glutamate and oxaloacetate to alpha-ketoglutarate and aspartate. Other organisms use transaminases for amino acid synthesis, too.
Nonstandard amino acids are usually formed through modifications to standard amino acids. For example, homocysteine is formed through the transsulfuration pathway or by the demethylation of methionine via the intermediate metabolite S-adenosyl methionine, while hydroxyproline is made by a posttranslational modification of proline.
Microorganisms and plants synthesize many uncommon amino acids. For example, some microbes make 2-aminoisobutyric acid and lanthionine, which is a sulfide-bridged derivative of alanine. Both of these amino acids are found in peptidic lantibiotics such as alamethicin. However, in plants, 1-aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is a key intermediate in the production of the plant hormone ethylene.
7) Amino Acid Dating:
Amino acid dating is a dating technique used to estimate the age of a specimen in paleobiology, molecular paleontology, archaeology, forensic science, taphonomy, sedimentary geology, and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed.
All biological tissues contain amino acids. All amino acids except glycine (the simplest one) are optically active, having a stereocenter at their α-C atom. This means that the amino acid can have two different configurations, “D” or “L” which are mirror images of each other. With a few important exceptions, living organisms keep all their amino acids in the “L” configuration. When an organism dies, control over the configuration of the amino acids ceases, and the ratio of D to L moves from a value near 0 towards an equilibrium value near 1, a process called racemization. Thus, measuring the ratio of D to L in a sample enables one to estimate how long ago the specimen died.
8) Unusual Amino Acids:
Several L-amino acids have physiological functions as free amino acids rather than as constituents of proteins. Examples are as follows:
1.β-Alanine is part of the vitamin pantothenic acid.
2. Homocysteine, homoserine, ornithine, and citrulline are intermediates in the biosynthesis of certain other amino acids.
3. Taurine, which has an amino group in the β-carbon and a sulfonic acid group instead of COOH, is present in the CNS and as a component of certain bile acids participate in digestion and absorption of lipids in the gastrointestinal tract.
4.y-Aminobutyric acid is an inhibitory neurotransmitter.
5. Hypoglycin A is present in unripe akee fruit and produces severe hypoglycemia when ingested.
6. Some D-amino acids are found in polypeptide antibiotics, such as gramicidins and bacitracin, and in bacterial cell wall peptides.
9) Conditional amino acids:
Although 11 of the amino acids are nonessential, humans may require some of them if they are under stress or have an illness. During these times, the body may not be able to make enough of these amino acids to keep up with the increased demand. These amino acids are “conditional,” which means that a person may require them in certain situations. People may sometimes wish to take essential amino acid supplements. It is best to seek advice from a doctor first regarding safety and dosage.
10) Incorporating essential amino acids into the diet:
Although it is possible to be deficient in essential amino acids, most people can obtain enough of them by eating a diet that includes protein.
The foods in the following list are the most common sources of essential amino acids:
- Lysine is in meat, eggs, soy, black beans, quinoa, and pumpkin seeds.
- Meat, fish, poultry, nuts, seeds, and whole grains contain large amounts of histidine.
- Cottage cheese and wheat germ contain high quantities of threonine.
- Methionine is in eggs, grains, nuts, and seeds.
- Valine is in soy, cheese, peanuts, mushrooms, whole grains, and vegetables.
- Isoleucine is plentiful in meat, fish, poultry, eggs, cheese, lentils, nuts, and seeds.
- Dairy, soy, beans, and legumes are sources of leucine.
- Phenylalanine is in dairy, meat, poultry, soy, fish, beans, and nuts.
- Tryptophan is in most high-protein foods, including wheat germ, cottage cheese, chicken, and turkey.
These are just a few examples of foods that are rich in essential amino acids. All foods that contain protein, whether plant-based or animal-based, will contain at least some of the essential amino acids.
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