Thus they rarely colonise the same tree. Aside from vleis and the private residential land that Harareans cultivate, considerable public land is used for agriculture in Harare: Study of various explanations of crime: It is not because women and feminists are fighting for whatever, it is that women don't want to be strong and independent in real life, they want to watch other women do it and live vicariously through the media. Computer application packages include Quickbooks, Peachtree, and Microsoft Excel. We did not explicitly model lag times.
In the structure shown at the top of the page, R represents a side chain specific to each amino acid. Amino acids containing an amino group bonded directly to the alpha carbon are referred to as alpha amino acids. The alpha amino acids are the most common form found in nature, but only when occurring in the L -isomer.
The alpha carbon is a chiral carbon atom, with the exception of glycine which has two indistinguishable hydrogen atoms on the alpha carbon.
While L -amino acids represent all of the amino acids found in proteins during translation in the ribosome, D -amino acids are found in some proteins produced by enzyme posttranslational modifications after translation and translocation to the endoplasmic reticulum , as in exotic sea-dwelling organisms such as cone snails.
In alternative fashion, the S and R designators are used to indicate the absolute stereochemistry. Amino acids are usually classified by the properties of their side chain into four groups.
The side chain can make an amino acid a weak acid or a weak base , and a hydrophile if the side chain is polar or a hydrophobe if it is nonpolar. The phrase " branched-chain amino acids " or BCAA refers to the amino acids having aliphatic side chains that are non-linear; these are leucine , isoleucine , and valine.
The negatively charged carboxylate ion predominates at pH values greater than the pKa of the carboxylic acid group mean for the 20 common amino acids is about 2. Because all amino acids contain amine and carboxylic acid functional groups, they share amphiprotic properties. But at pH between 2. This molecular state is known as a zwitterion , from the German Zwitter meaning hermaphrodite or hybrid. Amino acids exist as zwitterions also in the solid phase, and crystallize with salt-like properties unlike typical organic acids or amines.
The variation in titration curves when the amino acids can be grouped by category. 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.
The individual amino acids all have slightly different pKa values, so have different isoelectric points. For amino acids with charged side chains, the pKa of the side chain is involved. Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behaviour 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.
Amino acids are the structural units monomers that make up proteins. They join together to form short polymer chains called peptides or longer chains called either polypeptides or proteins. These polymers are linear and unbranched, with each amino acid within the chain attached to two neighboring amino acids.
Twenty-two amino acids are naturally incorporated into polypeptides and are called proteinogenic or natural amino acids. The remaining 2, selenocysteine and pyrrolysine , are incorporated into proteins by unique synthetic mechanisms. It is coded for with the codon UAG, which is normally a stop codon in other organisms.
Aside from the 22 proteinogenic amino acids , many non-proteinogenic amino acids are known. Those either are not found in proteins for example carnitine , GABA , levothyroxine or are not produced directly and in isolation by standard cellular machinery for example, hydroxyproline and selenomethionine. Non-proteinogenic amino acids that are found in proteins are formed by post-translational modification , which is modification after translation during protein synthesis.
These modifications are often essential for the function or regulation of a protein. For example, the carboxylation of glutamate allows for better binding of calcium cations ,  and collagen contains hydroxyproline, generated by hydroxylation of proline. Some non-proteinogenic amino acids are not found in proteins. Examples include 2-aminoisobutyric acid and the neurotransmitter gamma-aminobutyric acid.
D -isomers are uncommon in live organisms. For instance, gramicidin is a polypeptide made up from mixture of D - and L -amino acids. These compounds disrupt bacterial cell walls, particularly in Gram-positive bacteria. Only D -amino acids were found in Swiss-Prot database million amino acids analysed. The 20 amino acids that are encoded directly by the codons of the universal genetic code are called standard or canonical amino acids. A modified form of methionine N -formylmethionine is often incorporated in place of methionine as the initial amino acid of proteins in bacteria, mitochondria and chloroplasts.
Other amino acids are called non-standard or non-canonical. Most of the non-standard amino acids are also non-proteinogenic i. The two non-standard proteinogenic amino acids are selenocysteine present in many non-eukaryotes as well as most eukaryotes, but not coded directly by DNA and pyrrolysine found only in some archaea and one bacterium.
The incorporation of these non-standard amino acids is rare. For example, 25 human proteins include selenocysteine Sec in their primary structure,  and the structurally characterized enzymes selenoenzymes employ Sec as the catalytic moiety in their active sites.
When taken up into the human body from the diet, the 20 standard amino acids either are used to synthesize proteins and other biomolecules or are oxidized to urea and carbon dioxide as a source of energy. The other product of transamidation is a keto acid that enters the citric acid cycle. In humans, non-protein amino acids also have important roles as metabolic intermediates , such as in the biosynthesis of the neurotransmitter gamma-amino-butyric acid GABA.
Many amino acids are used to synthesize other molecules, for example:. Some non-standard amino acids are used as defenses against herbivores in plants. Amino acids are used for a variety of applications in industry, but their main use is as additives to animal feed. This is necessary, since many of the bulk components of these feeds, such as soybeans , either have low levels or lack some of the essential amino acids: The food industry is also a major consumer of amino acids, in particular, glutamic acid , which is used as a flavor enhancer ,  and aspartame aspartyl-phenylalaninemethyl ester as a low-calorie artificial sweetener.
The chelating ability of amino acids has been used in fertilizers for agriculture to facilitate the delivery of minerals to plants in order to correct mineral deficiencies, such as iron chlorosis. These fertilizers are also used to prevent deficiencies from occurring and improving the overall health of the plants. Similarly, some amino acids derivatives are used in pharmaceutical industry.
They include 5-HTP 5-hydroxytryptophan used for experimental treatment of depression,  L -DOPA L -dihydroxyphenylalanine for Parkinson's treatment,  and eflornithine drug that inhibits ornithine decarboxylase and used in the treatment of sleeping sickness.
Since , 40 non-natural amino acids have been added into protein by creating a unique codon recoding and a corresponding transfer-RNA: Nullomers are codons that in theory code for an amino acid, however in nature there is a selective bias against using this codon in favor of another, for example bacteria prefer to use CGA instead of AGA to code for arginine. This characteristic can be taken advantage of and used to create new selective cancer-fighting drugs  and to prevent cross-contamination of DNA samples from crime-scene investigations.
Amino acids are important as low-cost feedstocks. These compounds are used in chiral pool synthesis as enantiomerically pure building-blocks. Amino acids have been investigated as precursors chiral catalysts , e. Amino acids are under development as components of a range of biodegradable polymers. These materials have applications as environmentally friendly packaging and in medicine in drug delivery and the construction of prosthetic implants.
These polymers include polypeptides, polyamides , polyesters, polysulfides, and polyurethanes with amino acids either forming part of their main chains or bonded as side chains. These modifications alter the physical properties and reactivities of the polymers. As amino acids have both a primary amine group and a primary carboxyl group, these chemicals can undergo most of the reactions associated with these functional groups.
These include nucleophilic addition , amide bond formation, and imine formation for the amine group, and esterification , amide bond formation, and decarboxylation for the carboxylic acid group.
Several methods exist to synthesize amino acids. Nucleophilic substitution with ammonia then converts the alkyl bromide to the amino acid.
At the current time, the most-adopted method is an automated synthesis on a solid support e. As both the amine and carboxylic acid groups of amino acids can react to form amide bonds, one amino acid molecule can react with another and become joined through an amide linkage.
This polymerization of amino acids is what creates proteins. This condensation reaction yields the newly formed peptide bond and a molecule of water. In cells, this reaction does not occur directly; instead, the amino acid is first activated by attachment to a transfer RNA molecule through an ester bond. However, not all peptide bonds are formed in this way.
In a few cases, peptides are synthesized by specific enzymes. For example, the tripeptide glutathione is an essential part of the defenses of cells against oxidative stress.
This peptide is synthesized in two steps from free amino acids. This dipeptide is then condensed with glycine by glutathione synthetase to form glutathione. In chemistry, peptides are synthesized by a variety of reactions. One of the most-used in solid-phase peptide synthesis uses the aromatic oxime derivatives of amino acids as activated units. These are added in sequence onto the growing peptide chain, which is attached to a solid resin support.
In plants, nitrogen is first assimilated into organic compounds in the form of glutamate , formed from alpha-ketoglutarate and ammonia in the mitochondrion. In order to form other amino acids, the plant uses transaminases to move the amino group to another alpha-keto carboxylic acid. For example, aspartate aminotransferase converts glutamate and oxaloacetate to alpha-ketoglutarate and aspartate.
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 can 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. Amino acids must first pass out of organelles and cells into blood circulation via amino acid transporters , since the amine and carboxylic acid groups are typically ionized. Degradation of an amino acid, occurring in the liver and kidneys, often involves deamination by moving its amino group to alpha-ketoglutarate, forming glutamate.
This process involves transaminases, often the same as those used in amination during synthesis. In many vertebrates, the amino group is then removed through the urea cycle and is excreted in the form of urea. However, amino acid degradation can produce uric acid or ammonia instead.
For example, serine dehydratase converts serine to pyruvate and ammonia. The 20 amino acids encoded directly by the genetic code can be divided into several groups based on their properties. Important factors are charge, hydrophilicity or hydrophobicity , size, and functional groups.
The water-soluble proteins tend to have their hydrophobic residues Leu, Ile, Val, Phe, and Trp buried in the middle of the protein, whereas hydrophilic side chains are exposed to the aqueous solvent. Note that in biochemistry , a residue refers to a specific monomer within the polymeric chain of a polysaccharide , protein or nucleic acid.
The integral membrane proteins tend to have outer rings of exposed hydrophobic amino acids that anchor them into the lipid bilayer. In the case part-way between these two extremes, some peripheral membrane proteins have a patch of hydrophobic amino acids on their surface that locks onto the membrane.
In similar fashion, proteins that have to bind to positively charged molecules have surfaces rich with negatively charged amino acids like glutamate and aspartate , while proteins binding to negatively charged molecules have surfaces rich with positively charged chains like lysine and arginine. There are different hydrophobicity scales of amino acid residues.
Some amino acids have special properties such as cysteine , that can form covalent disulfide bonds to other cysteine residues, proline that forms a cycle to the polypeptide backbone, and glycine that is more flexible than other amino acids. Many proteins undergo a range of posttranslational modifications , when additional chemical groups are attached to the amino acids in proteins. Some modifications can produce hydrophobic lipoproteins ,  or hydrophilic glycoproteins. For example, the addition and removal of the fatty acid palmitic acid to cysteine residues in some signaling proteins causes the proteins to attach and then detach from cell membranes.
Two additional amino acids are in some species coded for by codons that are usually interpreted as stop codons:. In addition to the specific amino acid codes, placeholders are used in cases where chemical or crystallographic analysis of a peptide or protein cannot conclusively determine the identity of a residue.
They are also used to summarise conserved protein sequence motifs. The use of single letters to indicate sets of similar residues is similar to the use of abbreviation codes for degenerate bases. In addition, many non-standard amino acids have a specific code. For example, several peptide drugs, such as Bortezomib and MG , are artificially synthesized and retain their protecting groups , which have specific codes. To aid in the analysis of protein structure, photo-reactive amino acid analogs are available.
These include photoleucine pLeu and photomethionine pMet. From Wikipedia, the free encyclopedia. This article is about the class of chemicals. For the structures and properties of the standard proteinogenic amino acids, see Proteinogenic amino acid. A polypeptide is an unbranched chain of amino acids. The amino acid selenocysteine. Protein primary structure and Posttranslational modification.
Protein nutrient and Amino acid synthesis. Diagram of the molecular signaling cascades that are involved in myofibrillar muscle protein synthesis and mitochondrial biogenesis in response to physical exercise and specific amino acids or their derivatives primarily L -leucine and HMB. Biosynthetic pathways for catecholamines and trace amines in the human brain   .
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