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A sulfur-containing amino acid that is propanoic acid with an amino group at position 2 and a sulfanyl group at position 3." []

D-cysteine is an optically active form of cysteine having D-configuration. It is a cysteine and a D-alpha-amino acid. It is a conjugate base of a D-cysteinium. It is a conjugate acid of a D-cysteinate(1-). It is an enantiomer of a L-cysteine. It is a tautomer of a D-cysteine zwitterion. D-Cysteine, also known as D-cystein or DCY, belongs to the class of organic compounds known as cysteine and derivatives. Cysteine and derivatives are compounds containing cysteine or a derivative thereof resulting from reaction of cysteine at the amino group or the carboxy group, or from the replacement of any hydrogen of glycine by a heteroatom. It is a non-proteogenic sulfur-containing amino acid. D-Cysteine is known to be toxic to bacteria and several bacteria (and plants) have developed and enzyme called D-cysteine desulfhydrase (EC4.1.99.4). D-cysteine can be generated from D-Cysteine via cysteine racemase. D-Cysteine is a naturally occurring enantiomer of L-Cysteine. Cysteine is named after cystine, which comes from the Greek word kustis meaning bladder -cystine was first isolated from kidney stones. D-Cysteine exists in all living species, ranging from bacteria to humans. Outside of the human body, D-Cysteine has been detected, but not quantified in several different foods, such as chervils, fruits, lichee, nuts, and cherimoya.

Cysteine (Cys), also known as L-cysteine is an alpha-amino acid. These are amino acids in which the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (alpha carbon). Amino acids are organic compounds that contain amino (–NH2) and carboxyl (–COOH) functional groups, along with a side chain (R group) specific to each amino acid. L-alanine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Cysteine is found in all organisms ranging from bacteria to plants to animals. It is classified as an aliphatic, non-polar, sulfur-containing amino acid. Cysteine is an important source of sulfur in human metabolism, and although it is classified as a non-essential amino acid, cysteine may be essential for infants, the elderly, and individuals with certain metabolic disease or who suffer from malabsorption syndromes. Cysteine can occasionally be considered as an essential or conditionally essential amino acid. Cysteine is unique amongst the twenty natural amino acids as it contains a thiol group. Thiol groups can undergo oxidation/reduction (redox) reactions; when cysteine is oxidized it can form cystine, which is two cysteine residues joined by a disulfide bond. This reaction is reversible since the reduction of this disulphide bond regenerates two cysteine molecules. The disulphide bonds of cystine are crucial to defining the structures of many proteins. Cysteine is often involved in electron-transfer reactions, and help the enzyme catalyze its reaction. Cysteine is also part of the antioxidant glutathione. N-Acetyl-L-cysteine (NAC) is a form of cysteine where an acetyl group is attached to cysteine's nitrogen atom and is sold as a dietary supplement. Cysteine is named after cystine, which comes from the Greek word kustis meaning bladder (cystine was first isolated from kidney stones). Oxidation of cysteine can produce a disulfide bond with another thiol and further oxidation can produce sulphfinic or sulfonic acids. The cysteine thiol group is also a nucleophile and can undergo addition and substitution reactions. Thiol groups become much more reactive when they are ionized, and cysteine residues in proteins have pKa values close to neutrality, so they are often in their reactive thiolate form in the cell. The thiol group also has a high affinity for heavy metals and proteins containing cysteine will bind metals such as mercury, lead, and cadmium tightly. Due to this ability to undergo redox reactions, cysteine has antioxidant properties. Cysteine is important in energy metabolism. As cystine, it is a structural component of many tissues and hormones. Cysteine has clinical uses ranging from treating baldness to psoriasis to preventing smoker's hack. In some cases, oral cysteine therapy has proved excellent for treatment of asthmatics, enabling them to stop theophylline and other medications. Cysteine also enhances the effect of topically applied silver, tin, and zinc salts in preventing dental cavities. In the future, cysteine may play a role in the treatment of cobalt toxicity, diabetes, psychosis, cancer, and seizures (http://www.dcnutrition.com/AminoAcids/). Cysteine has been identified as a uremic toxin according to the European Uremic Toxin Working Group (PMID: 22626821).

Benzoate, also known as benzoic acid or benzenecarboxylate, is a member of the class of compounds known as benzoic acids. Benzoic acids are organic Compounds containing a benzene ring which bears at least one carboxyl group. Benzoate is soluble (in water) and a weakly acidic compound (based on its pKa). Benzoate can be found in a number of food items such as malus (crab apple), broccoli, pepper (c. annuum), and corn salad, which makes benzoate a potential biomarker for the consumption of these food products. Benzoic acid , C7H6O2 (or C6H5COOH), is a colorless crystalline solid and a simple aromatic carboxylic acid. The name is derived from gum benzoin, which was for a long time its only known source. Benzoic acid occurs naturally in many plants and serves as an intermediate in the biosynthesis of many secondary metabolites. Salts of benzoic acid are used as food preservatives and benzoic acid is an important precursor for the industrial synthesis of many other organic substances. The salts and esters of benzoic acid are known as benzoates .

The simplest member of the class of benzoates that is the conjugate base of benzoic acid, comprising a benzoic acid core with a proton missing to give a charge of -1." []

The univalent carboacyl group formed by loss of -OH from the carboxy group of salicylic acid." []