Asparagine (Asn) is one of the 20 most common natural amino acids on Earth. It has carboxamide as the side chain's functional group. Asparagine is not an essential amino acid, which means that it can be synthesized from central metabolic pathway intermediates in humans and is not required in the diet. The precursor to asparagine is oxaloacetate. Oxaloacetate is converted to aspartate using a transaminase enzyme. The enzyme transfers the amino group from glutamate to oxaloacetate producing alpha-ketoglutarate and aspartate. The enzyme asparagine synthetase produces asparagine, AMP, glutamate, and pyrophosphate from aspartate, glutamine, and ATP. In the asparagine synthetase reaction, ATP is used to activate aspartate, forming beta-aspartyl-AMP. Glutamine donates an ammonium group which reacts with beta-aspartyl-AMP to form asparagine and free AMP. Since the asparagine side chain can make efficient hydrogen bond interactions with the peptide backbone, asparagines are often found near the beginning and end of alpha-helices, and in turn motifs in beta sheets. Its role can be thought as "capping" the hydrogen bond interactions which would otherwise need to be satisfied by the polypeptide backbone. Glutamines have an extra methylene group and have more conformational entropy, and thus are less useful in this regard. Asparagine also provides key sites for N-linked glycosylation, modification of the protein chain with the addition of carbohydrate chains. A reaction between asparagine and reducing sugars or reactive carbonyls produces acrylamide (acrylic amide) in food when heated to sufficient temperature (i.e. baking). These occur primarily in baked goods such as french fries, potato chips, and roasted coffee. Asparagine was first isolated in 1806 from asparagus juice, in which it is abundant--hence its name--becoming the first amino acid to be isolated. The smell observed in the urine of some individuals after their consumption of asparagus is attributed to a byproduct of the metabolic breakdown of asparagine, asparagine-amino-succinic-acid monoamide. However, some scientists disagree and implicate other substances in the smell, especially methanethiol.