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A classic example for favoring the keto form can be seen in the equilibrium between vinyl alcohol and acetaldehyde (K = [enol]/[keto] ≈ 3 × 10 −7). In 1,3-diketones, such as acetylacetone (2,4-pentanedione), the enol form is more favored. The acid-catalyzed conversion of an enol to the keto form proceeds by proton transfer from O to carbon.
Molecular orbitals of an enolate, showing the occupancy corresponding to the anion. Enolate anions are electronically related to allyl anions. The anionic charge is delocalized over the oxygen and the two carbon sites. Thus they have the character of both an alkoxide and a carbanion. [5]
The mechanism of the enzyme requires removal of the α-proton of the 2-methylacyl-CoA to form a deprotonated intermediate (which is probably the enol or enolate [10]) followed by non-sterespecific reprotonation. [11] Thus either epimer is converted into a near 1:1 mixture of both isomers upon full conversion of the substrate.
Enolase is a member of the large enolase superfamily.It has a molecular weight of 82,000–100,000 daltons depending on the isoform. [3] [4] In human alpha enolase, the two subunits are antiparallel in orientation so that Glu 20 of one subunit forms an ionic bond with Arg 414 of the other subunit. [3]
KSI catalyzes the rearrangement of a carbon-carbon double bond in ketosteroids through an enolate intermediate at a diffusion-limited rate. [2] There have been conflicting results on the ionization state of the intermediate, whether it exists as the enolate [12] or enol. [13]
As a result, enolate ions are more common than enols in both laboratory and biological chemistry. Because they are resonance hybrids of two nonequivalent forms, enolate ions can be looked at either as vinylic alkoxides (C=C- O −) or as α-ketocarbanions (− C-C= O). Thus, enolate ions can react with electrophiles either on oxygen or on carbon.
In proteins and amino acids, the α-carbon is the backbone carbon before the carbonyl carbon atom in the molecule. Therefore, reading along the backbone of a typical protein would give a sequence of –[N—Cα—carbonyl C] n – etc. (when reading in the N to C direction). The α-carbon is where the different substituents attach to each ...
The generation of a protein sequence is much easier than the determination of a protein structure. However, the structure of a protein gives much more insight in the function of the protein than its sequence. Therefore, a number of methods for the computational prediction of protein structure from its sequence have been developed. [39]