Definitions

# covalent bond

Force holding atoms in a molecule together as a specific, separate entity (as opposed to, e.g., colloidal aggregates; see bonding). In covalent bonds, two atoms share one or more pairs of valence electrons to give each atom the stability found in a noble gas. In single bonds (e.g., HsinglehorzbondH in molecular hydrogen), one electron pair is shared; in double bonds (e.g., OdoublehorzbondO in molecular oxygen or H2CdoublehorzbondCH2 in ethylene), two; in triple bonds (e.g., HCtriplehorzbondCH in acetylene), three. In coordinate covalent bonds, additional electron pairs are shared with another atom, usually forming a functional group, such as sulfate (SO4) or phosphate (PO4). The number of bonds and the atoms participating in each (including any additional paired electrons) give molecules their configuration; the slight negative and positive charges at the opposite ends of a covalent bond are the reason most molecules have some polarity (see electrophile; nucleophile). Carbon in organic compounds can have as many as four single bonds, each pointing to one vertex of a tetrahedron; as a result, certain molecules exist in mirror-image forms (see optical activity). Double bonds are rigid, leading to the possibility of geometric isomers (see isomerism). Some types of bonds, such as the amide linkages that join the amino acids in peptides and proteins (peptide bonds), are apparently single but have some double-bond characteristics because of the electronic structure of the participating atoms. The configurations of enzymes and their substrates, determined by their covalent bonds (particularly the peptide bonds) and hydrogen bonds, are crucial to the reactions they participate in, which are fundamental to all life. Seealso aromatic compound; compare ionic bond.

In enzymology, a nonpolar-amino-acid-transporting ATPase is an enzyme that catalyzes the chemical reaction

ATP + H2O + nonpolar amino acidout $rightleftharpoons$ ADP + phosphate + nonpolar amino acidin

The 3 substrates of this enzyme are ATP, H2O, and nonpolar amino acid, whereas its 3 products are ADP, phosphate, and nonpolar amino acid.

This enzyme belongs to the family of hydrolases, specifically those acting on acid anhydrides acting on acid anhydrides to catalyse transmembrane movement of substances. The systematic name of this enzyme class is ATP phosphohydrolase (nonpolar-amino-acid-transporting).

## References

• Kuan G, Dassa E, Saurin W, Hofnung M, Saier MH Jr "Phylogenetic analyses of the ATP-binding constituents of bacterial extracytoplasmic receptor-dependent ABC-type nutrient uptake permeases". Res. Microbiol. 146 271–8.
• Saier MH Jr "Molecular phylogeny as a basis for the classification of transport proteins from bacteria, archaea and eukarya". Adv. Microb. Physiol. 40 81–136.
• Griffiths JK and Sansom CE "The Transporter Factsbook, Academic Press, San Diego, 1998".