attokatal
An SI unit of catalytic activity equal to 10⁻¹⁸ katals. Symbol: a
attokatal: one quintillionth of a katal, abbreviated aK
An attokatal (symbol aK) is the smallest standard SI unit of catalytic activity, representing 10-18 katals. It measures how fast an enzyme or catalyst converts substrate into product, scaled to the femtomolar range. This unit appears almost nowhere in routine industrial work but becomes essential in pharmaceutical research, biochemical assays, and analytical instrumentation where enzyme quantities are measured in picomoles or smaller.
The katal itself, adopted by SI in 1999, defines catalytic activity as the amount of product formed per unit time: one katal equals one mole of product per second. The attokatal follows the standard SI prefix convention, where "atto" means 10-18. Thus one attokatal represents the catalytic activity of an enzyme system that converts one attomole of substrate per second. For context, a typical enzyme molecule working at modest turnover rates exhibits catalytic activity in the picocatal to femtocatal range; attokatal scales describe enzyme populations at molecular or near-molecular concentrations.
Why this unit matters in practice: Researchers working with enzyme kinetics at extreme dilution, single-enzyme studies, or ultrasensitive immunoassays encounter attokatal values. An enzyme-linked immunosorbent assay (ELISA) using horseradish peroxidase labels or other enzymatic reporters may report enzyme concentrations and activity in units down to the attomolar level, making attokatal the natural choice for expressing their catalytic output. Biosensors and microfluidic devices with picoliter reaction volumes also push into this regime.
The attokatal exists formally in SI but lacks widespread adoption outside research settings. Clinical and industrial enzyme assays almost always report activity in units per milliliter or per gram, expressed in microkatal (μK) or smaller conventional units like international units per milliliter, which predate SI adoption. Metrological laboratories and standards bodies recognize the attokatal, but you will rarely see it on a certificate of analysis or in production specifications.