The comparison of TOFs and TONs of different (bio)catalysts should be
exercised with great caution [147], since TOFs only indicate how fast the catalyst
acts at the onset of the reaction within a short time span, but it does not tell anything
about its long-time performance.
The operational stability of a (bio)catalyst is described by the dimensionless
‘total turnover number’ (TTN), which is determined by the moles of product formed
by the amount of catalyst spent. In other words, it stands for the amount of product
which is produced by a given amount of catalyst during its whole lifetime. If the
TONs of repetitive batches of a reaction are measured until the catalyst is dead, the
sum of all TONs would equal to the TTN. TTNs are also commonly used to
describe the efficiency of cofactor recycling systems.
Total Turnover Number TTN
ð
Þ¼
Number of Substrates Converted
Number of Catalyst Molecules
Mol
Mol
Lifetime
ð
Þ
The efficiency of microbial transformations (where the catalytic activity of
enzymes involved cannot be measured) is characterized by the so-called ‘productivity number’ (PN) [148], defined as
Productivity Number PN
ð Þ ¼
Amount of Product Formed
Biocatalyst dry weight
ð
ÞÂTime
Mol
g  Time
which is the amount of product formed by a given amount of whole cells (dry
weight) within a certain period of time. This number resembles the specific activity
as defined for pure enzymes, but also includes several other important factors such
as inhibition, transport phenomena, and concentration.
1.4.4 Coenzymes
A remarkable proportion of synthetically useful enzymes require cofactors (coenzymes),
22 which have a molecular weight of only few hundred Da, in contrast to the
typical 50,000 Da of enzymes used in biotransformations. Coenzymes serve as
molecular shuttles to stabilize and transfer sensitive ‘chemical reagents’, for
instance redox-equivalents (e.g., complex hydrides, electrons, and activated oxygen
species), toxic intermediates (ammonia) and water-sensitive carbanion species.
Alternatively, ‘chemical energy’ is stored in energy-rich functional groups, such
as acid anhydrides in ATP or PAPS. As a rule of thumb, enzymes are bound to their
22 A ‘cofactor’ is tightly bound to an enzyme (e.g., FAD), whereas a ‘coenzyme’ can dissociate into
the medium (e.g., NADH). In practice, however, this distinction is not always made in a consequent manner.
1.4 Enzyme Properties and Nomenclature
23
exercised with great caution [147], since TOFs only indicate how fast the catalyst
acts at the onset of the reaction within a short time span, but it does not tell anything
about its long-time performance.
The operational stability of a (bio)catalyst is described by the dimensionless
‘total turnover number’ (TTN), which is determined by the moles of product formed
by the amount of catalyst spent. In other words, it stands for the amount of product
which is produced by a given amount of catalyst during its whole lifetime. If the
TONs of repetitive batches of a reaction are measured until the catalyst is dead, the
sum of all TONs would equal to the TTN. TTNs are also commonly used to
describe the efficiency of cofactor recycling systems.
Total Turnover Number TTN
ð
Þ¼
Number of Substrates Converted
Number of Catalyst Molecules
Mol
Mol
Lifetime
ð
Þ
The efficiency of microbial transformations (where the catalytic activity of
enzymes involved cannot be measured) is characterized by the so-called ‘productivity number’ (PN) [148], defined as
Productivity Number PN
ð Þ ¼
Amount of Product Formed
Biocatalyst dry weight
ð
ÞÂTime
Mol
g  Time
which is the amount of product formed by a given amount of whole cells (dry
weight) within a certain period of time. This number resembles the specific activity
as defined for pure enzymes, but also includes several other important factors such
as inhibition, transport phenomena, and concentration.
1.4.4 Coenzymes
A remarkable proportion of synthetically useful enzymes require cofactors (coenzymes),
22 which have a molecular weight of only few hundred Da, in contrast to the
typical 50,000 Da of enzymes used in biotransformations. Coenzymes serve as
molecular shuttles to stabilize and transfer sensitive ‘chemical reagents’, for
instance redox-equivalents (e.g., complex hydrides, electrons, and activated oxygen
species), toxic intermediates (ammonia) and water-sensitive carbanion species.
Alternatively, ‘chemical energy’ is stored in energy-rich functional groups, such
as acid anhydrides in ATP or PAPS. As a rule of thumb, enzymes are bound to their
22 A ‘cofactor’ is tightly bound to an enzyme (e.g., FAD), whereas a ‘coenzyme’ can dissociate into
the medium (e.g., NADH). In practice, however, this distinction is not always made in a consequent manner.
1.4 Enzyme Properties and Nomenclature
23