Chemistry Reference
In-Depth Information
diphosphate analogue (ADP) by ATPase. Likewise, the formation of micro-
tubules, which are responsible for transport within the cell as well as processes
of cellular reorganisation relevant to cell division, is enzymatically regulated.
Here, guanosine triphosphate (GTP) is hydrolysed to guanosine diphosphate
(GDP) by the enzyme GTPase to control the corresponding assembly and
disassembly of the monomeric proteins in microtubules.
17,19
In summary, en-
zymatic self-assembly and hydrogelation is a common mechanism to control
these natural self-assembly processes and provides a unique opportunity to
develop new dynamic materials based on these same principles.
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4.3 Biocatalytic Self-Assembly in Designed Biomaterials
A number of approaches have been used to trigger the self-assembly of small
molecules into supramolecular hydrogels. They include a variety of chemical
and physical means, which can broadly be divided into two types, depending on
whether the stimulus applied is a bulk or localised event. In the first type, self-
assembly is initiated by an overall change in environmental conditions, such as
pH or ionic strength.
20,21
The other type includes the locally applied stimulus
such as light or catalytic action of the enzymes that involves conversion to the
self-assembly molecule by a triggered change in chemical structure.
22,23
The
exploitation of the enzymes as selective biological stimuli to trigger the self-
assembly of small molecules has some unique advantages. These systems
always operate under mild and constant conditions and provide better control
over a self-assembly process and its kinetics due to the unique role of the enzyme,
which locally amplifies the concentration of the self-assembly building blocks.
The selectivity means that, in principle, a range of different triggers (responding
to different enzymes) can be applied within single systems, something that is very
dicult to achieve using the more conventional stimuli. In addition, the differ-
ence between metabolic profiles in health and disease commonly comes down to
different expression levels of certain key enzymes. If these enzymes can be ex-
ploited as triggers to control assembly or disassembly, new opportunities emerge
to dynamically respond to the onset of disease.
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4.4 Design of Biocatalytic Gelators
4.4.1 Basic Building Blocks
The idea of biocatalytic self-assembly is based on the fact that self-assembling
molecules are always amphiphilic in nature, and the balance between hydro-
phobic and hydrophilic elements dictates their self-assembly propensity. By
either cleaving or adding hydrophobic or hydrophilic elements from/to these
molecular structures, the tendency to assembly can be modified.
Most examples of the biocatalytic self-assembly are based on peptides and
their derivatives; these will be the focus of the chapter. There are 20 gene coding
amino acids in nature (Figure 4.1). Peptides generated from the combination of
these amino acids have different properties, depending on the nature of their
