, produced by
ClustalO, of five mammalian
histone H1 proteins. Sequences are the
amino acids for residues 120-180 of the proteins. Residues that are conserved across all sequences are highlighted in grey. Below each site (i.e., position) of the protein sequence alignment is a key denoting
conserved sites (*), sites with conservative replacements (:), sites with semi-conservative replacements (.), and sites with
non-conservative replacements ( ). This key uses
PAM250 as a similarity measure. There are 20 naturally occurring amino acids, however some of these share similar characteristics. For example,
leucine and
isoleucine are both
aliphatic, branched
hydrophobes. Similarly,
aspartic acid and
glutamic acid are both small, negatively charged residues. Although there are many ways to classify amino acids, they are often sorted into six main classes on the basis of their structure and the general chemical characteristics of their side chains (R groups). Physicochemical distances aim at quantifying the intra-class and inter-class dissimilarity between amino acids based on their measurable properties, and many such measures have been proposed in the literature. Owing to their simplicity, two of the most commonly used measures are the ones of Grantham (1974) and Miyata et al (1979). A conservative replacement is therefore an exchange between two amino acids separated by a small physicochemical distance. Conversely, a radical replacement is an exchange between two amino acids separated by a large physicochemical distance. == Impact on function ==