Interactions subunit binding motif

Tire tetrahydrobiopterin formed in this reaction is similar in structure to a reduced flavin. The mechanism of its interaction with 02 could reasonably be the same as that of 4-hydroxybenzoate hydroxylase. However, phenylalanine hydroxylase, which catalyzes the formation of tyrosine (Eq. 18-45), a dimer of 451-residue subunits, contains one Fe per subunit,113 313i whereas flavin monooxygenases are devoid of iron. Tyrosine hydroxylase416 193 and tryptophan hydroxylase420 have very similar properties. All three enzymes contain regulatory, catalytic, and tetramerization domains as well as a common Fe-binding motif in their active sites.413 421 4213... 

Domain IV is the C-terminal portion of the protein, and is also known as the calmodulin-like domain due to its similarity with calmodulin. It contains five EF-hand Ca2+-binding motifs, the fifth of which does not bind calcium but is involved in the dimerization with the small subunit, through the interaction with the analogous EF-hand motif in domain VI. The small subunit is composed of two domains (V and VI) separated by a region containing a proline-rich stretch. 

Scheme 5. Comparison of the ThDP-binding motif of PDCS.c. and PDCZ.m. [173]. The amino acid residues involved in the binding of cofactors are marked with. Those which are involved in polar subunit-subunit-interactions within the dimer are marked with + [51]. Amino acids which have been investigated by site-directed mutagenesis are underlined (Tables 4 and 5)...

We have reviewed the best-known prokaryotic plasmid-based resistance systems from the view of metal ion homeostasis. Plasmid-based systems are, with few exceptions, inducible and are regulated by activators (mercury and copper) or repressors (cadmium) in different cases. The metal-binding motifs used by these systems are frequently localized (e.g., Cys-Xaa-Xaa-Cys, Glu-His-His, and His-Xaa-His), but occasionally coordinating cysteine residues on different subunits interact (as in the MerR regulatory protein and mercuric reductase enzyme). 

Figure 8.11 The DNA-binding domain of 434 repressor. It is a dimer in its complexes with DNA fragments. Each subunit (green and brown) folds into a bundle of four a helices (1-4) that have a structure similar to the corresponding region of the lambda repressor (see Figure 8.7) including the helix-turn-helix motif (blue and red). A fifth a helix (5) is involved in the subunit interactions, details of which are different from those of the lambda repressor fragment. The structure of the 434 Cro dimer is very similar to the 434 repressor shown here.

Many biochemical and biophysical studies of CAP-DNA complexes in solution have demonstrated that CAP induces a sharp bend in DNA upon binding. This was confirmed when the group of Thomas Steitz at Yale University determined the crystal structure of cyclic AMP-DNA complex to 3 A resolution. The CAP molecule comprises two identical polypeptide chains of 209 amino acid residues (Figure 8.24). Each chain is folded into two domains that have separate functions (Figure 8.24b). The larger N-terminal domain binds the allosteric effector molecule, cyclic AMP, and provides all the subunit interactions that form the dimer. The C-terminal domain contains the helix-tum-helix motif that binds DNA. 

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