α-IPMS-14CR, with the additional 12 copies of the repeat units, is ~30% larger than α-IPMS-2CR. The lower Km (higher affinity for substrates) of α-IPMS-14CR is more difficult to understand. A report on the cystine protease CPB isoforms of Leishmania mexicana showed that variation in a few charged amino acid residues located outside of but close to the active site may influence

SB525334 cost the electrostatic potential on the surface of the proteins, resulting in different Km values [22]. In the case of α-IPMS-14CR, although the segment of the protein that includes the 14 copies of the repeat units is located in the C-terminal domain, it may come into close proximity with the active site due to its huge size. The amino acid composition of the repeat units may also be important. Since seven of the 19 residues in the repeat unit are hydrophilic and charged (Figure 5), they could affect

the electrostatic potential on the surface of the enzyme and, therefore, the enzyme’s affinity for its substrates. Figure 5 Amino acid sequence of α-IPMS containing two copies of the VNTR. The N-terminal domain (catalytic domain), residues 51–368, is colored red. Residues involved in substrate (α-KIV) binding are underlined: D81, H285, H287, N321, E309 and G320. The conserved GxGERxG motif (residues 314–320, H379 and Y410), which forms a groove possible for acetyl CoA binding, is this website underlined. Linker domain: subdomain I (residues 369–424) is colored blue; subdomain II (residues 434–490) is colored magenta. The C-terminal regulatory Thiazovivin in vitro domain (residues 491–644) is colored green. The two copies (one copy contains 19 amino acids, vtiaspaqpgeagrhasdp, at residues 575–612) of the repeat sequence are underlined. The hydrophilic and charged residues

are in bold. Residues involved in leucine binding are indicated in bold italics: L535, A536, V551, Y554, A565 and A567. 6-phosphogluconolactonase Mutation of residues G531, G533 and A536 (underlined) abolished feedback inhibition of α-IPMS in S. cerevisiae. The Y410F mutant form of M. tuberculosis α-IPMS was insensitive to feedback inhibition. The mechanism of l-leucine inhibition was suggested to be a slow-onset inhibition (time-dependent) [19]. After a rapid formation of an initial inhibitory complex (leucine binds to the regulatory domain), isomerization of the complex occurs, leading to a tightly bound complex. Evidence confirmed that an inhibitory signal is transmitted through the linker domain to the catalytic domain, as the Tyr410Phe mutant form of M. tuberculosis α-IPMS is insensitive to l-leucine feedback inhibition [23]. Mutations that abolish l-leucine feedback inhibition in S. cerevisiae α-IPMS are clustered around residues surrounding the l-leucine binding site (amino acids Leu-535, Ala-536, Val-551, Tyr-554, Ala-558, Ala565 and Ala-567; Figure 5) [9].