Impact of distant peptide substrate residues on enzymatic activity of SlyD.

Peptidyl-prolyl isomerases (PPIases) catalyze intrinsically slow and often rate-limiting isomerization of prolyl-peptide bonds in unfolded or partially folded proteins, thereby speeding up the folding process and preventing misfolding. They often possess binding and chaperone domains in addition to the domain carrying the isomerization activity. Although generally, their substrates display no identity in their amino acid sequence upstream and downstream of the proline with 20 possibilities for each residue, PPIases are efficient enzymes. SlyD is a highly efficient PPIase consisting of an isomerase domain and an additional chaperone domain. The binding of peptide substrates to SlyD and its enzymatic activity depend to some extend on the proline-proximal residues, however, the impact of proline-distant residues has not been investigated so far. Here, we introduce a label-free NMR-based method to measure SlyD activity on different peptide substrates and analysed the data in the context of obtained binding affinities and several co-crystal structures. We show that especially charged and aromatic residues up to eight positions downstream and three positions upstream of the proline and outside the canonical region of similar conformations affect the activity and binding, although they rarely display distinct conformations in our crystal structures. We hypothesize that these positions primarily influence the association reaction. In the absence of the chaperone domain the isomerase activity strongly correlates with substrate affinity, whereas additional factors play a role in its presence. The mutual orientation of isomerase and chaperone domains depends on the presence of substrates in both binding sites, implying allosteric regulation of enzymatic activity.

Is there an influence of perceptual or cognitive impairment on complex sentence processing in hearing aid users?

BACKGROUND: Hearing-impaired listeners often have difficulty understanding complex sentences. It is not clear if perceptual or cognitive deficits have more impact on reduced language processing abilities, and how a hearing aid might compensate for that. METHODS: In a prospective study with 5 hearing aid users and 5 normal hearing, age-matched participants, processing of complex sentences was investigated. Audiometric and working memory tests were performed. Subject- and object-initial sentences from the Oldenburg Corpus of Linguistically and audiologically controlled Sentences (OLACS) were presented to the participants during recording of an electroencephalogram (EEG). RESULTS: The perceptual difference between object and subject leading sentences does not lead to processing changes whereas the ambiguity in object leading sentences with feminine or neuter articles evokes a P600 potential. For hearing aid users, this P600 has a longer latency compared to normal hearing subjects. CONCLUSION: The EEG is a suitable method for investigating differences in complex speech processing for hearing aid users. Longer P600 latencies indicate higher cognitive effort for processing complex sentences in hearing aid users.