The structural basis of beta-peptide-specific cleavage by the serine protease cyanophycinase.

Cyanophycin, or poly-L-Asp-multi-L-Arg, is a non-ribosomally synthesized peptidic polymer that is used for nitrogen storage by cyanobacteria and other select eubacteria. Upon synthesis, it self-associates to form insoluble granules, the degradation of which is uniquely catalyzed by a carboxy-terminal-specific protease, cyanophycinase. We have determined the structure of cyanophycinase from the freshwater cyanobacterium Synechocystis sp. PCC6803 at 1.5-A resolution, showing that the structure is dimeric, with individual protomers resembling aspartyl dipeptidase. Kinetic characterization of the enzyme demonstrates that the enzyme displays Michaelis-Menten kinetics with a k(cat) of 16.5 s(-1) and a k(cat)/K(M) of 7.5x10(-6) M(-1) s(-1). Site-directed mutagenesis experiments confirm that cyanophycinase is a serine protease and that Gln101, Asp172, Gln173, Arg178, Arg180 and Arg183, which form a conserved pocket adjacent to the catalytic Ser132, are functionally critical residues. Modeling indicates that cyanophycinase binds the beta-Asp-Arg dipeptide residue immediately N-terminal to the scissile bond in an extended conformation in this pocket, primarily recognizing this penultimate beta-Asp-Arg residue of the polymeric chain. Because binding and catalysis depend on substrate features unique to beta-linked aspartyl peptides, cyanophycinase is able to act within the cytosol without non-specific cleavage events disrupting essential cellular processes.

Hearing loss in Chinese school children with Down syndrome.

OBJECTIVE: There is well-documented evidence in the literature concerning a high prevalence of deafness in children with Down syndrome (DS). The aim of this study was to examine the extent of hearing impairment and address the rehabilitation needs of a Chinese population with DS who were either in special schools or integrated into mainstream schools. METHODS: This study screened 92 children with DS at their own schools and 11 were reassessed in the University of Hong Kong Hearing Centre. Hearing status of the children with DS was analyzed on the basis of their screening and reassessment results for tympanometric, transient evoked otoacoustic emission (TEOAE) and pure tone audiometric examinations. RESULTS: A high point prevalence of hearing impairment (78% by ears or 90% by participants) in a Chinese school-aged sample of children with DS was noted. The most common degree of loss was mild to moderate. No significant gender difference, age effect or ear asymmetry was found for tympanometric failure or absence of TEOAE. An unfavorable mean speech intelligibility index score (0.2) was found for this group of children. CONCLUSIONS: Sound field amplification and suitable acoustic modifications to classrooms were recommended for Chinese children with DS in Hong Kong to improve their listening and learning environment. The point prevalence of hearing impairment in older children with DS in this study was in contrast to a previous local study on a younger age group. Further effort is needed to determine the role of possible aging effects on the type and prevalence of hearing impairment in populations with DS.

Length and content of family practice residency training.

BACKGROUND: Family practice residency programs are based largely on a model implemented more than 30 years ago. Substantial changes in medical practice, technology, and knowledge necessitate reassessment of how family physicians are prepared for practice. METHODS: We simultaneously surveyed samples of family practice residency directors, first-year residents, and family physicians due for their first board recertification examination to determine, using both quantitative and qualitative methods, their opinions about the length and content of family practice residencies in the United States. RESULTS: Twenty-seven percent of residency directors, 32% of residents, and 28% of family physicians favored extending family practice residency to 4 years; very few favored 2- or 5-year programs. There was dispersion of opinions about possible changes within each group and among the three groups. Most in all three groups would be willing to extend residency for more training in office-based procedures and sports medicine, but many were unwilling to extend residency for more training in surgery or hospital-based care. Residents expressed more willingness than program directors or family physicians to change training. Barriers to change included disagreement about the need to change; program financing and opportunity costs, such as loss of income and delay in debt repayment; and potential negative impact on student recruitment. CONCLUSION: Most respondents support the current 3-year model of training. There is considerable interest in changing both the length and content of family practice training. Lack of consensus suggests that a period of elective experimentation might be needed to assure family physicians are prepared to meet the needs and expectations of their patients.