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The overall goal for this review is to summarize the current body of knowledge about the structure and function of major known antigens of Streptococcus pneumoniae, a major gram-positive bacterial pathogen of humans. This information is then related to the role of these proteins in pneumococcal pathogenesis and in the development of new vaccines and/or other antimicrobial agents. S. pneumoniae is the most common cause of fatal community-acquired pneumonia in the elderly and is also one of the most common causes of middle ear infections and meningitis in children. The present vaccine for the pneumococcus consists of a mixture of 23 different capsular polysaccharides. While this vaccine is very effective in young adults, who are normally at low risk of serious disease, it is only about 60% effective in the elderly. In children younger than 2 years the vaccine is ineffective and is not recommended due to the inability of this age group to mount an antibody response to the pneumococcal polysaccharides. Antimicrobial drugs such as penicillin have diminished the risk from pneumococcal disease. Several pneumococcal proteins including pneumococcal surface proteins A and C, hyaluronate lyase, pneumolysin, autolysin, pneumococcal surface antigen A, choline binding protein A, and two neuraminidase enzymes are being investigated as potential vaccine or drug targets. Essentially all of these antigens have been or are being investigated on a structural level in addition to being characterized biochemically. Recently, three-dimensional structures for hyaluronate lyase and pneumococcal surface antigen A became available from X-ray crystallography determinations. Also, modeling studies based on biophysical measurements provided more information about the structures of pneumolysin and pneumococcal surface protein A. Structural and biochemical studies of these pneumococcal virulence factors have facilitated the development of novel antibiotics or protein antigen-based vaccines as an alternative to polysaccharide-based vaccines for the treatment of pneumococcal disease

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Hyaluronate lyase enzymes degrade hyaluronan, the main polysaccharide component of the host connective tissues, predominantly into unsaturated disaccharide units, thereby destroying the normal connective tissue structure and exposing the tissue cells to various endoand exogenous factors, including bacterial toxins. The crystal structures of Streptococcus pneumoniae hyaluronate lyase with tetra-and hexasaccharide hyaluronan substrates bound in the active site were determined at 1.52-and 2.0-Å resolution, respectively. Hexasaccharide is the longest substrate segment that binds entirely within the active site of these enzymes. The enzyme residues responsible for substrate binding, positioning, catalysis, and product release were thereby identified and their specific roles characterized. The involvement of three residues in catalysis, Asn 349 , His 399 , and Tyr 408 , is confirmed, and the details of proton acceptance and donation within the catalytic machinery are described. The mechanism of processivity of the enzyme is analyzed. The flexibility (allosteric) behavior of the enzyme may be understood in terms of the results of flexibility analysis of this protein, which identified two modes of motion that are also proposed to be involved in the hyaluronan degradation process. The first motion describes an opening and closing of the catalytic cleft located between the ␣-and ␤-domains. The second motion demonstrates the mobility of a binding cleft, which may facilitate the binding of the negatively charged hyaluronan to the enzyme.Streptococcus pneumoniae colonizes predominantly the upper respiratory tract of humans and is a major human pathogenic bacterium. It is one of the key causes of life-threatening disease such as pneumonia, bacteremia, and meningitis (1). It also causes less threatening diseases that are, however, very prevalent like otitis media and sinusitis (2). Pneumococci interact with the host and its tissues through the surface sugars (capsule) and a variety of usually surface-exposed protein molecules. These interactions are essential for the full pathogenicity of these bacteria and are likely involved in the diseasecausing processes. The proteins known to be involved in this interaction include, among others, hyaluronate lyase (3, 4), pneumolysin (5), pneumococcal surface protein A (6), and pneumococcal surface antigen A (7, 8).S. pneumoniae hyaluronate lyase (SpnHL) 1 primarily degrades hyaluronan (HA), the predominant polysaccharide component of animal and human connective tissues and the nervous system, into unsaturated disaccharide units as the end products (9). Cells in the connective tissues are embedded in the strikingly viscoelastic HA matrix. Hyaluronan is a polymeric glycan composed of linear repeats of a few hundred to as many as 20,000 or more disaccharide units of glucuronic acid and N-acetylglucosamine. The glycosidic linkage present within the disaccharide unit is ␤-1,3, whereas the disaccharide units are connected with the ␤-1,4-glycosidic linkage. The HA metabolism seems to be more si...

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