Krishna K. Bhandary

Structural relationship between the enzymatic and streptococcal binding sites of human salivary α-amylase

Previous studies have demonstrated that human salivary alpha-amylase specifically binds to the oral bacterium Streptococcus gordonii. This interaction is inhibited by substrates such as starch and maltotriose suggesting that bacterial binding may involve the enzymatic site of amylase. Experiments were performed to determine if amylase bound to the bacterial surface possessed enzymatic activity. It was found that over one-half of the bound amylase was enzymatically active. In addition, bacterial-bound amylase hydrolyzed starch to glucose which was then metabolized to lactic acid by the bacteria. In further studies, the role of amylases histidine residues in streptococcal binding and enzymatic function was assessed after their selective modification with diethyl pyrocarbonate. DEP-modified amylase showed a marked reduction in both enzymatic and streptococcal binding activities. These effects were diminished when DEP modification occurred in the presence of maltotriose. DEP-modified amylase had a significantly altered secondary structure when compared with native enzyme or amylase modified in the presence of maltotriose. Collectively, these results suggest that human salivary alpha-amylase may possess multiple sites for bacterial binding and enzymatic activity which share structural similarities.

Structural characteristics of human salivary statherin: a model for boundary lubrication at the enamel surface.

A three-dimensional structural model for salivary statherin in aqueous phase has been developed using structure prediction, circular dichroism, molecular modeling, and mechanics. The relevant structural features of statherin are N-terminal helix segment connected to a long poly-L-proline type II segment, which is followed by a short extended structure. Using this model, the hydroxyapatite binding ability of statherin has been explained. The hydroxyapatite binding region is comprised of the N-terminal acidic residues (Asp-pSer-pSer-Glu-Glu) and Glu-26, which are clustered together in space. Partial conformational unfolding and oriented aggregation of several statherin molecules at the enamel surface provides an amphipathic film that is responsible for the boundary lubrication exhibited by statherin.

Structure of a photodimer of 3-acetoxy-2-inden-1-one : 9,10-dioxoindano[2',3':4,3]cyclobuta[1,2-b]indan-4b,4c-diyl diacetate

C22H16O6, Mr = 376.37, monoclinic, P21/c, a = 9.555 (3), b = 15.664 (2), c = 12.300 (4) A, beta = 100.08 (2) degrees, V = 1812.5 (5) A3, Z = 4, Dx = 1.379 g cm-3, lambda(Cu K alpha) = 1.5418 A, mu = 8.0 cm-1, F(000) = 784, room temperature, R = 0.047, wR = 0.064 for 3203 observed reflections [I greater than 3 sigma (I)]. The molecule exists as the syn-trans isomer in the crystal. The crystal structure exhibits a number of C--H...O intermolecular contacts.

A new class of substituted 1,2,4-triazolo-1,3,4-thiadiazepines

A series of 3-aryloxymethyl-(5-nitro-2-furyl)-6-phenyl-1,2,4- triazolo[3,4-b][1,3,4]thiadiazepine compounds have been synthesized recently by a new route. Reported here are the structures of two such compounds with para-substituted aryloxymethyl groups: one has a chloro group, 3-(4-chlorophenyl-oxymethyl)-8-(5-nitro-2-furyl)-6-phenyl-1,2,4- triazolo[3,4-b][1,3,4]thiadiazepine, C22H14C1N5O4S, TD1, and the other a methyl group, 3-(p-tolyloxy-methyl)-8-(5-nitro-2-furyl)-6-phenyl-1,2,4- triazolo-[3,4-b][1,3,4]thiadiazepine, C23H17N5O4S, TD7. The nitrofuryl and phenyl groups on the thiadiazepine ring are each found to adopt a similar conformation in the two structures, whereas the aryloxymethyl substituents on the triazole rings are conformationally different from each other. Each thiadiazepine ring adopts a boat conformation with the S atom at the apex. The interplanar angle between the triazole ring and the thiadiazepine ring is 30 degrees for both compounds. The conformation of the aryloxy-methyl group is dependent on the intermolecular interactions that arise as a result of the polarity of the para substituent. The Cl group in TD1 is involved in a C-Cl...O non-bonded interaction with a Cl...O distance of 3.100 (3) A and a C-Cl...O angle of 138.4 (1) degree. TD7 has a stacking interaction involving the nitrofuryl groups.