Crystal structures of N-acetylmannosamine kinase provide insights into enzyme activity and inhibition.

Sialic acids are essential components of membrane glycoconjugates. They are responsible for the interaction, structure, and functionality of all deuterostome cells and have major functions in cellular processes in health and diseases. The key enzyme of the biosynthesis of sialic acid is the bifunctional UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase that transforms UDP-N-acetylglucosamine to N-acetylmannosamine (ManNAc) followed by its phosphorylation to ManNAc 6-phosphate and has a direct impact on the sialylation of cell surface components. Here, we present the crystal structures of the human N-acetylmannosamine kinase (MNK) domain of UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase in complexes with ManNAc at 1.64 Å resolution, MNK·ManNAc·ADP (1.82 Å) and MNK·ManNAc 6-phosphate · ADP (2.10 Å). Our findings offer detailed insights in the active center of MNK and serve as a structural basis to design inhibitors. We synthesized a novel inhibitor, 6-O-acetyl-ManNAc, which is more potent than those previously tested. Specific inhibitors of sialic acid biosynthesis may serve to further study biological functions of sialic acid.

GLIS, a bioactive proteoglycan fraction from Ganoderma lucidum, displays anti-tumour activity by increasing both humoral and cellular immune response.

AIMS: Ganoderma lucidum, a traditional Chinese medicine, is well known as a modulator of functions of the immune system as well as an anti-tumour agent. However, its active compounds and their molecular mechanisms of action are not well established. GLIS, a proteoglycan isolated from the fruiting body of G. lucidum, stimulates directly the activation of B lymphocytes. In this work, the immunoactivation capacities of GLIS as well as its anti-tumour effect were investigated in vitro and in vivo. MAIN METHODS: Tumour-bearing mice were prepared by inoculation of mouse sarcoma S180 cells into BALB/c mice. Lymphocytes and bone marrow-derived macrophages were isolated from spleen and tibia/femurs, respectively. After stimulation with GLIS different immune responses of these cells were analysed. Anti-tumour effect of GLIS was determined. KEY FINDINGS: After treatment with GLIS, spleen-derived B lymphocytes from tumour-bearing mice became activated, proliferated and produced large amounts of immunoglobulins. Bone marrow-derived macrophages from tumour-bearing mice also became activated after exposure to GLIS, and they produced important immunomodulatory substances, such as IL-1ß, TNF-α and reactive nitrogen intermediates, like NO. GLIS markedly increased phagocytosis of macrophages, and very importantly, it markedly raised the macrophage-mediated tumour cytotoxicity. Treatment of mice with GLIS caused an inhibition of mouse sarcoma S180 tumour growth by 60% in vivo. SIGNIFICANCE: These results indicate that GLIS exhibits a capacity to increase remarkably both humoral and cellular immune activities of tumour-bearing mice and inhibits tumour growth significantly. The anti-tumour effect of GLIS results from its capacity to increase the host's immune activity.