Propionibacterium acnes is an anaerobic gram-positive bacterium found in the niche of the sebaceous glands in the human skin, and is a causal pathogen of inflammatory skin diseases as well as periprosthetic joint infection. To gain effective control of P. acnes, a deeper understanding of the cellular metabolism mechanism involved in its ability to reside in this unique environment is needed. P. acnes exhibits typical cell membrane features of gram-positive bacteria, such as control of membrane fluidity by branched-chain fatty acids (BCFAs). Branching at the iso- or anteiso-position is achieved by incorporation of isobutyryl- or 2-methyl-butyryl-CoA via beta-ketoacyl acyl carrier protein synthase (KAS III) from fatty acid synthesis. Here, we determined the crystal structure of P. acnes KAS III (PaKAS III) at the resolution of 1.9 angstrom for the first time. Conformation-sensitive urea polyacrylamide gel electrophoresis and tryptophan fluorescence quenching experiments confirmed that PaKAS III prefers isobutyryl-CoA as the acetyl-CoA, and the unique shape of the active site cavity complies with incorporation of branched-short chain CoAs. The determined structure clearly illustrates how BCFA synthesis is achieved in P. acnes. Moreover, the unique shape of the cavity required for the branched-chain primer can be invaluable in designing novel inhibitors of PaKAS III and developing new specifically targeted antibiotics. (C) 2018 Elsevier Inc. All rights reserved.