REN and chronic obstructive pulmonary disease: Functional analysis of pulmonary microbiota based on the KEGG database indicated that amino acid-related enzymes, aminoacyl-tRNA biosynthesis, DNA replication proteins, folate biosynthesis, glycine, serine and threonine metabolism, histidine metabolism, mineral absorption, purine metabolism, renin-angiotensin system, riboflavin metabolism, ribosome, and sporulation pathways were more abundant in the COPD group than controls.