Involved in the maturation of [NiFe] hydrogenases. Involved in the biosynthesis of the Fe(CN)(2)CO cofactor (PubMed:15504408, PubMed:23022438). HypD may act as a scaffold on which the Fe(CN)(2)CO cofactor is formed (PubMed:23597401). In complex with HypC, accepts the cyanide ligand generated by HypF and HypE, and also coordinates the carbon monoxide ligand (PubMed:15504408, PubMed:23022438). Required for the formation of all three hydrogenase isoenzymes (Probable). Protein modification; [NiFe] hydrogenase maturation. Monomer (PubMed:15504408). Interacts with HypC (PubMed:12441107, PubMed:15504408, PubMed:23022438). Forms a complex with HypC, or HybG, and HypE (PubMed:15504408, PubMed:16412426). In addition to the iron-sulfur cluster, contains two additional oxygen-labile iron ions. Belongs to the HypD family.