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The HCP family of iron-sulfur proteins includes hybrid cluster protein (HCP), acetyl-CoA synthase (ACS), and carbon monoxide dehydrogenase (CODH), all of which contain [Fe4-S4] metal clusters at their active sites. These proteins have a conserved alpha-beta rossman fold domain. HCP, formerly known as prismane, is thought to play a role in nitrogen metabolism but its specific function is unknown. Acetyl-CoA synthase (ACS), is found in acetogenic and methanogenic organisms and is responsible for the synthesis and breakdown of acetyl-CoA. ACS forms a heterotetramer with carbon monoxide dehydrogenase (CODH) consisting of two ACS and two CODH subunits. CODH reduces carbon dioxide to carbon monoxide and ACS then synthesizes acetyl-CoA from carbon monoxide and CoA.