- A: a/b three-layered sandwiches
- X: B12 binding domain in carbon monoxide dehydrogenase corrinoid/iron-sulfur protein gamma subunit
- H: B12 binding domain in carbon monoxide dehydrogenase corrinoid/iron-sulfur protein gamma subunit
- T: B12 binding domain in carbon monoxide dehydrogenase corrinoid/iron-sulfur protein gamma subunit
- F: CdhD