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Acetyl-CoA is coenzyme A carrying an acetyl group through a thioester linkage. It is a central two-carbon donor in metabolism.

Condensation of acetyl-CoA with oxaloacetate forms citrate at the beginning of the citric acid cycle. The same carrier also supplies carbon for biosynthetic pathways, so its fate depends on the cell's metabolic state.^[ref]

Bert J. C. Janssen is the first author of a 2005 structural study of Complement C3 and its processed fragment C3c. The team determined crystal structures that connected the arrangement of protein domains to activation and regulation in the complement system.^[ref]

The structures helped explain how a reactive thioester, needed for later covalent attachment, can remain protected against premature hydrolysis. Comparing the native and processed forms also revealed substantial changes in domain arrangement.^[ref]

The comparison is specifically Complement C3 versus C3c. It is not a direct pair of structures showing native Complement C3 and active C3b. Keeping the fragments distinct matters because different stages of processing have different biological roles: an informative structural difference is not automatically the exact movement that creates an active intermediate.

Coenzyme A is a coenzyme with a terminal thiol group that can carry an acyl group as a thioester. Its nucleotide-containing portion is recognized by enzymes, while the sulfur-linked group participates in chemical transformations.

Acetyl-CoA and succinyl-CoA carry different acyl units on the same coenzyme framework. This reuse of a molecular carrier connects fatty acid metabolism with the citric acid cycle.^[ref]

Complement C3 is a central protein of the complement system. Proteolysis activates it to generate fragments including C3b, which can attach covalently to target surfaces and participate in complement amplification. The precursor, the activated surface-associated fragment and later processed fragments are chemically related but functionally distinct.^[ref]

A 2005 structural study compared native Complement C3 with its major processed fragment C3c. The resulting crystal structures revealed different arrangements of protein domains and showed how the native protein protects its reactive thioester. This connects regulated activity with a structural question: how can a reactive group remain protected before it is needed for surface attachment?^[ref]

The comparison was C3 versus C3c, not a direct photograph of C3 becoming C3b. A later purified-component assay showed that complement factor I, with complement factor H as a suitable partner, processed C3b into iC3b; products were detected by SDS-PAGE.^[ref]

Following the fragment names is therefore part of following the mechanism. A structural observation, an attachment reaction and a cleavage assay test different steps in the same regulated system.

Succinyl-CoA is coenzyme A carrying a four-carbon succinyl group through a thioester linkage. It is formed during oxidative decarboxylation of 2-Oxoglutarate in the citric acid cycle.

Conversion of succinyl-CoA to succinate can be coupled to formation of a nucleoside triphosphate. This is a direct energy-conserving step in the cycle, distinct from the later use of respiratory electron carriers to produce ATP.^[ref]

A thioester contains the linkage $\mathrm{R}-\mathrm{C}(=\mathrm{O})-\mathrm{S}-{\mathrm{R}}^{\prime }$, in which sulfur replaces the single-bonded oxygen of an ordinary ester. It is an acyl group joined to a sulfur-containing substituent; it is not the same structural class as a compound with a C=S double bond.

Coenzyme A carries acyl groups through a thioester linkage. Nucleophilic attack at the carbonyl carbon can transfer the acyl group, coupling chemical transformations through an activated carrier. The familiar usefulness of this linkage concerns the free-energy change of the overall transfer or hydrolysis reaction, rather than an isolated “high-energy bond” acting as a store of released energy when simply broken.