How Amazing Is Thiolase?

Introduction:

Thiolases are enzymes utilizing the unique thioester chemistry of coenzyme A (CoA)1 derivatives. These CoA- binding enzymes exploit the increased chemical reactivity of the C-C and C-H bonds near the thioester group in their catalysis. Thiolase contribute to fatty-acid β-oxidation in mitochondria.

Reaction : Acyl-CoA + acetyl-CoA <=> CoA + 3-oxoacyl-CoA

Article #1:

Biosynthetic thiolase catalyzes the formation of acetoacetyl-CoA from two molecules of acetyl- CoA. This is a key step in the synthesis of many biological compounds, including steroid hormones and ketone bodies. The thiolase reaction involves two chemically distinct steps; during acyl transfer, an acetyl group is transferred from acetyl-CoA to Cys89, and in the Claisen condensation step, this acetyl group is further transferred to a second molecule of acetyl-CoA, generating acetoacetyl-CoA.

Kursula, Petri, Juha Ojala, and Anne-Marie Lambeir. "The Catalytic Cycle of Biosynthetic Thiolase:  A Conformational Journey of an Acetyl Group through Four Binding Modes and Two Oxyanion Holes." Biochemistry 41.52 (2002): 15543-5556. Print.

Article #2:

The biosynthetic thiolase catalyzes a Claisen condensation reaction between acetyl-CoA and the enzyme acetylated at Cys89. Two oxyanion holes facilitate this catalysis: oxyanion hole I stabilizes the enolate intermediate generated from acetyl-CoA, whereas oxyanion hole II stabilizes the tetrahedral intermediate of the acetylated enzyme. The latter intermediate is formed when the R-carbanion of acetyl-CoA enolate reacts with the carbonyl carbon of acetyl-Cys89, after which C-C bond formation is completed.

Figure. Wild type thiolase active site geometry

Meriläinen, Gitte, Visa Markus Poikela, Petri Kursula, and Rik K. Wierenga. "The Thiolase Reaction Mechanism: The Importance of Asn316 and His348 for Stabilizing the Enolate Intermediate of the Claisen Condensation." Biochemistry 48.46 (2009): 11011–11025. Print.

Article #3:

1. The activities and relative 3-oxoacyl-CoA substrate specificities of oxoacyl-CoA thiolase were determined in a large number of animal tissues. The relative activities with different 3-oxoacyl-CoA substrates varied widely in different tissues and, in addition, the activity as measured with acetoacetyl-CoA (but not with other longer-carbon-chain acyl-CoA substrates) was activated by K+.

2. These properties were due to the presence, in different proportions in each tissue, of three classes of thiolase, all of which use acetoacetyl-CoA as substrate but which have different intracellular locations and substrate specificities and which differ also in kinetic and chromatographic behaviour.

3. Cytoplasmic thiolase activity was found to be widely distributed among different tissues and was due to an acetoacetyl-CoA-specific thiolase. This cytoplasmic activity was found to account for a significant proportion of the total tissue activity towards acetoacetyl-CoA in several tissues, and especially in the brain of newborn rats.

4. Mitochondrial thiolase activity towards acetoacetyl-CoA was due to two different classes of enzyme whose relative amounts varied with the tissue type. An oxoacyl-CoA thiolase of general specificity for the acyl-CoA substrate constituted one class, the other being a specific acetoacetyl-CoA thiolase that differed from its cytoplasmic counterpart in being greatly stimulated by K+.

5. This activation by K+ made it possible to calculate the tissue contents of mitochondrial acetoacetyl-CoA thiolase and mitochondrial oxoacylCoA thiolase from measurements of activity with acetoacetyl-CoA in tissue extracts under defined conditions.

6. The properties and the different thiolases and their tissue distribution is discussed with respect to their possible roles in metabolism.

Middleton, B. "The Oxoacyl-Coenzyme A Thiolases of Animal Tissues." Biochem.J. 132. (1973): 717-730. Print.