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.
