KDOaldolase

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Aldolase is an enzyme that catalyzes a reversible reaction. The enzyme decomposes aldose 1,6-diphosphate fructose into phosphotriose dihydroxyacetone phosphate (DHAP) and glycerol triphosphate (G3P). Aldolase can also produce DHAP from other (3S, 4R)-ketose 1-phosphate fructose 1-phosphate and heptaheptose 1,7-bisphosphate. The gluconeogenesis and Calvin cycle of the anabolic pathway use reverse reactions. Glycolysis is a catabolic pathway, which uses a positive reaction. Aldolase is divided into two types according to mechanism.

KDOaldolase

In enzymology, a 3-deoxy-D-manno-octulosonate aldolase (EC 4.1.2.23) is an enzyme that catalyzes the chemical reaction:3-deoxy-D-manno-octulosonate↔ pyruvate + D-arabinose. Hence, this enzyme has one substRate, 3-deoxy-D-manno-octulosonate, and two products, pyruvate and D-arabinose. This enzyme belongs to the family of lyases, specifically the aldehyde-lyases, which cleave carbon-carbon bonds.

Structure

Aldolase is a homotetramer, one of three aldolase isoenzymes (A, B and C) encoded by three different genes. The Aldolase gene contains 8 exons and 5'UTR IB. The key amino acids responsible for its catalytic function have been identified. The residue Tyr363 was used as an acid-base catalyst to protonate the C3 substrate, while Lys146 proposed the C-terminus of the strained structure of the conjugate base Tyr363 generated with a stable negative charge. Residue Glu187 is involved in many functions, including FBP aldolase catalysis, substrate binding, dehydration and acid-base catalysis in the process of substrate cleavage. Although Aldolase is located in the nucleus, it lacks any known nuclear localization signal (NLS).

Figure 1. Structure of Aldolase.

Mechanism

In mammalian aldolase, the key catalytic amino acid residues involved in this reaction are lysine and tyrosine. Tyrosine can act as an effective hydrogen receptor, while lysine can covalently bind and stabilize the intermediate. Many bacteria use two magnesium ions instead of lysine.

Figure 2. Reaction mechanism of aldolase.

Clinical significance

Aldolase is necessary for glycolysis in muscle as a "rapid response" pathway for production of adenosine triphosphate, independent of tissue oxygen. Aldolase catalyzes the conversion of fructose 1,6-diphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, an important reaction in the glycolytic breakdown of glucose to lactate in muscle. Aldolase is a tetramer whose primary structure depends upon the tissue from which it was synthesized (highest expression in liver, muscle, brain).Elevated values are found in muscle diseases, such as Duchenne muscular dystrophy, dermatomyositis, polymyositis, and limb-girdle dystrophy. While elevated creatinine kinase (CK) levels are more sensitive and specific for muscle disease, occasionally elevated aldolase is observed in some patients with myositis that have normal CK values.

Interpretation

Measuring serum muscle enzymes is common in the evaluation of patients with muscle weakness or muscle myalgia. When elevated, serum muscle enzymes can help differentiate muscle disease derived muscle weakness from a neurogenic cause. The highest levels of aldolase are found in progressive (Duchenne) muscular dystrophy. Lesser elevations are found in dermatomyositis, polymyositis, and limb-girdle dystrophy. In dystrophic conditions causing hyperaldolasemia, the increase in aldolase becomes less dramatic as muscle mass decreases.