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= Maltase-Glucoamylase (MGAM) =

PDB: 3L4W Maltase-Glucoamylase (PDB: 3L4W) is a hydrolase involved in the complete processing of starch into glucose molecules. A hydrolase catalyzes the following chemical reaction

$$A-B+H_2O\longrightarrow A-OH+B-H$$

In humans, maltase–glucoamylase (MGAM) is one of the two enzymes responsible for catalyzing the last stage of glucose release in amylose and amylopectin digestion. In the first step of starch digestion the hydrolases of saliva and pancreas (called endo-hydrolases) cleave the internal α-(1→4) bonds of starch into shorter linear and branched chains. The resulting mixture is then further hydrolyzed into glucose in the small intestine by two exo-hydrolases: maltase-glucoamylase (MGAM) and sucrose-isomaltase (SI). These exo-hydrolases can be found primarily in the lumen of small intestine in humans.

MGAM domains:
MGAM is a membrane-bound hydrolase enzyme that contains two catalytic domains: an N-terminal domain (NtMGAM) that is proximal to the membrane-bound end and a C-terminal domain (CtMGAM) (Figure 1). Both domains are classified under the glycosyl hydrolase family 31 (GH31). These two catalytic domains are structurally and functionally similar and both share a 40% of amino acid sequence.

Both subunits participate in the hydrolytic cleavage of α-(1→4) bonds in oligosaccharides. 20 years ago it had not yet been established whether NtMGAM and CtMGAM had different substrate specificities because there were few studies on isolated N- and C-terminal MGAM subunits. From a study developed with pig MGAM, it was inferred that domains have similar specificities and contain both maltase and glucoamylase activities. However, in present day with the development of studies of recombinant enzymes, it was revealed that, despite its structural similarity, the N-terminal catalytic domain had the highest activity against maltose, whereas the C-terminal domain has a broader substrate specificity and activity against glucose oligomers. The study of MGAM catalytic domains is important for nutritional and metabolic purposes because the exacerbated activity of this type of enzyme is related to high levels of blood sugar.

Mechanism of action:
The principal biological function of MGAM is to complete the digestion of starch in living organisms. In the small intestine of humans this enzyme works in synergy with sucrase-isomaltase (SI) to digest the full range of starches. The catalytic subunits of MGMA operate through a mechanism that results in the retention of configuration at the anomeric center of glucose. Both subunits (NtMGAM and CtMGAM) produce the cleavage of α-(1→4) linkage in oligosaccharides

The MGAM and SI present overlap in their hydrolase activities because both cleavage α-(1→4) linkages of linear molecules. Starch is composed of amylose and amylopectin (Figure 2). Since this oligosaccharide is the predominant component of human diet, it is the main substrate of MGAM and SI. In the processing of starch SI produce the hydrolytic cleavage of α-(1→4), α-(1→6) and α-(1→2) linkages meanwhile MGAM just cleavage α-(1→4) linkages (Figure 3). Nevertheless, MGAM can compensates the multi-activity of SI by showing higher hydrolytic activities.

Resemblance between MGAM and SI:
The MGAM and SI catalytic subunits share approximately 40–60% of amino acid sequence, both contain the catalytic site signature sequence WiDMNE, a characteristic of GH31 subgroup 4 members. It is believed that MGAM and SI genes have evolved by duplication of an ancestral gene, and therefore that is the principal reason of the resemblance of the catalytic subunits (N-terminal and C-terminal) of MGAM and SI. Both are more closely related in sequence to one another than are the N- and C-terminal subunits within the same protein (see Figure 1).

Inhibition of MGAM:
The inhibition of α-glucosidases as MGAM is a target for the treatment of Type 2 diabetes. Since the function of MGAM is mainly to release glucose from starch, in diabetic patients the inhibition of this enzyme could result in a decrease of blood sugar levels. From a therapeutic standpoint, the inhibition mechanism of MGAM is currently used to delay glucose production and therefore used as an aid in the treatment of Type 2 diabetes.

Acarbose and miglitol are α-glucosidase inhibitors currently available on the market. Acarbose has broad, but weak inhibitory activity against some α-glucosidases such as MGAM. A structural analysis of the NtMGAM–acarbose complex revealed that acarbose is bound to the NtMGAM active site mainly through side-chain interactions with its acarvosine unit, and therefore there are almost no interactions with its glycone rings. Since both inhibitors work by reversibly inhibiting MGAM, it is believed that the lower interaction between acarbose and the binding site of MGAM is the reason of its weak inhibitory activity. In present day, compounds with greater inhibitory activity are being developed with the purpose of creating more efficient drugs that could decrease blood sugar levels in patients suffering from Type 2 diabetes.