Adenylate kinases (AKs) are nucleoside monophosphate kinases, which catalyse the phosphorylation of AMP by using ATP or GTP as phosphate donors. In humans, nine different AK isoenzymes have been identified (AK1-9). This entry represents AK1. AK1 is the major cytosolic isoform and is present in all tissues with its highest expression levels in skeletal muscle, brain and erythrocytes [].
Adenylate kinases (AKs) are nucleoside monophosphate kinases, which catalyse the phosphorylation of AMP by using ATP or GTP as phosphate donors. In humans, nine different AK isoenzymes have been identified (AK1-9). This entry includes AK1 and AK5. AK1 is the major cytosolic isoform and is present in all tissues with its highest expression levels in skeletal muscle, brain and erythrocytes []. AK5 expression is limited to brain tissue [].
Aspartate kinase () (AK) catalyzes the first reaction in the aspartate pathway; the phosphorylation of aspartate. The product of this reaction can then be used in the biosynthesis of lysine or in the pathway leading to homoserine, which participates in the biosynthesis of threonine, isoleucine and methionine [].In bacteria there are three different aspartate kinase isozymes which differ in sensitivity to repression and inhibition by Lys, Met and Thr. AK1 and AK2 are bifunctional enzymes which both consist of an N-terminal AK domain and a C-terminal homoserine dehydrogenase domain. AK1 is involved in threonine biosynthesis and AK2, in that of methionine. The third isozyme, AK3 is monofunctional and involved in lysine synthesis. In archaea and plants there may be a single isozyme of AK which in plants is multifunctional.This entry represents a region encoding aspartate kinase activity found in both the monofunctional and bifunctional enzymes.Synonym(s): Aspartokinase
Bacteria, plants and fungi metabolise aspartic acid to produce four amino acids - lysine, threonine, methionine and isoleucine - in a series of reactions known as the aspartate pathway. Additionally, several important metabolic intermediates are produced by these reactions, such as diaminopimelic acid, an essential component of bacterial cell wall biosynthesis, and dipicolinic acid, which is involved in sporulation in Gram-positive bacteria. Members of the animal kingdom do not posses this pathway and must therefore acquire these essential amino acids through their diet. Research into improving the metabolic flux through this pathway has the potential to increase the yield of the essential amino acids in important crops, thus improving their nutritional value. Additionally, since the enzymes are not present in animals, inhibitors of them are promising targets for the development of novel antibiotics and herbicides. For more information see [].Aspartate kinase () (AK) catalyzes the first reaction in the aspartate pathway; the phosphorylation of aspartate. The product of this reaction can then be used in the biosynthesis of lysine or in the pathway leading to homoserine, which participates in the biosynthesis of threonine, isoleucine and methionine [].In bacteria there are three different aspartate kinase isozymes which differ in sensitivity to repression and inhibition by Lys, Met and Thr. AK1 and AK2 are bifunctional enzymes which both consist of an N-terminal AK domain and a C-terminal homoserine dehydrogenase domain. AK1 is involved in threonine biosynthesis and AK2, in that of methionine. The third isozyme, AK3 is monofunctional and involved in lysine synthesis. In archaea and plants there may be a single isozyme of AK which in plants is multifunctional.
This entry represents an S8 family domain found in thermitase as well as other thermostable subtilisin homologues, all of which are stablized by calcium ions [].Thermitase (MEROPS identifier S08.007) is a calcium-dependent, non-specific, serine protease from the thermophilic bacterium Thermoactinomyces vulgaris. The peptidase is maximally active between pH 7.5 and 9.5, and at 85 degrees Celsius. It cleaves a wide variety of proteins including native collagen and elastin []. The tertiary structure of thermitase is similar to that of subtilisin BPN' []. Mesentericopeptidase (MEROPS identifier S08.002) is from the mesophilic bacterium Bacillus mesentericus and has been crystallized []. Subtilisin Ak1 (MEROPS identifier S08.009) from Geobacillus stearothermophilis is stable at 70 degrees Celsius and maximally active at pH 7.5 []. From the crystal structure, subtilisin Ak1 possesses three calcium ions, only two of which correspond to positions in other thermostable subtilisins, and it is this third calcium that provides the greater thermostability []. Halolysin R4 (MEROPS identifier S08.102) from the halophilic archaean Haloferax mediterranei is also included in this entry, but unlike its bacterial relatives, its protein sequence has a long C-terminal extension which if removed abolishes peptidase activity [].This domain is part of a family of domains found in serine peptidases belonging to the MEROPS peptidase families S8 (subfamilies S8A (subtilisin) and S8B (kexin)) and S53 (sedolisin), both of which are members of clan SB [, , , ].Members of the peptidases S8 (subtilisin and kexin) and S53 (sedolisin) clan include endopeptidases and exopeptidases. The S8 family has an Asp/His/Ser catalytic triad similar to that found in trypsin-like proteases, but do not share their three-dimensional structure and are not homologous to trypsin. Serine acts as a nucleophile, aspartate as an electrophile, and histidine as a base. The S53 family contains a catalytic triad Glu/Asp/Ser with an additional acidic residue Asp in the oxyanion hole, similar to that of subtilisin. The serine residue here is the nucleophilic equivalent of the serine residue in the S8 family, while glutamic acid has the same role here as the histidine base. However, the aspartic acid residue that acts as an electrophile is quite different. In S53 it follows glutamic acid, while in S8 it precedes histidine. The stability of these enzymes may be enhanced by calcium, some members have been shown to bind up to 4 ions via binding sites with different affinity. There is a great diversity in the characteristics of their members: some contain disulfide bonds, some are intracellular while others are extracellular, some function at extreme temperatures, and others at high or low pH values [, , , ].
