Читать книгу Enzyme-Based Organic Synthesis - Cheanyeh Cheng - Страница 39

Acetyltransferases (ATs) catalyze the transfer of an acetyl group from acetyl coenzyme A (Acetyl‐CoA) to a variety of small molecules including lipids, amino acids, drugs, and sugars as well as large molecules such as proteins. The acetyl group donor (acetyl‐CoA) is generated in mitochondria from carbohydrate or amino acid catabolism by acetyl‐CoA synthetase utilizing acetate, CoA, and ATP [46, 47]. The small molecule melatonin synthesis using AT was a typical example of application in organic synthesis concerning the acetyl group transfer. Melatonin has been found in almost all organisms from photosynthetic bacteria to human beings. Melatonin is a potent direct free radical scavenger and antioxidant that strongly protects unicellular organisms, plants, and animals from oxidative insults. Thus, it is important to human health, agriculture applications, and in food and beverage industries [48]. Based on the classic melatonin synthetic pathway in animals, serotonin is first acetylated to form N‐acetylserotonin by arylalkylamine N‐acetyltransferase (AANAT) or arylamine N‐acetyltransferase (SNAT) and N‐acetylserotonin is, subsequently, methylated to form melatonin by N‐acetylserotonin O‐methyltransferase (ASMT) (Scheme 3.14) [48–50]. However, evidences strongly support that the two steps illustrated by Scheme 3.14 could be reversed and may be more important in certain organisms and under certain conditions [48].

Physostigmine is a pyrroloindole alkaloid and is a parasympathomimetic drug that reversibly inhibits acetylcholinesterase. Physostigmine has been used clinically to treat a wide variety of disorders including Alzheimer’s disease, glaucoma, delayed gastric emptying, and orthostatic hypertension. In addition, physostigmine can cross the blood–brain barrier, and this is used to counteract the effects on the central nervous system of overdoses of atropine, scopolamine, and other anticholinergic drugs [51–53]. Just like melatonin, physostigmine is derived from tryptophan; however, the biosynthetic pathway for physostigmine involves more steps that are characterized by unusual reaction cascade consisting of highly coordinated methylation and acetylation/deacetylation reactions [51]. The N‐acetylation of serotonin to produce N‐acetyl‐5‐hydroxytryptamine catalyzed by N‐acetyltransferase is the third reaction step of the reaction cascade that is the same step as the first step in Scheme 3.14 for melatonin synthesis.

Scheme 3.14 Synthetic pathway of melatonin from serotonin by two kinds of acetyltransferases.

Source: Based on Tan et al. [48]; Weissbach and Redfield [49]; Axelrod and Weissbach [50].

The GCN5‐related N‐acetyltransferase (GNAT) superfamily encompasses more than 10 000 related enzymes in all kingdoms of life and human acetyl‐CoA: glucosamine‐6‐phosphate N‐acetyltransferase 1 (GNA1) belongs to a member of it [54, 55]. In the cytosol, GNA1 catalyzes the transfer of an acetyl group from the acetyl coenzyme A donor substrate to the acceptor substrate glucosamine‐6‐phosphate (GlcN6P) to form N‐acetyl‐glucosamine‐6‐phosphate (GlcNAc6P) that is used as an intermediate in the biosynthesis of UDP‐GlcNAc. Since N‐butyryl glucosamine (GlcNBu), an analog of GlcNAc, has been shown the healing properties of bone and articular cartilage in animal’s arthritis, enzymatic synthesis of GlcNBu is important for biomedical applications [56–58]. Research results have shown that both acetyl and n‐butyl groups were transferred to GlcN6P by GNA1 to form corresponding GlcNAc6P and GlcNBu6P, respectively [54]. Therefore, GlcNBu6P can be easily and subsequently converted to GlcNBu by alkaline phosphatase.

Instead of small molecules acetylation, reversible protein acetylation by corresponding acetyltransferase is a ubiquitous means for the rapid control of diverse cellular processes [59, 60]. The acetyl group on acetyl‐CoA is transferred to the ε‐amino group of lysine residues of the acceptor protein by acetyltransferase. This is just the case for the acetylation of histones by histone acetyltransferases (HATs) that has an important role in transcriptional regulation by remodeling chromatin structure. The acetylation of histones may result in altering interactions between protein–protein and protein–DNA complexes due to the neutralization of positive charged lysine in the amino termini of histone proteins [61, 62]. In higher organisms, aberrant acetylation of lysine residues in histone tails correlates with diseases such as cancers and developmental disorders and may contribute to modulation of cell life span [63, 64]. In mammalian mitochondrial matrix, endoplasmic reticulum lumen, and peroxisomes, carnitine acetyltransferase (CrAT) catalyzes the reversible transfer of acetyl groups between acetyl‐CoA and l‐carnitine (β‐hydroxy‐γ‐trimethylammonium butyrate). The main function of carnitine is the transfer of long‐chain fatty acids to mitochondria for subsequent β‐oxidation. CrAT is homologous to other carnitine acyltransferases, particularly, to carnitine palmitoyltransferase 1 (CPT I) that serves the regulation of long‐chain fatty acid metabolism. Therefore, the reversibly catalyzed reaction between acetyl‐CoA and carnitine by CrAT makes CrAT a regulator for the cellular pool of CoA that, in turn, plays a role as a carrier of activated acetyl groups in the oxidation of energy metabolism substrates and in the synthesis of fatty acids and lipids. It is also known that the accumulation of fatty acyl‐CoAs in heart may induce apoptosis and inflammation and acetyl‐carnitine improves cognition in the brain [65–67].

Scheme 3.15 Protein acetylation with CRTase and DAMC without involving acetyl‐CoA.

Source: Arora et al. [71].

The model acetoxy‐coumarins (AC), 7,8‐diacetoxy‐4‐methylcoumarin (DAMC), was shown to possess radical scavenger property by interacting with free radical to remove its acetyl group and give the acetyl cation (CH₃CO⁺) and the phenoxyl radical [68]. The antioxidant action of DAMC is independent on the formation of parent 7,8‐dihydroxy‐4‐methylcoumarin (DHMC). Calreticulin (CR) catalyzes the transfer of acetyl groups from AC to certain proteins [69, 70]; thus, CR was termed calreticulin transacetylase (CRTase). The enzymatic acetylation of protein by CRTase is unique and characterized as without involving acetyl‐CoA. CRTase of rat tracheal smooth muscle cells (TSMC) was characterized the specificity of DAMC for acetylating and activating nitric oxide synthase (NOS) as illustrated by Scheme 3.15 [71]. Since the activated TSMC NOS will enhance NO in airway cells, and NO is believed to ameliorate the exacerbation of airway diseases such as asthma and coronary obstructive pulmonary diseases (COPD), AC may be expected to find therapeutic applications in respiratory diseases [71, 72].