SAM

Enzymes that use SAM

120 members of the SAM utilising MT family - Substrates vary Very little homology, conserved structure Substrate binding more varied than SAM binding

Pathway Source of 5' deoxyadenosyl radicals

3 independent studies on different Iron-sulfur proteins suggest that : SAM required - Acts as a source of free radicals in cells The enzymes for free radical availability also require 4Fe-4S system chelated by only 3 cysteines in a conserved seq. : CXXXCXXC Therefore 1 accessible co-ordination site Also needs an e- source Named : RADICAL SAM (100s ID by bioinformatics SAM binds free iron via its amino and carboxylate gp SAM receives an e- from reduced 4Fe-4S Cleavage then releases Methionine and 5-deoxyA radical 5-deoxyA radical - allows generation of other radicals These are then involved In various oxidation/ Reduction reactions e.g. RNR and SAM decarboxylase

Why methylation

Among the acceptors modified by S-adenosylmethionine are specific bases in DNA. The methylation of DNA protects bacterial DNA from cleavage by restriction enzymes (Section 9.3). The base to be methylated is flipped out of the DNA double helix into the active site where it can accept a methyl group from S-adenosylmethionine (Figure 24.15). A recurring S-adenosylmethionine-binding domain is present in many SAM-dependent methylases.

Pathway - Donation of aminoakyl group

Example a. Biosyn. Of 1-amino-cyclopropane -1-carboxylic acid Involved in ethylene Biosyn. In plants Ethylene Exists as a gas Acts as a hormone Regulates fruit Ripening, flower Opening and Repsonse to Flooding, Drought etc.. Quorum sensing- Regulate bioillumnicscence and pathogenicity Formation of 3-amino-3carboxypropyl-uridine Modified nucleoside of tRNA at the supplementary Loop 7-MeG-U-C seq. Formation of polyamines: Organic compounds with 2 or more amines Growth factors which act on ion channels Conversion of putrescine to spermidine

WHEN

First discovered in 1953 Major role : methyl-donor in methylation reactions Highly favoured- thermodynamic Role of methylation : Epigenetics and tRNA mod. Fetal development Brain function PPL methylation - fluidity

Pathway Donation of Amino group

Formation of DAPA (diaminopelarginic acid) From KAPA (7-keto-8-amino-pelargonic acid DAPA synthase - Involved in biotin synthesis KAPA DAPA The only Aminotransferase known to use SAM

Pathway- Cyclopropane formation

Found in PPL of eubacteria and some Eukaryotes Bacteria - formed as shown by CFA synthase CFA syn. Inhibited by SAM analogues Studies of the reaction have been hindered by the instability of the enzyme In enzyme active site Nucleophilic double bond Attacks electrophilic Me gp SAH released Carbocation formed An unknown basic aa in active site - accepts a proton, causing ring closure Cis isomer formed - no rotation = tight binding by the enzyme

Disease

Methione synthase requires methyl-tetra hydrofolate For methyl gp Low folate availability / mutation of MTHF synthesis gene = many major diseases Various cancers Pathogenic brain function Therefore few organisms lack SAM synthase Fungi : Pneumocystis does and gets SAM from host Acts as a parasite and dangerous in immunosurpressed patients

methionine synthas

Methionine can be regenerated by the transfer of a methyl group to homocysteine from N 5-methyltetrahydrofolate, a reaction catalyzed by methionine synthase (also known as homocysteine methyltransferase).

Methionine degradation

Methionine is converted into succinyl CoA in nine steps (Figure 23.2.7). The first step is the adenylation of methionine to form S-adenosylmethionine (SAM), a common methyl donor in the cell (Section 24.2.7). Methyl donation and deadenylation yield homocysteine, which is eventually processed to α-ketobutyrate. The enzyme α-ketoacid dehydrogenase complex oxidatively decarboxylates α-ketobutyrate to propionyl CoA, which is processed to succinyl CoA as described in Section 23.3.3.

Pathway- Donation of ribose group

Queuosine biosynthesis

Where

S-Adenosylmethionine is also the precursor of ethylene, a gaseous plant hormone that induces the ripening of fruit. SAdenosylmethionine is cyclized to a cyclopropane derivative that is then oxidized to form ethylene. The Greek philosopher Theophrastus recognized more than 2000 years ago that sycamore figs do not ripen unless they are scraped with an iron claw. The reason is now known: wounding triggers ethylene production, which in turn induces ripening. Much effort is being made to understand this biosynthetic pathway because ethylene is a culprit in the spoilage of fruit.

Riboswatches

SAM can bind RNA Riboswitches = highly structured RNA regions within untranslated regions of mRNAs that sense metabolites (e.g. SAM SAM binds mRNA - alters secondary loop formation = affects translation - may obscure ribosome binding site terminator element formed '-side of the P1 helix is used to form an antiterminatior element (AT) May play a role in novel antibiotics

S-Adenosylmethionine Is the Major Donor of Methyl Groups

Tetrahydrofolate can carry a methyl group on its N-5 atom, but its transfer potential is not sufficiently high for most biosynthetic methylations. Rather, the activated methyl donor is usually S-adenosylmethionine (SAM), which is synthesized by the transfer of an adenosyl group from ATP to the sulfur atom of methionine. The methyl group of the methionine unit is activated by the positive charge on the adjacent sulfur atom, which makes the molecule much more reactive than N 5-methyltetrahydrofolate. The synthesis of S-adenosylmethionine is unusual in that the triphosphate group of ATP is split into pyrophosphate and orthophosphate; the pyrophosphate is subsequently hydrolyzed to two molecules of Pi . S-Adenosylhomocysteine is formed when the methyl group of S-adenosylmethionine is transferred to an acceptor. S-Adenosylhomocysteine is then hydrolyzed to homocysteine and adenosine.