Biosynthesis General

We shall now discuss the biosynthesis of the terpenoids, an area of research which is linked p articularly with the names of Bloch, Lynen, Cornforth, and Popjak. We shall first consider general principles, from 

The biosynthesis begins with acetyl CoA which is combined with a second unit of acetyl CoA to give acetoacetyl CoA. A third acetyl CoA molecule is then added and the 6 C body so produced is hydrogenated to mevalonic acid by means of NADPH -I- H+ with the liberation of coenzyme A. Mevalonic acid, which was discovered as a growth factor for microorganisms, is an important intermediate. The active isoprene, iso-pentenyl pyrophosphate, is derived from it by decarboxylation, dehydration, and phosphorylation with ATP. 

Isopentenyl pyrophosphate (IPP) exists in equilibrium with its isomer, dimethylallyl pyrophosphate. The latter is the fuse without which terpenoid biosynthesis cannot be set in motion. This is because it is only 

If a further unit of IPP is now added to geranyl pyrophosphate, far-nesyl pyrophosphate, a sesquiterpene, is obtained. The addition occurs head to tail the CH2 group, the head of IPP, adds to the pyrophosphate end, the tail, of geranyl pyrophosphate. The resulting farnesyl pyrophosphate can be joined tail to tail to form an open chain triterpene. The latter, in turn, serves as the starting material for the synthesis of the cyclic triterpenes which include the steroids, compounds that are essential for all living organisms. 

Let us consider further the consequences of head-tail additions. If an additional molecule of IPP is added head-to-tail to farnesyl pyrophosphate geranylgeranyl pyrophosphate, a diterpene, is obtained. The series of events outlined above can now be repeated at a higher level of complexity geranylgeranyl pyrophosphate can either be converted to other diter-penes or two molecules of geranylgeranyl pyrophosphate can be joined tail-to-tail to give 40 C bodies. In this way tetraterpenes, i.e. carotenoids, are obtained. Further head-to-tail additions of IPP lead, finally, to the polyterpenes rubber, gutta-percha, and balata. 

---

U.9 Phospholipid biosynthesis (general).- A few only of the very many papers published in this area that appear to have particular mechanistic importance can be described here. Stable carbon isotope ratios ( C/ C) at natural abundance levels were determined for each of the major fatty acid components of the phospholipids of E. coli. The results were consistent with a model of lipid metabolism in which fatty acids were released from the fatty acid synthase in free form, and required re-activation to the acyl-acyl carrier protein prior to esterification. A close coupling of fatty acid and phospholipid synthesis was implied. The characteristics of fatty acid transfer from acyl-acyl carrier protein to sn-glyoerol- -phosphate in E. coli have been inves-... 

Antibiotics have a wide diversity of chemical stmctures and range ia molecular weight from neat 100 to over 13,000. Most of the antibiotics fall iato broad stmcture families. Because of the wide diversity and complexity of chemical stmctures, a chemical classification scheme for all antibiotics has been difficult. The most comprehensive scheme may be found ia reference 12. Another method of classifyiag antibiotics is by mechanism of action (5). However, the modes of action of many antibiotics are stiU unknown and some have mixed modes of action. Usually within a stmcture family, the general mechanism of action is the same. For example, of the 3-lactams having antibacterial activity, all appear to inhibit bacterial cell wall biosynthesis. 

Much of protein engineering concerns attempts to explore the relationship between protein stmcture and function. Proteins are polymers of amino acids (qv), which have general stmcture +H3N—CHR—COO , where R, the amino acid side chain, determines the unique identity and hence the stmcture and reactivity of the amino acid (Fig. 1, Table 1). Formation of a polypeptide or protein from the constituent amino acids involves the condensation of the amino-nitrogen of one residue to the carboxylate-carbon of another residue to form an amide, also called peptide, bond and water. The linear order in which amino acids are linked in the protein is called the primary stmcture of the protein or, more commonly, the amino acid sequence. Only 20 amino acid stmctures are used commonly in the cellular biosynthesis of proteins (qv). 

Generally, the most powerful method for stmctural elucidation of steroids is nuclear magnetic resonance (nmr) spectroscopy. There are several classical reviews on the one-dimensional (1-D) proton H-nmr spectroscopy of steroids (267). C-nmr, a technique used to observe individual carbons, is used for stmcture elucidation of steroids. In addition, C-nmr is used for biosynthesis experiments with C-enriched precursors (268). The availability of higher magnetic field instmments coupled with the arrival of 1-D and two-dimensional (2-D) techniques such as DEPT, COSY, NOESY, 2-D J-resolved, HOHAHA, etc, have provided powerful new tools for the stmctural elucidation of complex natural products including steroids (269). 

Biosynthesis of Tea Flavonoids. The pathways for the de novo biosynthesis of flavonoids in both soft and woody plants (Pigs. 3 and 4) have been generally elucidated and reviewed in detail (32,51). The regulation and control of these pathways in tea and the nature of the enzymes involved in synthesis in tea have not been studied exhaustively. The key enzymes thought to be involved in the biosynthesis of tea flavonoids are 5-dehydroshikimate reductase (52), phenylalanine ammonia lyase (53), and those associated with the shikimate/arogenate pathway (52). At least 13 enzymes catalyze the formation of plant flavonoids (Table 4). 

Occurrence, Fermentation, and Biosynthesis. Although a large number of Streptomjces species have been shown to produce carbapenems, only S. cattkja (2) and S. penemfaciens (11) have been reported to give thienamycin (2). Generally the antibiotics occur as a mixture of analogues or isomers and are often co-produced with penicillin N and cephamycin C. Yields are low compared to other P-lactams produced by streptomycetes, and titres are of the order of 1—20 p-g sohdusmL despite, in many cases, a great deal of effort on the optimization of the media and fermentation conditions. The rather poor stabiUty of the compounds also contributes to a low recovery in the isolation procedures. The fermentation and isolation processes for thienamycin and the olivanic acids has been reviewed in some detail (12). 

Thiostrepton family members are biosynthesized by extensive modification of simple peptides. Thus, from amino acid iacorporation studies, the somewhat smaller (mol wt 1200) nosiheptide, which contains five thiazole rings, a trisubstituted iadole, and a trisubstituted pyridine, is speculated to arise from a simple dodecapeptide. This work shows that the thiazole moieties arise from the condensation of serine with cysteiae (159,160). Only a few reports on the biosynthesis of the thiostrepton family are available (159,160). Thiostrepton is presently used ia the United States only as a poly antimicrobial vetetinary ointment (Panalog, Squibb), but thiazole antibiotics have, ia the past, been used as feed additives ia various parts of the world. General (158) and mechanism of action (152) reviews on thiostrepton are available. 

The proposed pathway for the biosynthesis of the avermectins (Fig. 3) has been described in a review (23). Some of the details are yet to be elucidated, although the steps, in general, are based on firm evidence from four types of studies incorporation of labeled precursors, conversion of putative intermediates by producing strains and blocked mutants, in vitro measurement of biosynthetic enzymes, and studies with enzyme inhibitors. The biosynthesis of the oleandrose units was elucidated from studies using and labeled glucose, which indicated a direct conversion of glucose to... 

The earliest references to cinnamic acid, cinnamaldehyde, and cinnamyl alcohol are associated with thek isolation and identification as odor-producing constituents in a variety of botanical extracts. It is now generally accepted that the aromatic amino acid L-phenylalanine [63-91-2] a primary end product of the Shikimic Acid Pathway, is the precursor for the biosynthesis of these phenylpropanoids in higher plants (1,2). 

The first step in the biosynthesis of eicosanoids from arachidonic acid is generally a lipoxygenation reaction. The resulting hydroperoxides (HPETE s) can undergo reduction to the corresponding alcohols (HETE s). Preparative routes to the 5-, 11-, and 15-HETE s and HPETE s have been developed as oudine below. 

---