The shikimic acid pathway

The shikimic acid pathway is responsible for the biosynthesis of a large number of aromatic compounds, particularly in plants. Most important for many mammals is the fact that plants manufacture the aromatic amino acids Phe (phenylalanine), Tyr (tyrosine), and Trp (tryptophan). These are ‘essential’ amino acids for humans—we have to have them in our diet as we cannot make them ourselves. So how do plants make aromatic rings? A clue to the chemistry involved comes from the structure of caffeoylquinic acid, a compound that forms about 13% of the soluble solids from coffee beans. A substantial proportion of instant coffee is caffeoylquinic acid.

This ester has two six-membered rings—one aromatic and one saturated. You might imagine making an aromatic ring by the dehydration (losing three molecules of water) of a cyclohexane triol and the saturated ring in caffeoylquinic acid looks a good candidate. It is now known that both rings (shown in black) come from the same intermediate, shikimic acid. This key intermediate has given its name to nature’s general route to aromatic compounds and many other related six-membered ring compounds: the shikimic acid pathway. This path way contains some of the most interesting reactions (from a chemist’s point of view) in biol ogy. It starts with an aldol reaction between phosphoenol pyruvate as the nucleophilic enol component and the C4 sugar erythrose 4-phosphate as the electrophilic aldehyde.

Hydrolysis of the phosphate releases the aldol product, a C7 α-keto-acid with one new stereo genic centre, which is in equilibrium with a hemiacetal, just like a sugar. This intermediate has the right number of carbon atoms for shikimic acid and the next stage is a cyclization. If we redraw the uncyclized C7 α-keto-acid in the right shape for cyclization we can see what is needed. The green arrow shows which bond needs to be formed. This bond could be formed by another aldol reaction, and there is an obvious route to the required enol by elimination of phosphate. However, this would require the removal of a proton (green in the diagram) that is not at all acidic.

The problem can be avoided if the hydroxyl group at C5 is first oxidized to a ketone (using NAD+ as the oxidant). Then the green proton is much more acidic, and the elimination becomes an E1cB reaction, similar to the one in the synthesis of PEP. True, the ketone must be reduced back to the alcohol afterwards but nature can deal with that easily. There are obvi ously several more steps to get to shikimic acid but all the C–C bonds are in place, the most significant of them being formed by aldol reactions.

اخر الاخبار

اخبار العتبة العباسية المقدسة

خدمات فكرية وثقافية عديدة قدمها مركز مُلتقى القمر الثقافي لزائري الأربعين

إحياءً لذكرى زيارة الأربعين.. العتبة العباسية المقدسة تواصل إقامة مجلسها العزائي

عبر كاميرات تعمل بالذكاء الاصطناعي.. قسم الاتصالات يوضح تفاصيل جديدة حول آلية عدِّ زائري الأربعين

شعبة الخطابة الحسينية النسوية تواصل نشاطاتها الفكرية والثقافية في مجمع الشيخ الكليني

المزيد

الآخبار الصحية

حجم جزء محدد من الجسم يعكس الصحة العامة ومخاطر الأمراض

تفشي عدوى بكتيرية بفرنسا يحتمل ارتباطها بنوع من الجبن الطري

دراسة حديثة تكشف ما تفعله الشاشات بـ"قلوب الأطفال" !

متلازمة المبنى المريض.. هكذا يتحول تكييف الهواء إلى خطر صحي ؟؟

المزيد

الآخبار التكنلوجية

كيف تتعامل مع حرارة الطقس أثناء القيادة؟

لماذا يجب عليك إيقاف تكييف السيارة قبل إطفاء المحرك؟

دراسة تكشف سببا محتملا وراء زيادة خطر إصابة الأطفال بالتوحد

تشيلي.. العثور على متحجرة حيوان ثديي صغير من زمن الديناصورات

المزيد

مواضيع ذات صلة

Pyrrolidine alkaloids are made from the amino acid ornithine

Alkaloids are made by amino acid metabolism

Phosphoenolpyruvate

Nature’s enolate equivalents: lysine enamines and coenzyme A

Reductive amination in nature

Nature’s NaBH4 is a nucleotide: NADH or NADPH

Oil and water do not mix

Lipids

Most sugars are embedded in complex carbohydrates

Glycosides in nature

Sugars can be fixed in one shape by acetal formation

Sugars normally exist in cyclic forms with much stereochemistry