Comprehensive Overview of Monolignol Biosynthesis and Lignin Formation Pathways

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course on pharmacognosy and metabolic engineering this is lecture 50 where I will discuss about the monol liol ligins

and ligant so the concepts covered will be biosynthesis of hydroxycinnamic acids

biosynthesis of monolignols new view of monolignol pathway and ligins and if time permits I will talk briefly on the

liance otherwise liant aspect will be covered in one of the subsequent classes so let's go to the next slide so

in the previous class I have given an overview of pH propanoid metabolism this is just to make a quick revision that uh

this started with phy alanin and uh by the action of the enzyme pal

Phile alanin converted into trinamic acid and trinamic acid is basically a C6 C3

compound and trinamic acid again converted into into parakum aric acid by an enzyme called camate 4

hydroxy uh so it acts fourth position o and it forms four hydroxycinnamic acid or parakum aric acid after that parakum

aric acid will get activated uh by addition of ko so it's a KO

lias so and then finally it forms the different monolignols so uh so our point of discussions today is about the ligin

and the precursor of ligin is the monolignol so monolignols are nothing but the precursor of lignin so that is

our mostly the point of discussion and uh this is basically the structure of hydroxycinnamic acids uh which I have

also shown in the previous class uh so for example how the structure changes I

would like to show you this is called trans camic acid

and once uh o is added this is camic acid or trans camic

acid and if one o is added here then it will become parakum aric acid or four karic

acid or four hydroxy camic acid same word so now this structure can be further modified

into by addition of one hydroxic group here so

now this structure is called cafeic acid or 34 dihydroxy camic

acid from this if you compare the structure between this karic acid and caic

acid then it is quite clear that a simple Hydrox silation

at three position of the Benin ring will convert foric acid to cafeic acid so

ideally one should say that there might be an enzyme called uh

c3h which maybe a cam 3 hydroxy which converts uh Kumar to Cafe it and uh lot of attention was paid on this and

initially it was thought to be a soluble enzyme but much later when molecular tools were explode then it was found

that this conversion is not that simple so in other word Kumar to Cafe conversion is not possible through this

way okay so we will see then how this conversion will happen but let us now go to the other structures for example this

peric acid ferulic acid this one was O is basically the

cafet now if you remove the O and add

ch3 so it will become feric acid so one thumb rule is this in order to have the

methylation first there will be Hydrox silation followed by the methylation so and ideally such conversion is possible

so a methy donor should be available and then it makes this one then subsequently there is another structure which is also

elucidated this is peric acid which is O addition here so when o added at

five position so it is called five hydroxy ferulic

acid and this five hydroxy furic acid can be converted into Copic acid as you see the structure here so this at five

position now this is replaced with a methoxy group so this is five position this is six position this is 1

2 3 4 so this will become Sy acids so again there will be a donor for this methy group so this is basically all the

hydroxamic acids and it was thought that hydroxy camic acids act as a precursor for several uh uh phenolic compounds

including the monolignols now hydroxy Camile alcohols are the monolignols because these are

the precursors one is the Conifer alcohol which basically coming from ferulic Acid because structure if you

remember the structure it is this one and the other one is basically uh the Kumari alcohol who are

only o group so which is basically originated from karic Acid so this is for Kumar alcohol or parar alcohol or

same and the copile is basically originated from Copic acid so the

structure as you see here so this is the copile alcohol so these three are the uh three major monol liol that

contribute to the formation of uh ligin okay so the overview of aromatic metabolism in Plants what I have started

talking from the last class so phos phenol pyate peros for phosphate that joins by the dhp synthes which produce

dhp and I have mentioned that this is a very active step because this is a regulatory step and eventually via

simate it forms Kismet from Kismet one roote goes to tyrosin other root goes to phy alanin and there is

another root which goes to tryptophan okay so and phy alanin is basically the starting point for uh Phil propanoid

metabolism although tyrosin can also be the starting point particularly for the monocots and we have also mentioned

about the pal enzyme which is phenyalanine ammonia lies which releases the ammonia from here and converting F

into camic acid so this camic acid usually trans camic acid and this subsequently by the action of cinam for

hydroxy it forms paric acid and subsequently other hydroxycinnamic acids this is a common structure and this step

is very important which is the activation of hydroxy I acid by the 4cl I have also mentioned this so this is

basically addition of qu Hester and this is now the activated compound from their different root uh

emerges for example uh the root towards flavonoid and antoin moves in this way the root towards ligin which Moves In

This Way the root towards other penal proponent that moves in this way so specific Branch points so this is

General overview of the aromatic acid metabolism and then now we will I have already mentioned about this

structures uh if you the paracom alcohol I have mention just in the previous slide so which is one of the

monolignols and the lignine Which derived from paracom alcohol dominate or the ligin which is

basically dominated by the paracom alcohol are called parah hydroxy pheny ligin or in brief it is

called H liin hydroxy fenile okay whereas the Conifer alcohol which is

basically coming from the ferulic acid as you see the structure here so Conifer alcohol that actually

contributes toward the formation of guile ligin g u a i a c y l guy lignin or it is called

gin whereas the third one is the one which derives from copit or the copile alcohol is

the uh monolignol that uh makes the serile liin residue or it is called s ligin now one point I must mention here

that H liin is basically dominated in the

monocots example is grasses whereas

the uh gin is found to be the only Lin in the Conifer that is gymnos SPM so Conifer

liin is only the Gin whereas s liin you will find

in angos sperms so angos sperms you will get a fine balance between S and G so a balance

between s also you will find some hin is also present so in monocot although H is

dominant but you will you can also get little bit of S and G but normally in the angos which is S and G and uh that

actually uh makes a lot of uh issues particularly in terms of delignification in terms of Li quality all these things

maybe I will discuss a little bit later so now this is what I am showing this is basically a old scheme or the classical

scheme of monal pathway so uh now this scheme is not fully valid now but it is important that we should know this and

then we will appreciate the what are the new discoveries made on this so this was basically published in the first uh year

of the journal which is called Trends in plant science so Trends in plant science is basically the the journal uh which

which started publishing in 1996 okay so what we what we see here is again the camic

acid by the action of camet 4 hydroxy it makes parakum aric acid then then c3h you see that c3h is

mentioned here because see that it is as early as 1996 so still at that time there was a uh there was a

uh there was an argument that in fact the c3h exists it may be a soluble enzyme so that is why but now what I

have mentioned that as such c3h is not uh functioning nowadays so Cafe acid can convert it into fuic acid by a methy

transference this is possible okay because you know that this this added and then feret five hydroxy basically

converts feric acid to five hydroxy ferulic acid at this position that is all right and then then subsequently the

co Mt which makes this position methy group added so these are all five hydroxamic acids I

have already mentioned now this karic acid by activated by making parar Al KO and contributing to paracom alcohol

whereas the feral Coy which originates from ferulic Acid contributed to Conifer alcohol and

the Copo Koy which originated from the Copic acid now contributed to

the formation of copile alcohol and I have mentioned about the respective ligins

formed by this monolignols like H G and S so here this scheme is pretty straightforward so what you what you see

here that basically there are three parallel pathway operating this one other one is this

one and the this one and the intermediates like cafet and

hydroxy hydroxy fer alcohols that is not directly involved in the monolignol formation perhaps it supplies the

intermediates from one branching point to other branching point so methylation pathway so methylation is

very important and there is no wrong in the methylation uh

discoveries uh so so now what what is known what is now known that

the karic acid so I better use this whiteboard that uh structure I'll show in the next uh slide because drawing

structure will take time I'll not finish in time so for karic acid or parakum Maric

acid cannot directly convert into Cafe acid but it is first converted into par karil qu this parar

qu subsequently converted into uh either paracom sikim or paracom cafet so it is

joining with a cemic acid so simic acid biosynthesis I have shown

in the previous class and also apart from simic Acid uh quinic acid can also joins because that also I have

mentioned in this uh queet secate pathway so here uh for the sake of our

discussion we'll concentrate only on the simate queet issue I will discuss maybe in the next class when when we'll talk

about other cic acid EST so paracom qu joins with the cumic acid and it forms the

uh it forms the karile sikic acid or

par karile simic

acid so this pararel simic acid uh will be actually I should make it in that color otherwise there is no point

of so this converted into Capo

capil simic acid so the Capo simic

acid and then this capile sumic acid what will happen from here

this leads to the formation of Cil

Co leaving the simic acid

out so basically there are three reaction the first and third reaction are catalyzed by an enzyme which is

called HCT hydroxy co uh this is camil Cil transfer so it's basically a

transfer which basically joins sikic acid and comaro Co make pararo simic acid and the next enzyme is basically

the c3h which converts paracom sikim to Cil so remember that Kumar and cile is what

is the difference between the structure if you if I briefly tell that is this Kumar only four position and

Cil addition of a hydroxy is at three position so this is Capo and this is parakum

so this Hydrox this addition is basically this is addition so which means a

hydroxy so c3h is nothing but a uh three hydroxy it added three position which converts CFO Kumar oil to

Cil so and then finally again the H C which uh converts capil

to capile co and this capile KO subsequently will be utilized by another enzyme leading to

the formation of another important structure which is called feru Co so when it is FAL Coy then basically

there will be a cc Mt enzyme will be there cile Co methy transference what will

happen that this position o will be replaced with a myoxy so that is why omt working here I think I think

it is clear to you it will be more clear in the next slide when I'll see the structure

so the take-home message is that Kumar to Cafe conversion is not simple it involves a complex roote using either

simate or quinic acid and making paracom simate that convert into Capal simate and here c3h plays important role and

then it converted again back to Cil koi so this is now cleared here in this as you see here that this is parakum aric

acid so parakum aric acid first with the 4cl enzyme it makes paracom qu that PR paracom qu now joining with the simic

acid and here is the HCT and which makes the paracom cimic acid and then it's the c3h what I have

shown so you see the structural difference as I have mentioned in the previous

slide that so this is only o and is a capo then o added additional o added here

this is additional that is why C3 3 Prime H is see 3 H is there okay and

then this converted to Again by HCT which makes capile qu and I have said that capile qu subsequently utilize in

making the fqu and the pathway moves however cqu can also form from cic Acid by another foral reaction okay so

that means this is what that Kumar qu to Cil qu way is a multi-step reaction this part that is called metah Hydrox

silation which is the complexity of the lignification pathway so this was all discovered in 2004 5 6 that that time

onwards so you will first see uh such uh Pathways coming uh into the Publications so much later in the book so from uh

2006 onwards uh in the publication so one such publication I will I have referred somewhere okay and then you see

that the H unit no problem with h unit for the G Unit that means G Unit is basically moving in this way

okay and the a unit is basically moving in this way that means the copal

deide keral deide karal pararal deide how it is formed it is formed from paracom

qu keral how it is formed it is formed from feral qu and the enzymes that catalyzes those reactions are this CCR

cam Co reduct is a generic name okay so and uh okay so that means the ferulic acid

is not directly involved in making ferulic acid is not directly involved in

making feral Co and the Copic acid is not directly involved

in making this Copo KO that means this is not usually not functioning so function is performing in this way so

this is again a new discovery and uh this is basically how this once the monolignols are formed they

basically moves out from the cytool to the cell wall space and then there lies the enzyme laes and peroxides and then

it forms different linkages beta4 linkages 405 linkages 55 linkages and then they form the detail polymer so the

uh so this is basically the uh paper which published in 2000 and 2001 in two very high impact journals one is Journal

biological chemistry and plant cell so the there they have mentioned these things so the first one is

basically uh talked about these two steps so that means Conifer alide converted into copal deide and the

second one is basically talks about there is an existence of a d so

and which converts this to this so now this will be clear here this is a simplified diagram so what I have shown

again just to make the concept clear parakum o qu making C qu but in order to make this you require H C two times and

C3 H okay and uh so basically what is here HCT

c3h again HCT then it makes cap Co then cap Co methy transfer basically o methy transfer

which makes feral Co feral Coy by the action of CCR calco reductors which makes ciferal deide and ciferal deide by

the action of CAD which makes Conifer alcohol that is one of the monol liol which converted into which is

contributing towards the Gin whereas the new pathway says that from Conifer Al deide syap alide is formed cnap alide is

not from the copit so through the enzymes called this C Mt which is f5h is basically named as this C 5H Conifer

alide 5 hydroxy is c a d 5H and the second one is called Al lomt aldhy Ile transfer which convert

one alide to another alide five hydroxide Conifer alide converted into syap alide by a methy transfer so the

reaction is like this this is one five hydroxy Conifer alide converted into cop alide uh so omt means omile

transfar as you see here that here is the addition and here this o but here it

becomes O3 okay and then by the action of Cad and

sad copile alcohol dehydrogen is uh so both Cad and sad can work and this makes copile alcohol and then they are

transported to the cell wall and oxidative polymerization takes place by the action of peroxid and lacas and

ultimately it formed the ligin so so this is basically the revised model so what I have said just

briefly go through it so that it will uh settle in your mind so karic acid Kumar KO from there it moves to parakum simate

to capile simate and then it comes back to cafeic acid then from here it goes to ferulic

acid then Conifer alide one root goes to uh Conifer deide to Conifer alcohol and the root for the syap oil goes in

this way and then eventually it forms the uh cop alide and copile Al and copile alcohol so this is basically the

joining of different scheme uh leading to the formation of different uh monol Lial so this is another way of

representation so this was actually first appeared in environmental chemistry Letters by a group and I

picked up this one that and then uh with further addition we make it so here basically they have mentioned all the

enzymes like c3h or C3 Prime H so that is converting this uh Kumar to keil which is called Kumar esta 3 Prime

hydroxy so so both C3 and C3 Prime H is used c3h is used when you are talking about the Kumar simate to Cil simate but

when you are talking about the comaro quinet to Cil quinet there C3 Prime H is used that is what a normal

convention okay and finally uh that apart from this H ligin gin and S ligin

recently there are arguments and evidences that uh cin which is from the Cil

can it is a possibility that that can also be formed and uh although these stepes are unclear but from Cil KO as

Cil KO contributes to feral qu so Cil KO can by the action of uh CCR can make cile

alide and cile alide by the action of CAD can make Cil alcohol so this is nothing but the structure of cile

alcohol so so that may contribute to the cin and other one is this that uh the feret five hydroxy or which is the

intermediate which uh which contributes to five hydroxy feret or uh uh this is or 5H we can also call this uh CD uh

this uh CD 5H so that I have mentioned previously so this ultimately makes what

this is the structure of five hydroxy five hydroxy feral deide five hydroxy feral deide is already produced

in the pathway and this five Hydrox fer alide simply one requires only as only a

specific CAD which converts five hydroxy feral deide or ciferal deide into this F hydroxy Conifer alcohol this is five

hydroxy Conifer alcohol this is the Conifer alcohol this is conifer alcohol and this is five

hydroxy Conifer alcohol so simply one CAD can convert the already this Conifer alide five

Hydrox Conifer alide is already produced by c d o c d 5H ciferal alide five hydroxy so this is

produced so this can move in another Direction and it eventually May contribute to five hydroxy ligin which

is called 5H ligin so these are the new View and this was published in Trends in plan science in the year of

2018 so this is a review appeared in 2018 so I have given here but anyway I did not give you the detailed references

so this is all about the uh classical and contemporary views of monol Lial pathway okay liin I will not liance I

will not discuss I'll discuss in a separate class with this I end this class thank you very much