Polysaccharides Flashcards by Unknown Unknown

What is a glycoconjugate?

Glycoconjugates occur when a carbohydrate is linked to another macromolecule (e.g. protein or lipid)

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What are the 2 main classes of PS in plants? Give example for each class.

cell wall PS (cellulose, pectic components and hemicelluloses)
storage PS (starch)

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In how many layer are plant cell walls divided in? What are they made of?

middle lamella (pectic compounds and proteins), primary cell wall (cellulose microfibrils embedded in a gel matrix of pectin compounds, hemicellulose and glycoproteins), secondary cell wall (lignin, cellulose and hemicellulose)

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What are the parameters that determine the structure of PS?

Nature and number of constituent monosaccharides
Type and anomeric configuration of the glycosidic linkage (α, β)
Linear or branched structure
Mw and 3-D conformation
The presence of non-sugar substituents

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What is starch made of?

amylose + amylopectin

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What is cellulose made of?

Cellulose is a linear PS made of β-(1->4)-Glc units

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What are some properties of cellulose?

Cellulose is insoluble
Cellulose chains can align and form insoluble crystalline microfibrils

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What are pectins?

Heterogenous group of PS that comprises homogalacturonan (HG), rhamnogalacturonan I (RG-I) and II and xylogalacturonans (XGA)

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What is the model proposed for pectins?

Model for pectin contains ”smooth regions” of HG and blocks of highly branched RG-I made of neutral sugar side chains such as galactans and arabinans

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How can HM-pectin and LM-pectin form gels?

HM-pectin are most used in food systems: they are able of forming gels in aqueous systems with high contents of soluble sugars and low pH values
LM-pectin form gels with bivalent salts (e.g.: Ca2+) in systems with low solids content and wide pH range

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What does the DB measure?

The Degree of Blockiness (DB) measures the length and number of blocks of non-esterified Gal acids present: two pectins with the same DM can have different structures possibles which may result in different functionalities

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What are the types of distribution of methyl-esters over the HG backbone?

random, sequential, blockwise or a combination

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What are xylans?

a hemicellulose
Xylans consist of a linear backbone of β-(1->4)-Xyl residues which can be substituted with α-L-arabinose, (4-O-methyl-)glucuronic acid and simultaneously with both; present in cereals

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What are mixed-linked glucans?

Mixed-linked (13,14) β-glucans are linear molecules of β-(14)-linked Glc units that are interrupted after 3 or 4 β-(14)-Glc with β- (13)-Glc units

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What does enzyme activity lead to?

In general, enzyme activity leads to cleavage of PS resulting in a decrease of the PS MW and formation of smaller PS fragments or oligosaccharides

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What are the 2 enzymes responsible for degrading HG in pectin?

endo-polygalacturonase (endo-PG): hydrolyzes α-1,4 glycosidic bond between 2 non-esterified GalA residues
pectin lyase (PL): splits between 2-methyl-esterified GalA residues via β-elimination introducing an unsaturated double bond at the non-reducing end of GalA-OS

What are the 2 classes of enzymes responsible for degrading xylans?

endo-β(1-4)-xylanases and β-xylosidases

In bread making, which enzyme class results in higher dough volume?

GH11 xylosidases solubilize large (insoluble) xylans resulting in higher dough volume

What are the 2 conditions necessary for starch gelatinization?

water + heat

Describe the starch gelatinization process.

When heated, hydrogen bonds holding the starch together weaken, allowing water to penetrate the starch molecule and causing them to swell until their peak thickness is reached
Water is absorbed in the amorphous part of the starch granule causing it to swell and leach amylose: swelling extends to crystalline regions transforming it into amorphous form
Cooling of gelatinized solution leads to realignment of amylose and amylopectin, causing the liquid to gel  retrogradation

What are the main chemical modifications of starch?

cross-linking, acid hydrolysis, oxidation/bleaching and esterification

What is the most used form of enzymatic modification of starch?

modification with α-amylase

What is the name of the reaction that converts starch to glucose and lower glucose oligomers?

depolymerization

What techniques are used to study structural changes at the MW level? at the monomeric level?

at the MW level? HPSEC
at the monomeric level? HPAEC and MS-based analysis of oligosaccharides

Which heat treatment have a high impact on the structure of pectin?

1. bleaching long-term at low temperatures (e.g., 30min at 40ºC) leads to decrease in Gal, Rha and Ara 2. sterilization --> loss of branching (e.g., methyl esters), depolymerization of main chain and decrease of MW

What is beta-elimination?

β-elimination leads to cleavage of glycosidic bonds and formation of a double bond within the sugar ring of the released PS Occurs in alkaline medium at high temperatures

What is Fenton-oxidation?

It’s a chemical reaction that occurs in the presence of oxygen, trace amounts of metals and reducing agent leading to the formation of reactive –OH radicals Can result in scission of the PS chain (lytic oxidation) or conversion of a –OH to a carbonyl group with scission (non-lytic oxidation)

How can Fenton-oxidation products be detected?

MS analysis

What are the steps in sample preparation of PS? What about extraction of pectins, hemicelluloses and mixed-beta-linked glucans?

slide 30-33

Based on which property of molecules does SEC separate them?

size

What is the elution order in SEC?

large molecules elute first

Based on which property of molecules does AEC separate them?

AEC separates neutrally charged molecules from negatively charged ones

How does precipitation contribute to fractionation of PS?

more branched PS are more soluble and will be retained in the supernatant whilst other molecules will precipitate

What are the mains steps used to analyse the sugar composition of monosaccharides?

acid hydrolysis + analysis with GC

What is a derivatization step needed prior to GC analyis of monosaccharides?

to make them volatile: convert them to alditols using reduction and acetylation step

What are the steps involved in glycosidic linkage analysis?

methylation, acid hydrolysis and reduction/acetylation

What does NMR provide info on?

NMR provides information on monosaccharides composition, α- and β- anomeric configurations, establishment of linkages pattern and sequences of sugar units

In HPAEC-PAD, which oligosaccharides bind best to the column?

oligosaccharides with more -OH groups and more negative charges

What does MALDI-TOF provide info on?

MALDI-TOF recognizes the DP of oligosaccharides as well as the substituent groups (e.g., methyl-esters) and can also recognizes molecules above 400 kDa BUT nothing about the distribution of these units, whether the units are isomeric or distinction between sugars