Carbohydrate Structure & Function Flashcards
What are three types of carbohydrates?
Simple sugars
Polysaccharides
Glycoconjugates
What is another name for carbohydrates?
Glycans
What are glycans?
Polyhydroxy aldehydes or ketones or substances that yield such compounds on hydrolysis
Very heterogeneous in size and composition
Named so because many have formula Cn(H2O)n, where n ≥ 3
Glycans = Carbohydrates
What is the most abundant biological molecules and how are they produced?
Glycans (Carbohydrates)
Produced from CO2 and H2O via photosynthesis in plants
What can carbohydrates covalently link to?
Proteins and lipids
What functions do carbohydrates fulfill?
Energy source and energy storage
Structural component of cell walls and exoskeletons
Informational molecules in cell-cell signaling
What are examples of simple sugars?
Monosaccharides
- glucose, galactose, mannose
Disaccharides
- sucrose, maltose, lactose
Oligosaccharides
- milk oligosaccharides, raffinose
What are examples of polysaccharides?
Glucose homopolymers
- cellulose, starch, glycogen
Disaccharide heteropolymers
- keratan, sulfate
Chitin is a homopolymer of N-acetyl glucosamine
What are examples of glycoconjugates?
Glycoproteins
- antibodies, viral coat proteins
Proteoglycans
- aggrecan, syndecan, glypican
Glycolipids
- blood antigens, membrane anchors
What are oligosaccharides?
Simple sugars that range from 3-20 unbranched and branched sugar residues
What are milk oligosaccharides?
They are derived from lactose
Lacto-N-tetrarose
Lacto-N-fucopenraose-1
What is the importance of human milk oligosaccharides?
Important functions in the infant intestinal tract
What does Lacto-N-tetrarose do in the infant intestinal tract?
Helps with digestion
Growth advantage to bifidobacteria
What does Lacto-N-fucopentaose 1 do in the infant intestinal tract?
Fools bacteria with itself so it binds to the carbohydrate instead of the receptor of the cell
What are examples of plant oligosaccharides?
Raffinose
Stachyose
Verbascose
What are plant oligosaccharides derived from?
Sucrose
Why can humans and non-ruminating animals (pigs, poultry) not digest plant oligosaccharides?
Humans and non-ruminating animals lack the a-galactosidase enzyme needed to hydrolyze the a-1,6 glycosidic bond.
What is beano?
A preparation of a-galactosidase that can aid in digestion of plant oligosaccharides, resulting in the release of free galactose and sucrose
Sucrose is further metabolized to glucose and fructose by the enzyme sucrase in the small intestine
These simple sugars are easily metabolized by our own enzymes
What are the types of polysaccharides?
Homopolysaccharides
Heteropolysaccharides
What are homopolysaccharides?
They can be unbranched (linear) and branched (every now and then)
What are heteropolysaccharides?
They can be unbranched with two monomer types and branched with multiple monomer types.
What do polysaccharides not have?
They do not have a defined molecular weight and composition
This is in contrast to proteins because unlike proteins, no template is used to make polysaccharides
What are the functions of polysaccharides?
Some serve as structural elements
Some serve as storage forms of monosaccharides that are used as fuels
What are examples of structural polysaccharides?
Cellulose (plant cell walls)
Chitin (animal exoskeletons)
What are examples of storage polysaccharides?
Starch in plants
Glycogen in animals
What is cellulose?
An unbranched homopolysaccharide consisting of ~1000 repeating units of a disaccharide cellobiose
What is cellobiose?
2 glucose (Glc) monomers linked by (B1–>4) glycosidic bonds
What bonds does cellulose have and where?
H-bonds between cellulose strands
What is cellulose a component of?
Component of the plant cell wall
What is the most abundant polysaccharide in nature?
Cellulose
What is nearly pure fibrous cellulose?
Cotton
Why are individual cellulose strands extensively hydrogen-bonded?
To build a polysaccharide fibril
What do most animals lack in terms of polysaccharides?
Most lack cellulase
What is cellulase?
The enzye required to hydrolyze the B1–>4 linkage between glucose molecules in cellulose
What is high in cellulose?
Plant material
What is plant material high in cellulose considered?
Considered “roughage in the diet”
- it passes through the digestive system without being degraded
What can fungi, bacteria, and protozoa do in terms of cellulose?
Secrete cellulase, that can use wood as source of glucose
What do ruminants and termites have in relation to cellulose?
They live symbiotically with microorganisms that produce cellulase
What is chitin?
Linear homopolysaccharide of N-acetyl Glucosamine (GlcNAc) units
- Contains B1–>4 glycosidic bonds
Structural exoskeletons in insects and crustaceans
Also found in cell walls of many types of fungi, including mushrooms
What does chitin provide?
Provides a strong body frame
What bonds does chitin have and where?
H-bonding within and between polysaccharide strands
What is starch?
The main storage polysaccharide in plants
What are the two forms of starch that plants synthesize?
Amylose
Amylopectin
What is amylose?
an unbranched (linear) polymer of ~100 (a1–>4) linked glucose units
humans can metabolize this called amylase to break it down
What is amylopectin?
a (a1–>4) linked glc polymer with (a1–>6) branching occuring every 15-30 residues
linear chain with branches
- homopolymers
Left handed
How is starch different from cellulose and chitin?
Starch is an important dietary source of glucose for animals
Why is starch an important dietary source of glucose for animals?
It can be hydrolyzed by the enzyme a-amylase which cleaves (a1–>4) glycosidic bonds
What is glycogen?
Branched homopolysaccharide of glucose
- just like amylopectin but is more heavily branched
Molecular weight reaches several millions
Glycogen bonded to only 1 glucose
In glycogen, what do glucose monomers form?
(a1–>4) linked linear chains
In glyocogen, describe its branch points.
(a1–>6) linkers every 8-12 residues
What is the main function of glycogen?
Main storage polysaccharide in animals
Where is there an abundance of glycogen?
Abundant in liver (~7-10% wet weight of liver) and in skeletal muscles
What does glycogen and starch often form?
Granules in cells
What do granules contain?
Enzymes that synthesize and degrade these polymers
What do glycogen and amylopectin have?
One reducing end but many non-reducing ends
Anomeric carbon has to attack at the non reducing end
Where do enzymatic processing occur?
Occurs simultaneously at many non-reducing ends
Therefore, more branch points provide more ends for Glc retrieval and storage
What is the advantage of storing glucose as starch or glycogen?
Greatly reduces osmotic pressure that would result from its storage in monomeric form
This is because osmotic pressure is proportional to the number of solute molecules (regardless of their given weight) in a given volume.
What are important roles does glycans have?
In addition to the energy and structural roles, it has an important role as functional groups in glycoconjugates
How much of all human proteins are glycoproteins?
~50%
How much of the human genome encodes enzymes required for the synthesis and degradation of carbohydrates and glycojungates?
~1%
What enzymes are required for the synthesis and degradation of carbohydrates and glycoconjugates?
Glycosyltransferases
Glycosidases
What are glycosyltransferases?
Covalently linked glycans to proteins and lipids
What are glycosidases?
Remove glycans through hydrolysis reactions
What are the types of glycoconjugates?
Glycoproteins
Proteoglycans
Glycolipids
What are glycoproteins?
Bulk of the macromolecule consists of protein
- Some are transported by membrane vesicles to the plasma membrane for insertion or secretion
- Others are targeted to cellular organelles such as mitochondria, peroxisomes
What are proteoglycans?
Bulk of the macromolecule consists of carbohydrates
In humans, where are proteoglycans found?
Extracellular matrix
In bacteria, where are proteoglycans found?
Cell wall (peptidoglycans)
Where are glycoproteins transported?
transported to the plasma membrane for insertion or secretion by membrane vesicles
What targets glycoproteins?
Cellular organelles such as mitochondria, peroxisomes target some glycoproteins (the other half that i’snt transported)
Glycan groups on glycoconjugates are recognize by glycan binding proteins called ______.
Lectins
What are lectins?
Special class of proteins
Glycan group-lectin interactions play important role in cell signaling and immunity
What are colored symbols for monosaccharides used as?
Used as building blocks of the glycan groups in most glycoconjugates
Simplifies representation of the glycan group of the glycoconjugate
Consortium for Functional Glycomics (CFG) notation
Where on the protein do the glycans attach?
N- and O-Linked glycosylation
N-Linked = Asn always (GlcNac)
O-Linked = Ser or Thr (GalNAc)
What is N-Linked Glycosylation?
Carbohydrate linkage occurs through the amide N atom in the side chain of Asn.
What does N-glycosylating enzymes most often attach to?
Often attach the sugar to asparagine (Asn) residues contained within the consensus sequence Asn-X-Ser/Thr (X can be any amino acid except Pro)
What is the consensus sequence?
Asn-X-Ser/Thr
(X can be any amino acid except Pro)
Matters how it is attached
What is O-Linked Glycosylation?
Carbohydrate linkage occurs through the O atom in the side chain of Ser or Thr residues
What is the consensus sequence for O-glycosylating enzymes?
Unlike N-glycosylating enzymes, the O-glycosylating enzymes do not have a preferred consensus sequence
TLDR: O-glycosylating enzymes does not have a consensus sequence
What are mucins?
O-linked glycoproteins are often called mucins
Mucins function to protect epithelial cells in the intestinal, urinary and respiratory tracts from physical damage and pathogens
Which enzymes determine the expression of human ABO blood groups?
a-1,3-N-acetylgalactosaminyltransferase (GTA)
a-1,3-galactosyltransferase (GTB)
These enzymes attach either a GalNAc (GTA) or Gal (GTB) sugar residue to a glycan subgroup called the O antigen
If neither GTA or GTB enzymes are present, which blood type group is it?
O-type blood
If only GTA enzyme is present, which blood type group is it?
A-type blood
Adds square (GalNAc)
If only GTB enzyme is present, which blood type group is it?
B-type blood
Adds circle (Gal)
If both GTA and GTB enzymes are present, which blood type group is it?
AB-type blood
Adds square and circle (GalNAc, Gal)
What is the O antigen?
Glycan subgroup where a-1,3-N-acetylgalactosaminyltransferase (GTA) and a-1,3-galactosyltransferase (GTB) attach to either a GalNAc (GTA) or Gal (GTB) sugar residue
Where is the O antigen present?
Present on glycoproteins and glycolipids on red blood cells
Which blood group is a universal red blood cell acceptor?
Type AB blood
Which blood group is a universal red blood cell donor?
Type O blood
Which blood group is a universal plasma acceptor?
Type O blood
Which blood group is a universal plasma donor?
Type AB blood
What happens if anti-A and anti-B antibodies meet anti-O antibodies?
They will attack each other
What are proteoglycans?
Proteins with multiple long glycosaminoglycan chains covalently attached to a relatively small core protein
Attached to a hyaluronic acid through a linker protein
Extracellular matrix proteoglycans and cell surface proteoglycans
Where do proteoglycans function?
Membrane-bound through a Glyco-phosphatidyl inositol (GPI) anchor or a transmembrane a helix on the protein
Secreted into the extracellular matrix (most function here)
What is a peptidoglycan?
Component of bacterial cell wall
Provides shape and protection against osmotic changes
- Linear chains of B(1–>4) linked MurNAc and GlcNAc tethered together by linkages between short peptides
MurNAc = N-Acetylmuramic acid GlcNAc = N-acetylglucosamine meso-DAP = Meso-diaminopimelic acid (derivative of lysozyme)
What does lysozyme do in a peptidoglycan?
Breaks B(1–>4) bond which can neutralize bacteria
What does penicillin do?
Blocks bacterial cell wall synthesis by inhibiting Transpeptidase
When transpeptidase is inactive, cell wall synthesis is inhibited and bacterial cells die.
Forms a suicide inhibitor complex between a Ser residue in transpeptidase and a carbonyl C in the B-lactam ring of penicillin
What is transpeptidase required for?
This enzyme is required to form the oligopeptide linkages between polysaccharide chains (gives it strength)
Why is penicillin considered a “suicide inhibitor”?
A suicide inhibitor is one that inhibits an enzyme and the mechanism of inhibition such that no conditions exist by which this inhibition can be overcome in the cell.
Penicillin forms an irreversible, covalent bond with transpeptidase.
The presence of this bond inactivates the enzyme and the irreversible nature of the bond makes penicillin a suicide inhibitor.
How has bacteria developed mechanisms to resist B-lactam antibiotics?
Penicillin-resistant bacteria express the enzyme B-lactamase, which inactivates penicillin by cleaving the B-lactam ring
What is the solution to bacteria with resistance to B-lactam antibiotics?
Synthetic compounds that block Transpeptidase activity without being a substrate for B-lactamase such as Methicillin
What new problem arose after the use of methicillin?
Methicillin-resistant S. aureus (MRSA) bacterial strains arose that express a variant form of the transpeptidase enzyme that is not inhibited by the antibiotic
This variant transpeptidase has no reaction with methicillin which allows transpeptidase to be active and allowing bacterial cells to live
