Carbohydrates Chapter 16 Outline
• the structure and characteristics of different sugars - mono-, di-, oligo-, and poly-saccharides
• reactions involving monosaccharides
• glycogen, starch, cellulose and bacterial cell walls • glycoproteins and lectins
• carbohydrates are one of the four major classes of _________________________ - the others are lipids, proteins, and nucleic acids
• carbs make up most of the organic matter on Earth - serve as fuel/energy source and energy storage
- form part of the structural framework of DNA and RNA
- form the structural support of plant cell __________
- carry information and allow functionality by being added to proteins/enzymes - typically allow cell-cell communication and interaction
• complex carbohydrates are built from single ______________________ subunits called monosaccharides
- oligosaccharides: ________ and polysaccharides: ________________
• there are different types of monosaccharides and each type can usually take on different conformations
Monosaccharides
• these are the simplest carbs and the subunits for chains of saccharides
- they are literally “carbon ______________________” (C-H2O)n
• they include the most easily used fuel source and a key component of DNA and RNA
• the smallest monosaccharides contain three carbons and are called ____________
• simple monosaccharides with 4, 5, 6, and 7 carbons are called tetr-, pent-, hex-, and heptoses, respectively
- these, like amino acids, can be of different orientations and configurations and like amino acids
are designated ____ and ____
• there are two classes of sugars: ketoses and _________________ - don’t worry about the difference… I don’t…
• _______-carbon sugars – from both classes – are the most common
• the predominant form of monosaccharides (especially the longer ones) is not an open chain, but a ring
• cyclized rings are more ________________________ favorable
• for glucose, the C-1 (aldehyde) group in the open chain form _______________ the C-5 hydroxyl to form an intramolecular (hemiacetal) bond
• for fructose, the C-2 (keto) group in the open chain form attacks either the C-6 or C-5 hydroxyl in the same molecule to form a intramolecular (hemiketal) bond
- if the C-6 is attacked, a six-membered ring is formed (pyranose) - if the C-5 is attacked, a five-membered ring is formed (furanose)
Different Conformations of Carbohydrate Rings
• six-membered pyranose rings are not planar – not two-dimensional
• instead they can take one of two possible conformations: __________ or ______________ • can you tell which one is more favorable…?
• in either form there are axial and equatorial bonds
- axial groups will sterically interfere with each other if they emerge on the same side of the ring
- equatorial groups are less ________________________
• in the chair form all axial groups are H – small and uncrowded - favored • furanose rings are also not planar
- they can be ________________________ where four of the five members of the ring are planar with the fifth jutting out – envelope form
- different carbons can be the fifth one out of plane
- in this case the carbon out of plane determines the name: C-?-_________
• monosacchrides can interact with alcohols and amines to form modified monosaccharides (e.g., glycoside with glycosidic bond)
- nucleosides are combinations of ribose sugars and ___________________
• these modified forms can often have very different chemistries/reactivities increasing the diversity of carbohydrates
Redox Reactions with Sugars and Glycosides
• redox reactions with sugars play a large role in biochemistry • as always, with redox reactions, energy is transferred
- IT’S ALL ABOUT THE ENERGY!
• the oxidation of sugars provides the majority of the _______________ for making ATP (remember: sugars are oxidized to CO2 and water)
• the reverse – reduction of CO2 and water to make sugars is the basis of ___________________
• in sugars, aldehyde groups groups are oxidized to __________________ • this oxidation caused something else to be reduced
- therefore, these sugars are reducing agents
- they are called REDUCING ______________
Phosphorylation of Sugars in Important in Energy Maintenance
• phosphorylating sugars is very common and extremely important
• the first step in the breakdown of glucose to produce energy (which we will discuss at length
starting next week) is the conversion of glucose to glucose-6-___________________
- this locks them in the cell (charged molecules cannot diffuse across the lipid bilayer of the cell membrane)
- this makes them much more _________________ so they will be more likely to form linkages with other molecules
Glycosides
• a sugar hydroxyl group can bond with the carbon of another sugar - this forms a GLYCOSIDIC LINKAGE or BOND
- what is left is a GLYCOSIDE
• if the sugars are linked through an oxygen atom, they are ____-glycosides
• sugars can be bound to other molecules with glycosidic bonds that share a nitrogen – these are
_____-glyosides (as seen in dNTPs)
• glycosidic bonds between individual sugar molecules are how oligo- and polysaccharides are made (they result in chains of sugars)
- but many different bonds are possible…
• the chemistry of these chains depends on the sugars and types of bond
Complex Carbohydrates - Oligosaccharides
• glycosidic bonds can also give rise to straight chains or _______________________ chains
• ________ or more monosaccharides linked together form an oligosaccharide
• disaccharides – two mono-saccharides joined by a glycosidic bond - are extremely common
- sucrose (table sugar) is a disaccharide of glucose and __________________ - sucrase (enzyme) cleaves sucrose into its subunits
- lactose (milk sugar) is a disaccharide of glucose and ___________________ - lactase hydrolyzes lactose into its constituents
Glycogen and Starch – Stored, but Ready, Glucose
• polysaccharides are polymeric oligosaccharides – very large! - individual glucose monomers are stored in this way
• _______________________ is stored glucose in animal cells •
• ______________ is stored glucose in plant cells - there are two forms of starch:
- amylose: unbranched - amylopectin: branched
• over half the total amount of carbs ingested by people is in the form of starch
• both forms of starch are rapidly hydrolyzed to glucose by α-_______________ (found in saliva) - leave a piece of pasta or bread on your tongue for awhile…
Cellulose – Structural Glucose
• another polysaccharide of glucose found in plants
• unbranched polymer forming long straight chains
• _________________ are formed when parallel chains interact with each other via H-bonds – this creates an extremely strong matrix or mesh
• because we mammals lack ___________________, we cannot digest wood - but wood is still jam-packed with sugar for those who can eat it…
• the only difference between cellulose and starch is the type of glycosidic bonds between glucose subunits!
Polysaccharides and Bacterial Cell Walls
• most cell walls are made of polysaccharides
• heteropolysaccharides are the major component of ______________________ cell walls
• interestingly, these are cross-linked with _________________
• in the cell wall of Staphylococcus aureus, an oligomer of four amino acids is bound to the sugar (red)
• these are cross-linked to an oligomer of five glycines (blue) which is cross-linked to another sugar
• this extensive cross-linking results in a durable 3D network (a mesh) that protects the bacterium
Enzymes Responsible for Oligosaccharide Assembly
• glycosyltransferases catalyze the formation of the ____________________ bonds that link monosaccharides
- but, amazingly, each pair of sugars being linked needs its own specific enzyme (not like other biopolymers!)
• this requires a large number of individual glycosyltransferases • it also explains observations such as blood type…
Glycoproteins
• when a ____________________________ is covalently attached to a protein, you have a glycoprotein (these are much more protein than sugar)
• many glycoproteins can be found in the cell membrane where they function in cell-cell signaling and cell adhesion
- many secreted proteins are also glycosylated
• the different blood types found in the human population - A, B, O, AB (even +/-) - are due to glycosylated proteins found on the membranes of red blood cells
• proteins with a very high carbohydrate content (85-95%) are called ______________________
Glycoproteins and Lectins
• the diversity of glycoproteins is generated by:
- the different monosaccharides that can be joined together
- the configuration of the ____________________
• many more different oligosaccharides can be generated from four sugars than can oligopeptides from four amino acids
• why the complexity of glycoproteins?
- nothing is that complex for no reason – what do they do?
• lectins are proteins that ______________ to specific carbohydrate structures - lectins are expressed on one cell’s surface with the glycoprotein on another - these ‘partners’ then come together for an interaction
• lectins are found throughout life: animals, plants, and microorganisms • in animals, lectins facilitate cell-cell contacts
• lectins and carbs interact through numerous weak interactions that allow specificity (strength in numbers; Velcro)
• there are different classes of lectins containing proteins that share similar properties
• _______________________ are one class of lectins
- these proteins adhere immune system cells to sites of injury for the inflammatory response - different subtypes adhere to lymph-node, endothelium, or blood platelet cells
• ________________ are also crucial for embryonic adhesion to the ___________________ wall after fertilization (not an immune response role…)
• the cells of the uterine wall only express oligosaccharides for a short time – after selectin binding, pathways are activated to allow embryo implantation
• E. coli adheres to the wall of the ______________________ system through lectins • viruses also gain entry to host cells by co-opting this system
- influenza hemagglutinin recognizes cell-surface glycoproteins
- the interaction is so strong that once the virus is inside the cell another viral protein must
________________ the glycosidic bonds of the glycoprotein to free the virus from the interaction
Summary
• types of monosaccharides
• cyclic monosaccharides and their conformations (__________, _____________, ____________)
• glycosidic bonds (___- and ____-glycosides) • oligosaccharides
- sucrose and lactose • polysaccharides
- gylcogen and starch as a glucose storage device - cellulose for structure
- bacterial cell walls
• glycoproteins, the sugar core for display and the lectins that bind them
