top of page

Carbohydrates

image.png

Introduction

  • major source of energy from our diet (>50%)

  • composed of C,H,O

  • also called saccharides which means "sugars"

  • not all are digestible by human enzymes

  • produced by plants in photosynthesis

  • glucose is synthesized in plants from CO2, H2O and energy from the sun

  • oxidized in living cells to produce CO2, H2O and energy

image.png

Types of Carbohydrates

Monosaccharides

  • simplest carbohydrates

  • 3-6 carbons

  • carbonyl group and several hydroxyl group

  • aldoses or ketoses

Disaccharides

  • consist of two monosaccharides

Polysaccharides

  • consist of several monosaccharides

image.png
image.png
image.png

Fisher Projection

  • is used to represent carbohydrates

  • places the oxidized group at the top

  • shows chiral carbons as the intersection of vertical and horizontal lines

  • the -OH group on the chiral carbon furthest from the carbonyl group determines an L or D isomer

  • left is assigned the letter L for the L-form

  • right is assigned the letter D for the D-form

image.png

Chirality

Chiral Carbon

  • a carbon atom that is attached to four different types of atoms or groups of atoms

image.png
image.png

More than one chiral carbon?

image.png

Cyclic Structures

  • are the prevalent form of monosaccharides with 5 or 6 carbons

  • form with the hydroxyl group on the highest numbered chiral carbon reacts with the carbonyl group (aldehyde or ketone) 

    • hemiacetal​

    • hemiketal

  • ring structures are useful when visualizing the formation of disaccharides or polysaccharides

    • aldehydes typically form 6 sided structures​

    • the ketohexose fructose forms a 5 sided ring

image.png

After cyclization, D-glucose exists in 2 isomeric: α-D-glucopyranose and β-D-glucopyranose (called anomers) depending on orientation of hydroxyl group at C1 (the hemiacetal carbon)

image.png

Monosaccharides

  • contain functional groups that can undergo reactions

  • aldose: the aldehyde group can be oxidized to a carboxylic acids

  • the carbonyl group in an aldose or a ketose can be reduced (gain H to form a hydroxyl group)

  • even though monosaccharides are mostly in cyclic form, the aldehyde group of the open structure oxidizes easily

  • reduction and oxidation always occur in pairs

    • when the CHO is oxidized to COOH, another compound can be reduced​

    • when a monosaccharide can reduced another substance, it is called a reduced sugar

REDUCING SUGARS​

  • are monosaccharides that oxidize to give a carboxylic acid

  • include the monosaccharides glucose, galactose and fructose

    • fructose can rearrange the keto group and the hydroxyl group on carbon 1 to form an aldehyde which can be oxidized​

      • fructose (ketose) -> glucose (aldose) -> oxidized​

Disaccharides

  • two monosaccharides can link together to form a disaccharide

  • form a glycosidic bond

  • a molecule of water is removed

image.png

MALTOSE

  • known as malt sugar

  • composed of two D-glucose sugars

  • obtained from the hydrolysis of starch

  • linked by an α -1,4 - glycosidic bond from the α -OH on C1 of the first glucose and -OH on the second glucose

LACTOSE

  • β-D-galacose and α or β-D-glucose

  • contains a β-1,4-glycosidic bond

SUCROSE

  • consists of α-D-glucose and β-D-fructose

  • has a α,β-1,2-glycosidic bond

PROPERTIES

  • maltose

    • second glucose has a free -OH group on carbon 1, it can open up to give a free aldehyde​

    • reducing sugar

  • lactose

    • glucose has a free -OH group on carbon 1, it can open up to give a free aldehyde​

    • reducing sugar

  • sucrose

    • glycosidic bond between carbon 1 of glucose and carbon 2 of fructose cannot open up to give an aldehyde​

    • not a reducing sugar

Polysaccharides

  • many monosaccharide units joined together

  • complex carbohydrates

  • includes fiber

    • soluble​

    • insoluble

  • starch (plants)

  • glycogen (animals)

  • polymers of D-glucose

  • include amylose and amylopectin, starches made of α -D-glucose

  • include glycogen (animal starch in muscle) made of α -D-glucose

  • include cellulose (plants and wood) which is made of β -D-glucose

image.png

AMYLOSE

  • a polymer of α-D-glucose

  • linked by α-1,4 glycosidic bonds

  • a continuous chain

AMYLOPECTIN

  • a polymer of α-D-glucose molecules

  • is a branched-chain polysaccharide

  • has α-1,4 glycosidic bonds between the glucose units

  • has α-1,6 glycosidic bonds to branches

GLYCOGEN

  • polysaccharide that stores α-D-glucose in muscle and liver

  • important energy reserve

  • similar to amylopectin, but is more highly branched

image.png

CELLULOSE

  • a polysaccharide of β -D-glucose units in linear chains

  • has β-1,4 glycosidic bonds

  • cannot be digested by humans because we do not have proper enzymes

Digestion of Starch

image.png
image.png

DIGESTION OF STARCHES

  • breakdown of α-1,4 glycosidic bonds: hydrolysis

    • mouth (minimal)​

      • salivary α-amylase​

      • products: maltose, dextrins

    • stomach: nothing happens (too acidic)​

    • small intestine

      • pancreatic α-amylase​

      • products: glucose, maltose, isomaltose, maltotriose

DIGESTION OF DISACHARIDES

  • brush border

  • enzymes

    • lactase​

      • lactose -> galactose + glucose​

    • sucrase

      • sucrose -> glucose + fructose​

    • maltase

      • maltose -> glucose + glucose​

    • isomaltase

      • isomaltose (​α-1,6 bond) -> glucose + glucose

image.png
image.png

Absorption

Active transport - glucose and galactose

  • active transport entry into enterocytes

    • SGLT1​

      • sodium glucose transport protein​

      • symport

      • dependent on Na+/K+ ATPase pump

      • fate: ≈60% to blood via GLUT2, ≈25% to blood by diffusion, ≈15% leaks back to lumen

      • serves the energy needs of enterocytes

  • facilitates diffusion exit from enterocytes

    • GLUT2​

      • a transmembrane carrier protein that enables passive glucose movement across cell membranes​

      • principal transporter for transfer of glucose between liver and blood, and for renal glucose reabsoprtion

image.png

Active transport - fructose

  • entry into enterocytes: GLUT5

    • slow and less efficient

  • exit from enterocytes: GLUT2

    • in liver, phosphorylated and trapped​

      • no circulating fructose in bloodstream​

      • a downhill fructose gradient

image.png

Glucose Transporters (GLUT)

  • highly polar glucose cannot move across membrane, so carrier/transporter needed

  • GLUTs are present in nearly all cells

  • no energy required

  • 14 found so far

    • integral protein​

    • a specific combing site

    • reversible conformational change

    • transport other molecules ex vitamin C

image.png

Insulin

  • has major impacts on metabolism

    • not only works on CHO​

  • an important anabolic hormone

    • 2nd messenger involved in signaling pathway​

  • promotes glucose uptake by body cells (assimilation)

    • secretion increased during absorptive state​

      • increase glucose in plasma​

      • increase AA in plasma

    • secretion decreased during postabsorptive state

image.png
image.png

Blood Glucose

  • in the body

    • glucose has a normal blood level of 70-90mg/dL​

    • a glucose tolerance test measures blood glucose for several hours after ingesting glucose

image.png

Glycemic Index (GI)

  • the increased blood glucose over the baseline during a 2hr period following ingesting of a defined amount of CHO compared with the same amount of CHO in a reference food

image.png
image.png
image.png

Glycemic Load (GL)

  • predictive

    • estimates how much a food will raise blood glucose after ingestion​

  • accounts for both how much CHO is in the food and how much each gram of CHO raises blood glucose

    • GL = GI x CHO (g)/100​

    • a weighted measure - instead of defined for each food, can be calculated for any serving of size of food

  • GI and GL

    • useful in evaluating risks for obesity and chronic metabolic diseases; higher, longer elevation [glc]​ ≈ greater risk

  • CHO absorption - GI

    • starches that are considered porous (have a large surface-to-volume ratio are readily digested by amylase)​

    • in more dense starch products, amylase cant get to the substrate as easily

Assimilation

image.png

  • what happens to glucose in cells?

    • excess glucose leaves liver​

    • goes to all tissues

      • especially brain​

      • muscle (needs insulin)

      • adipose tissue (needs insulin)

    • liver and muscle can store a small amount of glucose as glycogen

      • liver plays an important role in maintaining normal blood glucose level

      • muscle does not contribute to blood glucose directly

©2023 by Syracuse University Dr.Margaret Voss

bottom of page