Unit 2 - Part 3

starch

carbohydrate that is made by plants and used for storage

amylose

long chains of glucose with alpha(1-4) bonds

amylopectin

glucose chains with alpha(1-4) bonds plus alpha(1-6) branch points

What are the components of starch

amylose and amylopectin

Where can starch be stored in plants?

chloroplasts, leukoplasts, and plant embryos/seeds

Where in chloroplast is starch stored

photosynthetic/light receiving cells

When can leukocytes store starch?

especially in plants that don’t photosynthesize

How do plant embryos/seeds store starch?

store starch and proteins as nutrients for early growth before they can photosynthesize

What are the uses for carbohydrates in plants

glycolysis and to make sucrose

What does glycolysis produce?

2 pyruvates, 2 NADH, and 2 ATP per glucose

How do roots and seeds get their energy?

they can’t photosynthesize, so they get sucrose from photosynthetic tissues and use it for energy or store it as starch

proplastid

organelle found in plant cells that can develop into plastids

What can proplastids develop into?

chloroplasts (in leaf cells), amyloplasts (in root cells), and seeds of next generation

glycogen

has similar branching network to amylopectin and is a major carbohydrate storage form in animals

How does amylose and glycogen structure differ in structure?

they have alpha (1-4) bonds that form right angles to chair form of glucose which produces a twisted coil, making terminal glucose accessible to pancreatic enzymes

Pancreatic enzymes

break down complex carbs into disaccharides that are digested by enterocytes and absorb into blood stream

pancreatic alpha amylase

secreted into the duodenum of the small intestine to digest starch and glycogen by hydrolyzing alpha (1-6) and alpha (1-4) bonds

enterocytes

absorptive cells in duodenum that form the brush-border membrane

Why is there increased electrostatic repulsion in the alpha position than the beta position?

because the 2 OH- groups are closer

villus (villi plural)

folded structure of the intestine

microvilli

many small projections on enterocytes

How do intestinal villi and microvilli affect the uptake of nutrients?

by increasing the surface area of the small intestine and increasing the uptake of nutrients

disaccharidases

membrane proteins on enterocytes that break down each substrate into monomers

What are the 3 disaccharidases?

sucrase, maltase, and lactase

What products does sucrase form?

glucose and fructose

What bonds does sucrase hydrolyze?

alpha (1-2) glucosidic

What products does maltase form?

glucose and glucose

What bonds does maltase hydrolyze?

alpha (1-4)

What products does lactase form?

glucose and galactose

What bonds does lactase hydrolyze?

beta(1-4) galactosidic

When is lactase produced in mammals?

only in infant development, but 15% of humans have lactase persistence

Describe the pathway of glucose absorption into the blood

SGLT1 → enterocyte → Glut 2 → blood

What is the ratio of Na+ to glucose for SGLT1?

it brings in 2 Na+ for every glucose

What type of transporter is SLGT1

secondary active transporter

Describe the pathway of fructose absorption into the blood

Glut 5 → enterocyte → Glut 2 → blood

Na+/K+ pump for glucose absorption into blood

uses ATP to maintain the Na+ gradient needed for glucose uptake by SGLT1

What type of transporter is the Na+/K+ pump?

primary active transporter

types of transport

1. primary active transport

2. secondary active transport

3. facilitated diffusion

4. simple (passive) diffusion

symporter and an example

a transporter that moves 2 substances together

SGLT1 brings in Na+ and glucose

primary active transport

directly uses ATP

secondary active transport

uses an ion gradient created by primary active transporter

facilitated diffusion

protein-assisted movement down a concentration gradient

simple (passive) diffusion

moves particles from a high concentration to a low concentration without a protein

Which types of transport move things from a low to high concentration?

primary and secondary active transport

Which types of transport move things from a high to low concentration?

facilitated and simple (passive) diffusion

Glut1

moves glucose from blood through endothelial cells to subendothelial tissues (important at the blood brain barrier)

Glut 2

moves glucose and fructose from enterocytes to blood and into liver and Islet beta cells

Glut 3

moves glucose from blood to neurons in the brain

Which Glut has a low affinity for glucose?

Glut 2

Glut 4

moves glucose from blood to muscle cells and adipose

Which Glut is induced by insulin?

Glut 4

Glut 5

moves fructose into enterocyte

How do GLUTs work?

glucose binds when GLUT is open to the outside which causes a change in conformation that allows glucose to cross the membranes

Where are integral membrane proteins translated at?

rER

What happens to the amino acid chain that forms a Glut as it is coming out of the ribosome that causes SRP54 to bind?

it is hydrophobic because of all of the leucines but it also forms an alpha helix

How many alpha helices does a Glut contain?

12

Describe the topological structure of Glut

contains 12 membrane alpha helices, an extracellular/lumenal region, and cytoplasmic regions

N-linked glycosylation

occurs in the first lumenal loop as Glut protein is translated into the lumen of the rER and interferes with signal peptidase cleaving the signal peptide

What happens to the uncleaved signal peptide in Glut during N-linked glycosylation?

it slides out of the translocon and becomes the anchor peptide (first transmembrane alpha helix)

What is the purpose of alpha helices in the Glut?

they connect extracellular/lumenal domain to the cytoplasmic domain

Km values

substrate concentration at ½ Vmax

What does a lower Km mean and an example?

higher affinity for glucose (Glut 3)

What does a higher Km mean and an example?

lower affinity for glucose (Glut 2)

Glucose-binding site of Glut

involves specific regions of Glut (specifically helix 7)

What amino acids contribute to glucose-binding site of Glut?

glutamines, asparagines, and tryptophan

How does the amino acids involved differ from Glut 1,3,4 and Glut 2?

Glut 1,3,4: Ser287 bonds with histidine to stablize glucose-binding site

Glut 2: Ala287 bonds with histidine to stablize glucose-binding site

How does the subsitution of Ala287 in Glut 2 for Ser287 affect the binding?

produces a more flexible/wobbly helix 7 with a poorer glucose binding site to allow Glut 2 to have a different substrate behavior

How can Glut 2 bind both glucose and fructose?

its helix 7 has 2 conformational sites

Why is the anchor peptide formed?

because the I of the amino acid sequence that forms a Glut can’t fit into the active site of signal peptidase and P of the amino acid sequence cannot be stretched

What is similar between glutamines and arginines that contribute to glucose binding sites?

they both have amides

Does Glut 2 have a higher affinity for binding glucose or fructose?

glucose

What consequence can develop from the fact that Glut 2 has a higher affinity for glucose than fructose?

it takes a high concentration of fructose and glucose has priority over fructose which can cause fructose to build up which is bad for you

Dolichol Phosphate

15-19 isoprene rings that N-acetylglucosamine attaches to on the cytoplasmic side of the rER and sugar gets built on

What is the structure of N-linked glycosylation?

2 N-acetylglucosamine, lots of mannose, and 3 glucoses at the end

What are the steps to building the carbohydrate precursor that becomes Glut?

1. N-acetylglucosamine sugars are added to dolichol phosphate

2. several mannoses are added

3. partially built precursor is flipped across the membrane

4. more mannose is added

5. 3 glucoses are added

Does the building of the carbohydrate precursor start in the cytoplasmic side or the lumenal side?

cytoplasmic side (outside of the rER)

oligosaccharyl transferase

transfers entire carbohydrate precursor to the oxygen of the asparagine (N) in a protein and it becomes a protein

What occurs for the Glut protein to be able to go to the Golgi?

as the protein matures, 3 glucoses leave which causes calreticulum to release it so it can be transported to the Golgi

What will happen if a Glut has no N-linked glycosylation?

the protein will never leave the rER

While Gluts are being transported to the cis Golgi, how does it face?

the lumenal side stays lumenal

What does Q288 and Q289 do for Gluts?

directly H-bond or stabalize the entire array of H-bonds to position glucose and help alternate modes

What are some roles of glucose?

form glycogen, make fat, activate glycolysis, cause insulin to activate translation (build muscle), and provide energy for the brain

What can glucose do once it reaches the liver?

form glycogen, make fat, activate glycolysis

What is fructose destined to do once it reaches the liver?

make fat

Why does insulin activate insertion of Glut 4?

because it indicates hyperglycemia

What do Glut 4 receptors do at hypoglycemic conditions?

they are withdrawn into the submembrane vesicles so that muscle cells do not take limited glucose because the brain needs it

glucose sparing

body conserves available glucose for cells/organs that require it (brain)