carbohydrate biosynthesis

CARBOHYDRATE BIOSYNTHESIS April 14, 2015

BC368

Biochemistry of the Cell II

vs. HCl

Chloroplast vesicles isolated in some kind of physiological buffer.

Question 4. See pages 786-787.

Vesicles soaked in pH 4 medium.

Effect of proton ionophore?

Effect of DCMU?

Effect of dark?

“MY PHILODENDRON ISN’T DOING SO WELL.”

An older philodendron is brought into the nursery in very bad condition. It is seriously etiolated and has not grown well for three months. Although the owner moved to Eastern Pennsylvania from Iowa five months earlier, light levels, humidity, and temperature are essentially similar. You are working at the nursery to help earn extra money while attending graduate school in biochemistry and conduct several tests on the plant leaves to further assess the condition of the plant:

A) With this information in hand, what questions would you ask the owner?

B) In light of the answers you receive, what other tests would you do?

C) What would you recommend for treatment?

Starch levels Very lowRubisco activity Below normal in vivo levelsMg2+ levels About normalStromal NADPH levels Slightly higher than normal

ANABOLISM VS. CATABOLISM

Anabolic and catabolic pathways share many of the same reactions, but irreversible reactions are bypassed.

ANABOLISM VS. CATABOLISM

Anabolic and catabolic pathways undergo coordinate control.

Anabolic and catabolic pathways share many of the same reactions, but irreversible reactions are bypassed.

ATP hydrolysis drives biosynthetic processes even when precursor concentrations are low.

Example of anabolic pathway = gluconeogenesis, the synthesis of glucose from non-carbohydrate precursors, which helps to maintain glucose homeostasis.

GluconeogenesisRed blood cells

Lactate

One of the two pathways by which the liver maintains blood sugar during times of fasting.

Glucose Homeostasis

Normally, blood sugar is kept fairly constant by the liver.

Blood sugar rises after food consumption (postprandial period)

Glucose Homeostasis

First line of defense against a fall in blood sugar is glycogen breakdown.

Glycogen stores are depleted after an overnight fast

Glucose Homeostasis

Gluconeogenesis becomes significant after about 10 hours of fasting (or during exercise to process lactate).

Glucose Homeostasis

Hypoglycemia can result from fasting coupled with hard work.

Central role of oxalo- acetate in gluconeogenesis

Anything that can undergo net conversion to oxaloacetate can result in glucose.

Note that acetyl-CoA does NOT result in glucose.

Lactic acid

~Fig 14-16

Glycerol

Gluconeo- genesis vs. glycolysis

Fig 14-17

ΔG = -70 kJ/mol

ΔG = -16 kJ/mol

Bypass #1

Fig 14-17

Pyruvate carboxylase rxn

Fig 14-18

Acetyl-CoA is a positive effector

Transport as malate

No transporter for oxaloacetate, so it is converted to malate for transport to cytosol.

Enzyme is malate dehydrogenase.

Reverse reaction in cytosol to regenerate OA, also producing NADH.

PEP carboxykinase rxn

Fig 14-18

Bypass II: FBPase-1 rxn

Bypass III: glucose 6 phosphatase rxn

Gerty & Carl Cori

Lactate entry

Lactate entry

Gerty & Carl Cori

Lactate entry

Lactate entry requires a relatively high NAD+/NADH ratio in the cytosol.

No need to go through malate because reducing equivalents are formed in cytosol by LDH.

Case Study

Peter agrees to run a half marathon with his friend from the cross country team. He is undertrained for the race, and his legs feel quite tired and heavy when he is done. Afterwards, his friend suggests that they go out for a few beers before heading back to campus. What does Peter tell him?

(OA)

(OA)

(OA)

Some animals are highly dependent on gluconeogenesis.

Some animals are highly dependent on gluconeogenesis.

Reciprocal Regulation

Fig 15-22

Regulation of pyruvate carboxylase

Acetyl-CoA acts in a reciprocal manner on pyruvate carboxylase and pyruvate dehydrogenase complex.

High acetyl-CoA stimulates gluconeogenesis, although it is not used directly in the process.

Fig 15-18

FBPase-1/PFK-1 reciprical control

Regulation by F26BP

Fig 15-18

F26BP stimulates PFK-1

F26BP inhibits FBPase-1

Origin of F26BP

Single protein with two domains.

Activity of PFK-2/FBPase-2 is under complex hormonal control

Fig 15-19Regulation of [F26BP]

You are a first-year resident called to the post-anesthesia care unit by a nurse who is concerned about a patient she just received from the operating room. The anesthesiologists and surgeons are all busy in the operating rooms due to the recent arrival of multiple trauma patients, so you need to deal with this problem on your own.

The nurse tells you that shortly after arriving in the post-anesthesia care unit, the patient's blood pressure and heart rate began to rise. His temperature is also rising and is currently 40°C. According to the most recent arterial blood gas reading, the patient is acidotic with elevated CO2.

Because you are stumped by these symptoms, the nurse gently informs you that she believes the patient may have malignant hyperthermia, a life-threatening syndrome that occurs during or immediately after general anesthesia.

Along with considering how to treat the condition, you wonder what caused the condition to develop. What is the origin of the patient’s increased body temperature?

Case Study

Malignant hyperthermia is genetic disorder that causes a rapid increase in body temperature and muscle rigidity when the patient is given general anesthesia.

Symptoms result from a hypercatabolic state because of a mutation in a calcium channel of muscle. The channel opens when bound by certain anesthetics, causing an influx of Ca2+.

Heat results from the following reactions.

Malignant Hyperthermia

PFK-1

FBPase-1

In-Class Problem

Consider a substrate cycle operating with enzymes X and Y in this pathway:

a) Under intracellular conditions, the activity of enzyme X is 100 pmol/106 cells/s and that of enzyme Y is 90 pmol/106 cells/s. What are the direction and rate of metabolic flux between B and C?

b) Calculate the effect of metabolic flux rate and direction after each of the following:i) Adding an activator that increases the activity of X by

20%ii) Adding an inhibitor that decreases the activity of Y by

20%iii) Adding both the activator and the inhibitor

Central role of glucose-6-P in metabolism

Gluconeogenesis

Glucose

Glycogen synthesis overviewGlucose-6-phosphate

UDP-glucose pyrophosphorylase

phosphoglucomutase

Step 1: phosphoglucomutase

Step 2: UDP-glucose pyrophosphorylase

Activated glucose donor

UDP-glucose

Sugar nucleotides are quite common as “activated” monosaccharides primed for further reaction

UDP-glucoseFig 15-29

Step 3: Glycogen synthase

Branching enzyme

GlycogeninFig 15-33

GlycogeninFig 15-33

Regulation via phosphorylation

Phosphorylation inactivates glycogen synthase.

Regulation via phosphorylation

Dephosphorylation activates glycogen synthase.

Regulation via phosphorylation

Insulin inactivates glycogen synthase kinase, activating glycogen synthase.

Role of insulinFig 15-39

Regulation via phosphorylation

Insulin activates protein phosphatase 1 (PP1), turning on glycogen synthase.

Glucagon and epinephrine inactivate protein phosphatase 1 (PP1), turning off glycogen synthase.

Hormone effects

Fig 15-41