MCAT Chapter 9: Carbohydrate Metabolism I: Glycolysis, Glycogen, Glucogenesis, and the Pentose Phosphate Pathway

What are the important tissues of GLUT 2? GLUT 4?

Liver and pancreas

Adipose tissue and muscle

What are the Km values of GLUT 2? GLUT 4?

High (~15 mM)

Low (~5 mM)

Is GLUT 2 saturated at normal glucose levels? GLUT 4?

No-cannot be saturated under normal physiological conditions

Yes-saturated when glucose levels are only slightly above 5 mM

Is GLUT 2 responsive to insulin? GLUT 4?

No (but serves as glucose sensor to cause release of insulin in pancreatic beta cells)

Yes

How does insulin promote glucose entry into cells?

GLUT 4 is saturated when glucose levels are only slightly above 5 mM, so glucose entry can only be increased by increasing the number of transporters. Insulin promotes the fusion of vesicles containing performed GLUT 4 with the cell membrane.

What is the function and key regulators of Hexokinase?

Hexokinase phosphorylates glucose to form glucose 6-phosphate, "trapping" glucose in the cell. It is inhibited by glucose 6-phosphate. It is irreversible.

What is the function and key regulators of Glucokinase?

Glucokinase also phosphorylates and "traps" glucose in liver and pancreas cells, and works with GLUT 2 as part of the glucose sensor in Beta-islet cells. In liver cells it is induced by insulin. It is irreversible.

What is the function and key regulators of PFK-1?

PFK-1 catalyzes the rate-limiting step of glycolysis, phosphorylating fructose 6-phosphate to fructose 1, 6-biphosphate using ATP. It is inhibited by ATP, citrate and glucagon. It is activated by AMP, fructose 2,6-biphosphate and insulin. It is irreversible.

What is the function and key regulators of Glyceraldehyde-3-phosphate dehydrogenase?

Glyceraldehyde-3-phosphate dehydrogenase generate NADH while phosphorylating glyceraldehyde 3-phosphate to 1,3-biphosphoglycerate. it is reversible.

What is the function of 3-phosphoglycerate kinase?

3-phosphoglycerate kinase performs a substrate-level phosphorylation, transferring a phosphate from 1,3-biphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate. It is reversible.

What is the function of pyruvate kinase?

Pyruvate kinase performs another substrate-level phosphorylation, transferring a phosphate from phosphoenolpyruvate (PEP) to ADP, forming ATP and pyruvate. It is activated by fructose 1,6-biphosphate. It is irreversible.

Why must pyruvate undergo fermentation for glycolysis to continue?

Fermentation must occur to regenerate NAD+, which is limited in supply in cells. Fermentation generates no ATP or energy carriers; it merely regenerates the coenzymes needed in glycolysis.

Why is it necessary that feta; hemoglobin does not bind 2,3-BPG?

The bidning of 2,3-BPG decreases the hemoglobin's affinity for oxygen. Fetal hemoglobin must be able to "steal" oxygen from maternal hemoglobin at the placental interface; therefore, it would be disadvantageous to lower its affinity for oxygen.

What enzyme is responsible for trapping galactose in the cell? "trapping enzyme"

Galactose is phosphorylated by galactokinase, trapping it in the cell.

What enzyme in galactose metabolism results in a product that can feed directly into glycolysis, linking the two pathways? "Linking enzyme"

Galactose-1-phosphate uridyltransferase produces glucose 1-phosphate, a glycolytic intermediate, thus linking the two pathways.

Which enzyme is responsible for trapping fructose in the cell? "Trapping enzyme"

Fructose is phosphorylated by fructokinase, trapping it in the cell (with a small contribution from hexokinase)

What enzyme in fructose metabolism results in a product that can feed directly into glycolysis, linking the two pathways? "linking enzymes"

Aldolase B produces dihydroxyacetone phosphate (DHAP) and glyceraldehyde (which can be phosphorylated to form glyceraldehyde 3-phosphate), which are glycolytic intermediates, thus linking the two pathways.

What are the reactants of the pyruvate dehydrogenase complex? What are the products?

Pyruvate, NAD+, and CoA are the reactants of the PDH complex. Acetyl-CoA, NADH, and CO2 are the products.

How does acetyl CoA affect PDH complex activity?

Acetyl CoA inhibits the PDH complex. As a product of the enzyme complex, a buildup of acetyl-CoA from either the citric acid cycle or fatty acid oxidation signals that the cell is energetically satisfied and that the production of acetyl-CoA should be slowed or stopped. Pyruvate can then be used to form other products, such as oxaloacetate for use in gluconeogenesis.

What is GLUT 2?

A low affinitt transporter in hepatocytes and pancreatic cells. It captures the excess glucose primarily for storage after a meal from the intestine. When the glucose concentration drops below the Km for the transporter, much of the remainder leaves the liver and enters the peripheral circulation. Km for GLUT 2 is about 15mM, so liver will pick up glucose in proportion to its concentration in the blood. Along with glycolytic enzymes, it serves as a glucose sensor for insulin release.

What is Km?

It is the concentration of substrate when an enzyme is active at half of its maximum velocity (Vmax). The lower the Km the higher the enzyme's affinity for the substrate.

What is GLUT 4?

Is in adipose tissue and muscle and responds to the glucose concentration in peripheral blood. The rate of glucose transport in these two tissues is increased by insulin, which stimulates the movement of additional GLUT 4 transporters to the membrane by a mechanism involving exocytosis. The Km of GLUT 4 is close to the normal glucose levels in the blood (5mM), the transporter is saturated when blood glucose levels are just a bit higher than normal. Only way to increase the intake of glucose by GLUT 4 is by increasing the # of transporters on the surface of cells!

What is the rate-limiting enzyme for glycolysis?

Phosphofructokinase-1

What is the rate-limiting enzyme for fermentation?

Lactate Dehydrogenase

What is the rate-limiting enzyme for glycogenesis?

Glycogen synthase

What is the rate-limiting enzyme for glycogenolysis?

Glycogen phosphorylase

What is the rate-limiting enzyme for gluconeogenesis?

Fructose-1,6-biphosphatase

What is the rate limiting enzyme for pentose phosphate pathway?

Glucose-6-phosphate dehydrogenase

Where is hexokinase present? Glucokinase?

In most tissues

Present in hepatocytes and pancreatic beta=islet cells (along with GLUT 2, acts as the glucose as the glucose sensor)

Is the Km high or low for hexokinase? Glucokinase?

Low Km(reaches max velocity at low [glucose])

High Km (acts on glucose proportionally to its concentration)

What molecule inhibits hexokinase? What induces Glucokinase?

Inhibited by glucose 6-phosphate

Induced by insulin in hepatocytes

What is feed-forward activation?

Product of an earlier reaction of glycolysis (fructose 1,6-biphosphate) stimulates or prepares a later reaction in glycolysis (by activating pyruvate kinase)

What are the irreversible steps of glycolysis?

(H)ow (G)lycolysis (P)ushes (F)orward the (P)rocess: (K)inases

(H)exokinase

(G)lucokinase

(P)(F)K-1

(P)yruvate (K)inase

What cellular respiration pathway occurs in erythrocytes (red blood cells)?

Anaerobic glycolysis represents the only pathway for ATP production, yielding a net 2 ATP per glucose.

What does adapting to high altitude (low pO2) involve?

Increased respiration

Increased oxygen affinity for hemoglobin (initial)

Increased rate of glycolysis

Increased [2,3-BPG] in RBC (over a 12-24 hour period)

Normalized oxygen affinity for hemoglobin restored by the increased level of 2,3-BPG

Increased hemoglobin (over days to weeks)

What is the BOHR effect?

A right shift of the oxygen dissociation curve (high 2,3-BPG, low pH, High [H+], and high pCO2) These occur during exercise. "Exercise is the RIGHT thing to do"

What are epimerases?

Enzymes that catalyze the conversion of one sugar epimer to another

What is an epimer?

Diastereomers, differ at exactly one chiral carbon.

What is glycogen?

A branched polymer of glucose, a storage form of glucose. Glycogen synthesis and degradation occur primarily in liver and skeletal muscles. It is stored in the cytoplasm as granules. Each granule has a central protein core with polyglucose chains radiating outward to form a sphere. If the chains are branched, the glucose density is highest at the periphery of the granule, allowing more rapid release of glucose on demand.

Stored in liver: source of glucose that is mobilized in between meals to prevent low blood sugar

Stored in muscles: energy reserve for muscle contraction.

What is the synthesis of glycogen granules called?

Glycogenesis (the rate limiting enzyme is glycogen synthase)

What is the difference between hydrolase and phosphorylase?

Breaks bonds using an inorganic phosphate instead of water.

What does the debranching enzyme do?

It is made up of two enzymes with different functions: one moves the terminal end of a glycogen chain to the branch point and one removes the glucose monomer actually present at the branch point.

What is an isoform?

Slightly different versions of the same protein; in the case of glycogen enzymes, there are often different isoforms of the enzymes in the liver and muscle.

What is the term for the genetic defiencies leading to accumulation of glycogen in one or more tissues? (ex. von Gierke's disease)

Glycogen Storage Disease

What is the structure of glycogen? What type of glycosidic links exist in a glycogen granule?

Glycogen is made up of a core protein of glycogenin with linear chains of glucose emanating out from the center, connected by alpha-1,4 glycosidic links. Some of these chains are branched, which requires alpha-1,6 glycosidic links.

What are the two main enzymes of glycogenesis and what does each accomplish?

Glycogen synthase attaches the glucose molecule from UDP-glucose to the growing glycogen chain, forming an alpha 1-4 link in the process. Branching enzyme creates a branch breaking an alpha 1,4 link in the growing chin and moving a block of oligoglucose to another location in the glycogen granule. The oligoglucose is then attached with an alpha 1,6 link.

What are the two main enzymes of glycogenolysis?

Glycogen phosphorylase removes a glucose molecule from glycogen using a phosphate, breaking the alpha 1,4 link and creating glucose 1-phosphate. Debranching enzyme moves all of the glucose from a branch to a longer glycogen chain by braking an alpha 1,4 link and forming a new alpha 1,4 link to the longer chain. The branchpoint is left behind; this is removed by breaking the alpha 1,6 link to form a free molecule of glucose.

How does the liver maintain glucose levels in blood during fasting?

Glycogenolysis or Gluconeogenesis.

What are the important substrated for gluconeogenesis?

Glycerol 3-phosphate (from stored fats, or triacylglycerols, in adipose tissue)

Lactate (from anaerobic glycolysis)

Glucogenic amino acids (from muscle proteins)

What does insulin do?

Acts to lower blood sugar levels.

What are the counterregulatory hormones that act to raise blood sugar levels by stimulating glycogenolysis and gluconeogenesis?

Glucagon, epinephrine, cortisol and growth hormone

What is a glucogenic amino acid?

All amino acids except leucine and lysine

Can be converted into intermediates that feed gluconeogenesis

What is a ketogenic amino acid?

Can be converted into ketone bodies, which can be used as an alternative fuel, particularly during periods of prolonged starvation.

What is pyruvate carboxylase of gluconeogenesis? What activates it?

Is a mitochondrial enzyme that is activated by acetyl-CaA (from beta oxidation), it produces oxaloacetate in the citric acid cycle .

What is phosphoenolpyruvate carboxykinase (PEPCK)?

Of gluconeogenesis, it is found in the cytoplasm and is induced by glucagon and cortisol, which generally acts to raise blood sugar levels. It coverts oxaloacetate (product from pyruvate carboxylase) to phospoenolpyruvate (PEP) in a reaction that required GTP.

What is fructose 1,6 biphosphatase?

Found in the cytoplasm, it is a key control point of gluconeogenesis and represents the rate-limiting step of the process. It reverses the reaction of phosphofructokinase-1, the rate limiting step of glycolysis, by hydrolyzing phosphate from fructose 1,6-biphosphate to produce fructose 6-phosphate.

What opposes kinases?

Phosphatases

What is fructose 2,6 biphosphate?

inhibits fructose 1,6 biphosphatase (high levels of ATP imply that the cell is energetically satisified enough to produce glucose for the rest of the body, whereas high levels of AMP imply that a cell needs energy and cannot afforf to produce energy for the rest of the body before satisfyin gits own requirements. It is sometimes thought of as a marker for satisfactory energy levels in liver cells, it helps these cells override the inhibition of phosphofructokinase-1 that occurs when high levels of acetyl-CoA are formed, signaling to the liver cell that it should shift its function from burning to storing fuel.

What is glucose-6-phosphatase?

Found only in the lumen of the ER in liver cells. Is transported into the ER and free glucose is transported back into the cytoplasm from where it can diffuse out of the cell using GLUT transporters. Enzyme of gluconeogenesis. Is used to circumvent glucokinase and hexokinase, which convert glucose to glucose 6-phosphate.

Why can't RBC carry out aerobic respiration?

No mitochondria present

What is the Cori cycle?

GLucose is converted to lactate in RBC and lactate is converted to glucose in liver cells

Under what physiological conditions should the body carry out gluconeogenesis?

Occurs when an individual has been fasting for >12 hours. To carry out gluconeogenesis, hepatic and renal cells must have enough energy to drive the process of glucose creation, which requires sufficient fat stores to undergo beta oxidation.

What are four enzymes unique to gluconeogenesis? Which irreversible glycolytic enzymes do they replace?

Gluconeogenic enzyme: pyruvate carboxylase, replace, pyruvate kinase

Gluconeogenic enzyme: Phosphoenolpyruvate carboxykinase (PEPCK) replace pyruvate kinase

Gluconeogenic enzyme: Fructose 2,6 biphosphate replaces phosphofructokinase-1

Gluconeogenic enzyme: glucose 6 phosphatase replaces glucokinase

How does acetyl Co-A shift the metabolism of pyruvate?

Acetyl CoA inhibits pyruvate dehydrogenase complex while activating pyruvate carboxylase. The net effect is to shift from burning pyruvate in the citric acid cycle to creating new glucose molecules for the rest of the body. The acetyl-CoA for this regulation comes predominantly from beta-oxidation, not glycolysis.

What is the function of pentose phosphate pathway (PPP)?

Or hexose monophosphate (HMP) shunt occurs in the cytoplasm of all cells where it produces NADPH and serves as a source of ribose 5-phosphate for nucleotide synthesis.

What is the rate limiting enzyme of pentose phosphate pathway (PPP)?

glucose 6-phosphate dehydrogenase (G6PD), it is induced by insulin because the abundance of sugar entering the cell under insulin stimulation will be SHUNTED into both fuel utilization pathways (glycolysis and aerobic respiration) as well as fuel storage pathways (fatty acid synthesis, glycogenesis and the PPP). Inhibited by its product NADPH and is activated by one of its reactants NADP+

What is the difference between NADPH and NADH?

NAD+ is an energy carrier while NADPH is used in biosynthesis, in the immune system and to help prevent oxidative damage.

What is NADPH?

primarily acts as an electron donor in a number of biochemical reactions, can be thought of as a potent reducing agent because it helps other molecules be reduced (and thus is oxidized itself during the process).

Functions:

1) Biosynthesis, mainly of fatty acids and cholestrol

2) Assisting in cellular bleach production in certain white blood cells, thereby contributing to bactericidal activity

3) Maintenance of a supply of reduced glutathione to protect against reactive oxygen species (acting as the body's natural antioxidant)

What is Glutathione?

A reducing agent that can help reverse radical formation before damage is done to the cell.