MCAT Biochemistry - Carbohydrate Metabolism I: Glycolysis, Glycogen, Gluconeogenesis, and the Pentose Phosphate Pathway

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Glucose entry into cells

driven by concentration and is independent of sodium

Normal glucose concentration in peripheral blood

5.6 mM (normal range: 4–6 mM)

glucose transporters

4, GLUT 1 through GLUT 4

GLUT 2

low-affinity transporter in hepatocytes and pancreatic cells; captures the excess glucose primarily for storage; pick up glucose in proportion to its concentration in the blood (first-order kinetics); along with the glycolytic enzyme glucokinase, serves as the glucose sensor for insulin release

K_m = ~15 mM

GLUT 4

in adipose tissue and muscle; responds to the glucose concentration in peripheral blood; constant rate of glucose influx because they will be saturated if blood glucose is even slightly higher (zero-order kinetics); insulin stimulates the movement of additional transporters to the membrane by exocytosis

K_m= 5 mM (normal glucose levels in blood)

Glycolysis

cytoplasmic pathway that converts glucose into two pyruvate molecules, releasing a modest amount of energy captured in two substrate-level phosphorylations and one oxidation reaction; may occur anaerobically, although some of the available energy is lost

all cells must be able to do it; RBC lack mitochondria and depend on this for energy, cancer cells use this more often than healthy cells

Hexokinase

phosphorylates glucose to prevent leaving via the transporter

widely distributed in tissues

Low K_m

inhibited by its product, glucose 6-phosphate

irreversible

Glucokinase

phosphorylates glucose to prevent leaving via the transporter

found only in liver cells and pancreatic β-islet cells

High K_m

Induced by insulin in hepatocytes

irreversible

rate-limiting enzymes for Carbohydrate Metabolism Pathways

Glycolysis: phosphofructokinase-1

Fermentation: lactate dehydrogenase

Glycogenesis: glycogen synthase

Glycogenolysis: glycogen phosphorylase

Gluconeogenesis: fructose-1,6-bisphosphatase

Pentose Phosphate Pathway: glucose-6-phosphate dehydrogenasen

Phosphofructokinase-1 (PFK-1)

rate-limiting enzyme and main control point in glycolysis: irreversible

fructose 6-phosphate is phosphorylated to fructose 1,6-bisphosphate using ATP

inhibited by ATP (high energy) and citrate (intermediate of citric acid cycle), and activated by AMP (low energy) and fructose 2,6-bisphosphate (F2,6-BP) (allows these cells to override the inhibition caused by ATP)

Insulin stimulates and glucagon inhibits in hepatocytes

Phosphofructokinase-2 (PFK-2)

converts a tiny amount of fructose 6-phosphate to fructose 2,6-bisphosphate (F2,6-BP)

Insulin stimulates and glucagon inhibits

mostly found in liver

Glyceraldehyde-3-phosphate dehydrogenase

catalyzes an oxidation and addition of inorganic phosphate (P_i) to its substrate, glyceraldehyde 3- phosphate → production of a high-energy intermediate 1,3-bisphosphoglycerate and the reduction of NAD+ to NADH

3-Phosphoglycerate kinase

transfers the high-energy phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate

substrate-level phosphorylation

ADP is directly phosphorylated to ATP using a high-energy intermediate; not dependent on oxygen; only means of ATP generation in an anaerobic tissue

Pyruvate Kinase

last enzyme in aerobic glycolysis; catalyzes a substrate-level phosphorylation of ADP using the high-energy substrate phosphoenolpyruvate (PEP); irreversible

activated by fructose 1,6-bisphosphate from PFK-1

feed-forward activation

the product of an earlier reaction stimulates, or prepares, a later reaction

fermentation

anaerobic method of replenishing NAD+

lactate dehydrogenase

oxidizes NADH to NAD+, replenishing the oxidized coenzyme for glyceraldehyde-3-phosphate dehydrogenase

reduces pyruvate to lactate, both three-carbon molecules

used when oxygenation is poor (during strenuous exercise in skeletal muscle, a heart attack, or a stroke)

yeast fermentation

conversion of pyruvate (three carbons) to ethanol (two carbons) and carbon dioxide (one carbon); replenishing NAD+

Dihydroxyacetone phosphate (DHAP)

used in hepatic and adipose tissue for triacylglycerol synthesis; formed from fructose 1,6-bisphosphate; isomerized to glycerol 3-phosphate, which can then be converted to glycerol

1,3-Bisphosphoglycerate (1,3-BPG)

high-energy intermediates used to generate ATP by substrate-level phosphorylation using 3-Phosphoglycerate kinase; only ATP gained in anaerobic respiration

phosphoenolpyruvate (PEP)

high-energy intermediates used to generate ATP by substrate-level phosphorylation using Pyruvate Kinase; only ATP gained in anaerobic respiration

Adaptation to high altitudes (low pO₂)

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)

bisphosphoglycerate mutase

in RBC, produces 2,3-bisphosphoglycerate from 1,3-BPG; moves phosphate group from 1-position to 2-position

2,3-bisphosphoglycerate (2,3-BPG)

binds allosterically to the β-chains of hemoglobin A (HbA) and decreases its affinity for oxygen; rightward shift in the curve is sufficient to allow unloading of oxygen in tissues, but still allows 100 percent saturation in the lungs; does not bind well to fetal hemoglobin `(HbF)

lactose/galactose metabolism

sucrose/fructose metabolism

Note:

• Galactokinase

• Galactose-1-phosphate uridyltransferase

galactose

derived from lactose by lactase; reaches the liver through the hepatic portal vein

Lactose

sugar found in milk; hydrolyzed to galactose and glucose by lactase

lactase

brush-border enzyme of the duodenum; hydrolyses lactose to galactose and glucose

galactokinase

phosphorylates galactose to galactose 1-phosphate; trapping it in hepatocytes

galactose-1-phosphate uridyltransferase

onverts galactose 1-phosphate to glucose 1-phosphate with an epimerase

Epimerases

enzymes that catalyze the conversion of one sugar epimer to another

epimers

diastereomers that differ at exactly one chiral carbon

galactosemia.

Genetic deficiencies of galactokinase or galactose-1-phosphate uridyltransferase; lead to cataracts

Cataracts

conversion of excess galactose in the blood to galactitol in the lens of the eye by aldose reductase; ; Accumulation of galactitol in the lens causes osmotic damage

polyol

a carbon chain with many alcohol groups

Primary lactose intolerance

hereditary deficiency of lactase; bacterial fermentation of lactose, which produces a mixture of CH4, H2, and small organic acids; result in the movement of water into the intestinal lumen

symptoms: include vomiting, bloating, explosive and watery diarrhea, cramps, and dehydration

Secondary lactose intolerance

precipitated at any age by gastrointestinal disturbances that cause damage to the intestinal lining, where lactase is found; bacterial fermentation of lactose, which produces a mixture of CH4, H2, and small organic acids; result in the movement of water into the intestinal lumen

symptoms: include vomiting, bloating, explosive and watery diarrhea, cramps, and dehydration

Fructose

found in honey and fruit and part of sucrose; absorbed into the hepatic portal vein; also metabolized in renal proximal tubules

sucrose

common table sugar; hydrolysed by glucose and fructose by sucrase

sucrase

duodenal brush-border enzyme; hydrolyses sucrose into glucose and fructose

fructokinase

phosphorylates fructose to fructose 1-phosphate in hepatocytes

aldolase B

cleaves fructose 1-phosphate into glyceraldehyde and DHAP; products downstream from PFK in gkycolysis

dihydroxyacetone phosphate (DHAP)

products of fructose metabolism

acetyl-CoA

product of pyruvate; necessary for citric acid cycle

pyruvate dehydrogenase complex (PDH)

converts pyruvate to Acetyl CoA; irreversible; requires multiple cofactors and coenzymes, including thiamine pyrophosphate, lipoic acid, CoA, FAD, and NAD+

activated by insulin in liver; not responsive to hormones in nervous system

Glycogen

a branched polymer of glucose used for storage; primarily in liver (source of glucose that is mobilized between meals to prevent low blood sugar) and skeletal muscle (stored as an energy reserve for muscle contraction); stored in the cytoplasm as granules: composed entirely of linear chains have the highest density of glucose near the core, while branched glucose density is highest at the periphery of the granule, allowing more rapid release of glucose on demand

starch

long α-linked chains of glucose; plant storage of excess sugar

Glycogenesis

synthesis of glycogen granules; begins with a core protein called glycogenin

1. begins with glucose 6-phosphate converted to glucose 1-phosphate

2. glucose 1-phosphate is then activated by coupling to a molecule of uridine diphosphate (UDP)

3. integration into the glycogen chain by glycogen synthase when glucose 1-phosphate interacts with uridine triphosphate (UTP)

   1. forming UDP-glucose and a pyrophosphate (PP_i)

Glycogen synthase

rate-limiting enzyme of glycogen synthesis; forms the α-1,4 glycosidic bond found in the linear glucose chains of the granule

stimulated by glucose 6-phosphate and insulin

inhibited by epinephrine and glucagon through a protein kinase phosphorylation

Branching enzyme

introduces α-1,6-linked branches into the granule as it grows; hydrolyzes one of the α-1,4 bonds to release a block of oligoglucose, which is then moved and added in a slightly different location where it forms an α-1,6 bond to create a branch

glycogenolysis

process of breaking down glycogen

glycogen phosphorylase

rate-limiting enzyme of glycogenolysis; breaks only α-1,4 glycosidic bonds, releasing glucose 1- phosphate from the periphery of the granule using an inorganic phosphate; forms glucose 1-phosphate then converted to glucose 6-phosphate by mutase

activated by glucagon in the liver, activated by AMP and epinephrine in skeletal muscle

inhibited by ATP

Debranching enzyme

two-enzyme complex that deconstructs the branches in glycogen that have been exposed by glycogen phosphorylase; Breaks an α-1,4 bond adjacent to the branch point and moves the small oligoglucose chain that is released to the exposed end of the other chain; Forms a new α-1,4 bond; Hydrolyzes the α-1,6 bond, releasing the single residue at the branch point as free glucos

von Gierke’s disease

defect in glucose-6-phosphatase; periods of extremely low blood sugar between meals; buildup of glucose 6-phosphate enlarges and damages liver over time

Isoforms

slightly different versions of the same protein

glycogen storage diseases

metabolic genetic deficiencies characterized by accumulation or lack of glycogen in one or more tissues

gluconeogenesis

metabolic pathway that results in the biosynthesis of glucose from certain non-carbohydrate carbon substrates; liver and kidney; requires expenditure of ATP that is provided by β-oxidation of fatty acids

promoted by glucagon and epinephrine

inhibited by insulin

fasting

glycogen reserves drop dramatically in the first 12 hours

gluconeogenesis increases

After 24 hours, it represents the sole source of glucose

Glucogenic amino acids

can be converted by individual pathways to citric acid cycle intermediates, then to malate, following the same path from there to glucose

all except leucine and lysine, esp. alanine

ketogenic amino acids

can be converted into ketone bodies, which can be used as an alternative fuel

Important substrates for gluconeogenesis

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

• Lactate (from anaerobic glycolysis)

• Glucogenic amino acids (from muscle proteins)

lactate dehydrogenase

Lactate is converted to pyruvate

alanine aminotransferase

Alanine is converted to pyruvate

glycerol-3-phosphate dehydrogenase

Glycerol 3-phosphate is converted to dihydroxyacetone phosphate (DHAP)

Pyruvate carboxylase

mitochondrial enzyme that is activated by acetyl-CoA (from β-oxidation); product, oxaloacetate (OAA), is a citric acid cycle intermediate and cannot leave the mitochondrion; reduced to malate, which can leave the mitochondrion via the malate–aspartate shuttle

Phosphoenolpyruvate carboxykinase (PEPCK)

converts OAA to phosphoenolpyruvate (PEP); requires GTP; PEP continues in the pathway to fructose 1,6-bisphosphate

cytoplasm

induced by glucagon and cortisol

Fructose-1,6-bisphosphatase

key control point of gluconeogenesis; represents the rate-limiting step of the process; reverses the action of phosphofructokinase-1 by removing phosphate from fructose 1,6-bisphosphate to produce fructose 6-phosphate

activated by ATP

inhibited by AMP and fructose 2,6-bisphosphate

Glucose-6-phosphatase

Glucose 6-phosphate is transported into the ER, and free glucose is transported back into the cytoplasm where it can diffuse out of the cell using GLUT transporters

lumen of the endoplasmic reticulum in liver cells

pentose phosphate pathway (PPP) / hexose monophosphate (HMP) shunt

production of NADPH and serving as a source of ribose 5-phosphate for nucleotide synthesis

1. glucose-6-phosphate dehydrogenase; begins with glucose 6-phosphate, ends with ribulose 5-phosphate; irreversible → NADPH

2. transketolase and transaldolase; beginning with ribulose 5-phosphate, reversible reactions that produce an equilibrated pool of sugars for biosynthesis, including ribose 5-phosphate; can feed back into glycolysis

cytoplasm of all cells

glucose-6-phosphate dehydrogenase (G6PD)

rate-limiting enzyme of pentose phosphate pathway; first step

induced by insulin

activated by one of its reactants, NADP+

inhibited by its product, NADPH

G6PD deficiency / favism

X-linked disorder; most common inherited enzyme defect

susceptible to oxidative stress, certain oxidizing compounds (antibiotics, antimalarial medications, fava beans) or infections can lead to high concentrations of reactive oxygen species

some malaria resistance

ribose 5-phosphate

backbone of nucleic acids; isomerised from ribulose 5-phosphate in PPP

NADPH

electron donor in a number of biochemical reactions; potent reducing agent

• Biosynthesis, mainly of fatty acids and cholesterol

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

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

Hydrogen peroxide, H₂O₂

produced as a byproduct in aerobic metabolism, and can break apart to form hydroxide radicals, OH•, that can attack lipids, including those in the phospholipids of the membrane

Glutathione

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