biochem exam 3

What is glycolysis?

The sequence of 10 enzyme-catalyzed reactions that metabolizes ONE molecule of glucose into TWO molecules of pyruvate, with a NET production of 2 ATP and 2 NADH. It occurs in the CYTOPLASM of virtually all cells (prokaryotic and eukaryotic). It is ANAEROBIC — it does NOT require oxygen.

Where does glycolysis occur in eukaryotic cells?

In the cytoplasm(Cytosol).

How many stages does glycolysis have, and what happens in each?

Two stages.

Stage 1 (trapping and preparation: Glucose is phosphorylated and cleaved into two 3-carbon units. NO ATP is generated. 2 ATP are invested

Stage 2 (energy Harvest:) The 3-carbon fragments are oxidized to pyruvate. ATP and NADH are produced. 4 ATP are generated (net gain =2 ATP)

What is the NET reaction of glycolysis?

Glucose + 2 Pi + 2 ADP + 2 NAD⁺ → 2 pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O

How much free energy is released in the anaerobic conversion of glucose to pyruvate?

About −90 kJ/mol (−22 kcal/mol).

How many total ATP are produced in glycolysis and how many are consumed?

4 ATP produced, 2 ATP consumed (invested in Stage 1) → NET GAIN of 2 ATP.

How many NADH are produced in glycolysis?

2 NADH (one per 3-carbon unit, from the GAP dehydrogenase step).

Why is glucose the favored fuel for all life forms?

Three reasons:

1. It is one of several monosaccharides formed from formaldehyde under PREBIOTIC conditions (ancient evolutionary origin).

2. It is the MOST STABLE hexose — all hydroxyl groups AND the hydroxymethyl group are in the EQUATORIAL position, minimizing steric clashes.

3. It has a LOW TENDENCY to nonenzymatically GLYCATE proteins (react with amino groups on proteins) compared to other monosaccharides.

What is glucose the ONLY fuel for in the human body?

1. The BRAIN under non-starvation conditions. 2. RED BLOOD CELLS at all times.

What is gluconeogenesis?

The process by which metabolic products such as PYRUVATE and LACTATE are salvaged to SYNTHESIZE GLUCOSE. It is the reverse direction of glycolysis (though not the exact same pathway).

What are glucose transporters and how do they work?

They are membrane proteins that mediate THERMODYNAMICALLY DOWNHILL (passive/facilitated diffusion) movement of glucose across plasma membranes of animal cells. They do NOT require ATP.

GLUT1 — tissue location, KM, function?

Location: ALL mammalian tissues. KM: ~1 mM. Function: BASAL glucose uptake.

GLUT2 — tissue location, KM, function?

Location: LIVER and PANCREATIC β cells. KM: 15–20 mM (HIGH → only active when glucose is ABUNDANT). Function: In pancreas — regulates insulin secretion. In liver — removes EXCESS glucose from blood.

GLUT3 — tissue location, KM, function?

Location: ALL mammalian tissues. KM: ~1 mM. Function: BASAL glucose uptake.

GLUT4 — tissue location, KM, function?

Location: MUSCLE and FAT cells. KM: ~5 mM. Function: Amount in plasma membrane INCREASES with endurance training (important for exercise adaptation).

GLUT5 — tissue location, KM, function?

Location: SMALL INTESTINE. KM: ~15 mM. Function: Primarily a FRUCTOSE transporter.

What does a LOW KM value for a transporter mean biologically?

Low KM = HIGH AFFINITY for glucose → transporter is active even when glucose concentrations are LOW. GLUT1 and GLUT3 (KM ~1 mM) provide basal uptake at all times.

What does a HIGH KM value for a transporter mean biologically?

High KM = LOW AFFINITY → transporter is only significantly active when glucose is ABUNDANT. GLUT2 (KM 15–20 mM) ensures liver and pancreas only respond when blood glucose is HIGH.

What are the 3 IRREVERSIBLE steps of glycolysis, and what makes them irreversible?

1. Hexokinase (Step 1), 2. Phosphofructokinase/PFK-1 (Step 3), 3. Pyruvate kinase (Step 10).

They are irreversible because they have LARGE NEGATIVE ΔG values — they are highly thermodynamically favorable in the forward direction only. Under cellular conditions, they essentially cannot run in reverse. They are all phosphoryl group transfers.

Step 1 of glycolysis — what happens, enzyme, product, reversible?

Glucose → Glucose 6-phosphate (G-6P).

Enzyme: HEXOKINASE (or GLUCOKINASE in liver/pancreas).

ATP is consumed (phosphoryl group transferred from ATP to glucose).

IRREVERSIBLE under cellular conditions. Large negative ΔG.

Why is phosphorylation of glucose by hexokinase important? (Two reasons)

1. G-6P CANNOT pass back through the plasma membrane → traps glucose inside the cell.

2. The phosphoryl group ACTIVATES glucose for further metabolism (gives it high phosphoryl-transfer potential).

What cofactor does hexokinase (and all kinases) require, and why?

Mg²⁺ (or another divalent metal ion). It forms a COMPLEX with ATP to minimize the net negative charge of ATP, facilitating the phosphoryl transfer.

escribe the conformational change hexokinase undergoes upon glucose binding.

A:

Hexokinase has TWO LOBES. When glucose binds, the lobes move TOWARD each other (the cleft CLOSES). This is an example of INDUCED FIT. The glucose becomes surrounded by protein except for the carbon accepting the phosphoryl group. Water is excluded from the active site, enhancing specificity and preventing ATP hydrolysis.

How is hexokinase regulated?

It is inhibited by its PRODUCT, G-6P (product inhibition). When G-6P accumulates (because PFK is inactive), hexokinase is inhibited → stops glucose phosphorylation.

Step 2 of glycolysis — what happens, enzyme, product, reversible?

Glucose 6-phosphate (G-6P) → Fructose 6-phosphate (F-6P).

Enzyme: PHOSPHOGLUCOSE ISOMERASE.

This is an ISOMERIZATION — conversion of an ALDOSE into a KETOSE.

REVERSIBLE.

Why is the isomerization of G-6P to F-6P (Step 2) crucial?

Because only 3-CARBON molecules are metabolized in the later stages of glycolysis. The conversion to fructose allows the molecule to be properly cleaved (by aldolase in Step 4) into two interconvertible 3-carbon units.

Step 3 of glycolysis — what happens, enzyme, product, reversible?

Fructose 6-phosphate → Fructose 1,6-bisphosphate (F-1,6-BP).

Enzyme: PHOSPHOFRUCTOKINASE-1 (PFK-1).

ATP is consumed (second phosphorylation).

IRREVERSIBLE under cellular conditions.

Why is PFK-1 considered the MOST IMPORTANT regulatory enzyme in glycolysis?

It catalyzes the RATE-LIMITING, COMMITTED step of glycolysis. It is irreversible, it is allosterically regulated by multiple molecules, and it controls the overall FLUX through the pathway. Once F-1,6-BP is made, the pathway is committed.

What are the ACTIVATORS and INHIBITORS of PFK-1?

INHIBITORS: High ATP (binds regulatory site, lowers affinity for F-6P), CITRATE (in liver, signals biosynthetic precursors are abundant), LOW pH.

ACTIVATORS: AMP (competes with ATP at regulatory site but does NOT inhibit), ADP, Fructose 2,6-bisphosphate (F-2,6-BP, especially in liver), F-1,6-BP (via pyruvate kinase feedforward).

Why does it make biological sense that HIGH ATP INHIBITS PFK-1, even though PFK-1 makes ATP? (Discussion Q)

HOMEOSTASIS. When ATP is already abundant, the cell does not need more. Inhibiting PFK-1 (the committed step) shuts down the entire glycolytic pathway, conserving glucose. The cell is maintaining EQUILIBRIUM — producing just enough ATP. This is allosteric feedback inhibition for energy balance.

How does ATP allosterically inhibit PFK-1?

ATP binds to a REGULATORY SITE (distinct from the catalytic site). This lowers the enzyme's AFFINITY for its substrate, fructose 6-phosphate (shifts the sigmoidal kinetic curve to the right).

How does AMP reverse ATP inhibition of PFK-1?

AMP COMPETES with ATP for the same regulatory binding site. When AMP is bound instead of ATP, the enzyme is NOT inhibited — it remains active. AMP itself does not inhibit.

Step 4 of glycolysis — what happens, enzyme, products, reversible?

Fructose 1,6-bisphosphate → Glyceraldehyde 3-phosphate (GAP) + Dihydroxyacetone phosphate (DHAP).

Enzyme: ALDOLASE.

READILY REVERSIBLE. (Important for gluconeogenesis going backward.)

What happens to DHAP after aldolase cleaves F-1,6-BP?

DHAP is isomerized to GAP by TRIOSE PHOSPHATE ISOMERASE (TPI/TIM). At equilibrium, 96% is DHAP, but GAP is continuously removed by the next step, driving the reaction forward.

Step 5 of glycolysis — what happens, enzyme, reversible?

Dihydroxyacetone phosphate (DHAP) → Glyceraldehyde 3-phosphate (GAP).

Enzyme: TRIOSE PHOSPHATE ISOMERASE (TPI, also called TIM).

RAPID and REVERSIBLE.

At equilibrium: 96% DHAP, 4% GAP. But GAP is pulled forward into Stage 2.

Why is triose phosphate isomerase important for glycolysis efficiency?

It converts DHAP (which cannot continue glycolysis) into GAP (which CAN). This means BOTH 3-carbon fragments from aldolase can enter Stage 2, allowing the cell to metabolize the ENTIRE glucose molecule (all 6 carbons).

Step 6 of glycolysis — what happens, enzyme, products, significance?

GAP → 1,3-Bisphosphoglycerate (1,3-BPG).

Enzyme: GAP DEHYDROGENASE (Glyceraldehyde 3-phosphate dehydrogenase).

This is an OXIDATION-REDUCTION reaction.

NAD⁺ is reduced to NADH. Inorganic phosphate (Pᵢ) is incorporated (NOT ATP).

1,3-BPG is an ACYL PHOSPHATE (mixed anhydride of phosphoric acid and a carboxylic acid) — it has a HIGH phosphoryl-transfer potential (greater than ATP).

What makes Step 6 (GAP → 1,3-BPG) so important conceptually?

It illustrates the ESSENCE of energy metabolism: the energy of CARBON OXIDATION is captured as HIGH PHOSPHORYL-TRANSFER POTENTIAL. The reaction is the sum of:

1. Oxidation of the aldehyde (GAP) to a carboxylic acid by NAD⁺ (ΔG°' ≈ −50 kJ/mol, VERY favorable).

2. Joining of carboxylic acid + orthophosphate to form acyl phosphate (ΔG°' ≈ +50 kJ/mol, VERY unfavorable).

These are COUPLED via a THIOESTER INTERMEDIATE, which preserves the free energy from oxidation in the high-energy acyl phosphate bond.

What is the thioester intermediate in Step 6 and why is it important?

During the GAP dehydrogenase reaction, a THIOESTER intermediate forms between the substrate and a cysteine residue on the enzyme. This intermediate PRESERVES the free energy from the aldehyde oxidation, preventing it from being lost as heat. The energy is then used to drive formation of the acyl phosphate product (1,3-BPG).

Which step of glycolysis produces NADH? (Discussion Q)

Step 6 — catalyzed by GAP DEHYDROGENASE. NAD⁺ is reduced to NADH as GAP is oxidized.

Step 7 of glycolysis — what happens, enzyme, products?

1,3-Bisphosphoglycerate (1,3-BPG) → 3-Phosphoglycerate (3-PG) + ATP.

Enzyme: PHOSPHOGLYCERATE KINASE.

This is SUBSTRATE-LEVEL PHOSPHORYLATION — ATP is formed by direct transfer of a phosphoryl group from a high-energy substrate (1,3-BPG) to ADP.

REVERSIBLE (important: this is also a step in gluconeogenesis).

What is substrate-level phosphorylation?

The formation of ATP by direct transfer of a phosphoryl group from a HIGH-ENERGY SUBSTRATE to ADP, without involving the electron transport chain or ATP synthase. Occurs in glycolysis at Steps 7 and 10.

Step 8 of glycolysis — what happens, enzyme?

3-Phosphoglycerate → 2-Phosphoglycerate.

Enzyme: PHOSPHOGLYCERATE MUTASE.

A REARRANGEMENT (mutase moves the phosphoryl group from carbon 3 to carbon 2). Reversible.

Step 9 of glycolysis — what happens, enzyme, significance?

2-Phosphoglycerate → Phosphoenolpyruvate (PEP) + H₂O.

Enzyme: ENOLASE.

This is a DEHYDRATION — a double bond is introduced, creating an ENOL PHOSPHATE.

PEP has an EXTREMELY HIGH phosphoryl-transfer potential (higher than ATP).

Reversible.

Step 10 of glycolysis — what happens, enzyme, products, reversible?

Phosphoenolpyruvate (PEP) → Pyruvate + ATP.

Enzyme: PYRUVATE KINASE.

Second substrate-level phosphorylation.

IRREVERSIBLE under cellular conditions (very large negative ΔG).

How is pyruvate kinase regulated in MUSCLE?

INHIBITED by high ATP (energy charge is high, no need for more glycolysis).

ACTIVATED by FRUCTOSE 1,6-BISPHOSPHATE (F-1,6-BP) — this is FEEDFORWARD STIMULATION: when upstream glycolysis increases, F-1,6-BP builds up and pre-activates pyruvate kinase to handle the oncoming flux.

How is pyruvate kinase regulated in LIVER?

The L isozyme in liver is inhibited by ALANINE (signals amino acid/building block availability) and by REVERSIBLE PHOSPHORYLATION triggered by GLUCAGON (via cAMP cascade, when blood glucose is low). This prevents the liver from consuming glucose that the brain and muscles need.

What is feedforward stimulation/activation?

A regulatory mechanism where an UPSTREAM metabolite ACTIVATES a DOWNSTREAM enzyme to prepare for increased flux. Example: F-1,6-BP activates pyruvate kinase before PEP even arrives.

What are the three possible fates of pyruvate produced by glycolysis?

1. ETHANOL fermentation (anaerobic, in yeast/bacteria).

2. LACTATE fermentation (anaerobic, in muscle/bacteria).

3. Complete OXIDATION to CO₂ and H₂O via the CITRIC ACID CYCLE + ELECTRON TRANSPORT CHAIN (aerobic, in mitochondria, much more ATP).

What is fermentation (definition)?

ATP-generating processes in which ORGANIC COMPOUNDS act as BOTH donors AND acceptors of electrons. They are REDOX-NEUTRAL pathways — no net oxidation or reduction overall. They do NOT require oxygen.

Why is NAD⁺ regeneration a BOTTLENECK in glycolysis?

NAD⁺ is present in LIMITED amounts (derived from vitamin NIACIN) and is consumed at Step 6 (GAP dehydrogenase). If NAD⁺ is not regenerated, Step 6 stops, and ALL of glycolysis halts because GAP cannot be oxidized. The entire energy-generating Stage 2 depends on having NAD⁺ available.

How does LACTATE FERMENTATION regenerate NAD⁺?

Pyruvate is REDUCED to LACTATE by NADH, in a reaction catalyzed by LACTATE DEHYDROGENASE. This oxidizes NADH back to NAD⁺, which can then re-enter glycolysis at Step 6.

Net: Glucose + 2 Pᵢ + 2 ADP → 2 lactate + 2 ATP + 2 H₂O

How does ETHANOL FERMENTATION regenerate NAD⁺?

TWO STEPS:

1. Pyruvate → Acetaldehyde + CO₂ (catalyzed by PYRUVATE DECARBOXYLASE, requires coenzyme THIAMINE PYROPHOSPHATE/TPP).

2. Acetaldehyde + NADH → Ethanol (catalyzed by ALCOHOL DEHYDROGENASE) → regenerates NAD⁺.

Net: Glucose + 2 Pᵢ + 2 ADP + 2 H⁺ → 2 ethanol + 2 CO₂ + 2 ATP + 2 H₂O

Is glycolysis aerobic or anaerobic? Does it stop without oxygen?

Glycolysis itself is ANAEROBIC and does NOT require oxygen. However, it needs NAD⁺. Without oxygen, NAD⁺ must be regenerated by FERMENTATION (lactate or ethanol). Without both oxygen AND fermentation, NADH would accumulate, NAD⁺ would be depleted, and glycolysis would halt. So: glycolysis does not stop without oxygen — it stops only if NAD⁺ cannot be regenerated.

What is the difference between a facultative anaerobe and an obligate anaerobe?

FACULTATIVE ANAEROBE: can metabolize aerobically when O₂ is present and ferment when O₂ is absent. OBLIGATE ANAEROBE: CANNOT survive in the presence of O₂ at all.

Name three obligate anaerobes and the infections they cause.

1. Clostridium tetani → TETANUS (lockjaw).

2. Clostridium botulinum → BOTULISM (severe food poisoning).

3. Clostridium perfringens → GAS GANGRENE (gas produced distorts/destroys tissue).

(Also: Bartonella henselae → Cat scratch fever; Bacteroides fragilis → abdominal/pelvic/pulmonary infections)

What is Triose Phosphate Isomerase Deficiency (TPID) and what causes it?

A RARE inherited deficiency in the enzyme TRIOSE PHOSPHATE ISOMERASE (TPI/TIM), which normally converts DHAP → GAP at Step 5 of glycolysis. Without active TPI, DHAP ACCUMULATES in cells instead of being converted to GAP.

What are the clinical symptoms of TPID?

It is a MULTISYSTEM disorder presenting in EARLY CHILDHOOD:

- Congenital HEMOLYTIC ANEMIA.

- Progressive NEUROMUSCULAR disorder.

- CARDIOMYOPATHY (inflammation and damage to heart muscle).

- Can lead to DEATH in early childhood in severe cases.

What is the impact of TPID on ATP production?

Without TPI, DHAP cannot be converted to GAP. Since GAP is the direct substrate for Stage 2 of glycolysis, HALF of the carbons from glucose CANNOT be metabolized to yield ATP. Effectively, only ONE of the two 3-carbon units (the GAP from aldolase) enters Stage 2. ATP production is HALVED relative to healthy individuals (1 ATP net per glucose instead of 2, though other complications arise). Without adequate ATP, neuromuscular function is COMPROMISED.

How does DHAP accumulation lead to protein damage in TPID?

Accumulated DHAP is converted to METHYLGLYOXAL, a HIGHLY REACTIVE molecule that COVALENTLY BONDS to amino groups on proteins. This yields ADVANCED GLYCATION END PRODUCTS (AGE), which INHIBIT PROTEIN FUNCTION. Extensive loss of protein function contributes to the pathologies of TPID, including early death.

What else is AGE implicated in beyond TPID?

AGE (advanced glycation end products) is implicated in AGING, ARTERIOSCLEROSIS (thickening and hardening of artery walls), and DIABETES.

What is aerobic glycolysis (the Warburg effect)?

The phenomenon in which TUMOR CELLS metabolize glucose to LACTATE even in the PRESENCE OF OXYGEN. Rapidly growing cancer cells preferentially use glycolysis over oxidative phosphorylation, even when O₂ is available.

What selective advantages does aerobic glycolysis give tumors?

1. LACTIC ACID secretion ACIDIFIES the tumor environment → facilitates tumor INVASION.

2. Increased G-6P provides substrates for the PENTOSE PHOSPHATE PATHWAY → generates NADPH (biosynthetic reducing power for rapid cell growth).

3. As tumors grow faster than blood vessels, O₂ drops → aerobic glycolysis REDUCES DEPENDENCE on oxygen.

How are tumors detected using the Warburg effect?

Using a NON-METABOLIZABLE glucose analog, 2-¹⁸F-2-deoxy-D-glucose, detectable by a combination of PET (positron emission tomography) and CAT (computer-aided tomography). Tumors with HIGH glucose uptake appear bright, and this also monitors TREATMENT EFFECTIVENESS.

What is HIF-1, and how does it relate to cancer AND exercise?

HIF-1 (Hypoxia-Inducible Transcription Factor 1) is activated by LOW OXYGEN (hypoxia).

In CANCER: Rapid tumor growth causes hypoxia, activating HIF-1, which:

- Increases expression of most GLYCOLYTIC ENZYMES.

- Increases GLUT1 and GLUT3 transporters.

- Increases VEGF (vascular endothelial growth factor) → stimulates blood vessel growth (angiogenesis) to nourish the tumor.

In ANAEROBIC EXERCISE TRAINING: Also activates HIF-1, enhancing glycolytic capacity and stimulating blood vessel growth → improved athletic performance.

What is bevacizumab and how does it fight cancer?

A MONOCLONAL ANTIBODY that binds to VEGF and prevents ANGIOGENESIS (blood vessel growth) in tumors. Without new blood vessels, a tumor cannot grow and either DIES or remains harmlessly small. Approved for GLIOBLASTOMAS (fast-growing cancers of the CNS from glial cells).

What causes lactose intolerance?

A deficiency of the enzyme LACTASE, which normally cleaves the glycosidic bond in LACTOSE (milk sugar), releasing GLUCOSE and GALACTOSE. Without lactase, lactose cannot be absorbed.

What happens to undigested lactose in the colon?

Intestinal microorganisms FERMENT lactose to LACTIC ACID while generating METHANE (CH₄) and HYDROGEN GAS (H₂). Effects:

- GAS → gut distension and FLATULENCE.

- LACTATE draws water into intestine → DIARRHEA.

- Severe cases: HINDERS absorption of fats and proteins.

Is lactose intolerance abnormal?

No. A DECREASE in lactase is NORMAL during development in ALL mammals. In most adult humans, lactase activity declines to ~5–10% of birth levels after weaning. Lactose TOLERANCE in adults evolved INDEPENDENTLY at least FOUR TIMES in different human populations in the last 10,000 years (linked to dairy farming). Adults with active lactase produced ~20% more fertile offspring.

What is galactosemia?

A disruption of galactose metabolism. The most common form, CLASSIC GALACTOSEMIA, is an inherited deficiency in GALACTOSE 1-PHOSPHATE URIDYL TRANSFERASE

Q: What are the symptoms of classic galactosemia?

- Markedly ELEVATED blood galactose; galactose in URINE.

- Infants FAIL TO THRIVE.

- VOMITING or DIARRHEA after consuming milk.

- Enlarged LIVER and JAUNDICE (can progress to CIRRHOSIS).

- CATARACTS (clouding of the eye lens due to galactitol accumulation).

- LETHARGY and impaired MENTAL DEVELOPMENT.

How do cataracts form in galactosemia?

Without the transferase, galactose accumulates in the lens. ALDOSE REDUCTASE reduces galactose to GALACTITOL. Galactitol is POORLY METABOLIZED and ACCUMULATES. Water diffuses into the lens for OSMOTIC BALANCE → cataracts (pathological protein aggregation and clouding).

Why does excessive fructose consumption raise concerns about metabolic disease?

The fructose metabolism pathway (via FRUCTOKINASE + TRIOSE KINASE) BYPASSES the key regulatory step PFK-1. So GAP and DHAP are produced in an UNREGULATED fashion from fructose. Excess pyruvate → acetyl CoA → FATTY ACIDS → stored in ADIPOSE TISSUE (obesity) or LIVER (fatty liver). Also linked to INSULIN INSENSITIVITY.

What do recent meta-analyses say about fructose's role in metabolic disease?

Recent meta-analyses indicate the widely held belief in the SIGNIFICANT NEGATIVE EFFECTS OF FRUCTOSE SPECIFICALLY is NO LONGER STRONGLY SUPPORTED. Overall caloric/sugar overconsumption (from any source) remains the key issue. The TYPE of sugar is less important than the AMOUNT.

What are the THREE irreversible, highly regulated enzymes that serve as control points in glycolysis?

1. HEXOKINASE (Step 1).

2. PHOSPHOFRUCTOKINASE-1 / PFK-1 (Step 3) — the MOST IMPORTANT.

3. PYRUVATE KINASE (Step 10).

All three catalyze phosphoryl transfers and have very negative ΔG values.

What is the dual role of the glycolytic pathway?

1. Degrades glucose to GENERATE ATP.

2. Provides BUILDING BLOCKS for biosynthetic reactions.

What is the primary regulatory signal for glycolysis in SKELETAL MUSCLE?

The ATP/AMP RATIO. When ATP falls (energy is being used), glycolysis is STIMULATED. When ATP is high, glycolysis is INHIBITED.

How does adenylate kinase amplify signals about ATP levels?

ADP + ADP ⇌ ATP + AMP (catalyzed by ADENYLATE KINASE).

Small % changes in [ATP] cause LARGER % changes in [AMP]. This means AMP is a MORE SENSITIVE indicator of energy status than ATP itself. This magnification provides TIGHTER REGULATION of PFK-1 (AMP is an activator).

Why does pH inhibit PFK-1 in muscle during intense exercise?

During fast-twitch muscle anaerobic activity, LACTIC ACID accumulates, dropping pH. LOW pH AUGMENTS the inhibitory effect of ATP on PFK-1, further slowing glycolysis. This prevents futile cycling and protects against excessive acidification.

How does the liver's role in glucose metabolism differ from muscle?

Muscle uses glycolysis primarily to generate ATP for CONTRACTION. The liver maintains BLOOD GLUCOSE HOMEOSTASIS: it STORES glucose as GLYCOGEN when glucose is plentiful, RELEASES glucose when supplies are low, and uses glucose for BIOSYNTHESIS. Liver regulation is therefore more COMPLEX.

What is GLUCOKINASE (hexokinase IV) and how does it differ from hexokinase?

Glucokinase is an ISOZYME of hexokinase found in LIVER and PANCREATIC β CELLS.

Differences from hexokinase:

- KM for glucose is ~50-fold HIGHER (low affinity → only active when glucose is ABUNDANT).

- NOT inhibited by its product G-6P.

- Regulated by GLUCOKINASE REGULATORY PROTEIN (GKRP): when glucose is LOW, GKRP sequesters glucokinase in the NUCLEUS until glucose increases.

This ensures glucose is not wasted and gives the BRAIN and MUSCLES first access when glucose is limited.

What is Fructose 2,6-bisphosphate (F-2,6-BP) and what does it do?

F-2,6-BP is a SIGNAL MOLECULE and POTENT ACTIVATOR of PFK-1 in the LIVER. It is NOT a glycolytic intermediate — it is a regulatory molecule. When blood glucose is HIGH, fructose 6-phosphate is abundant, which accelerates synthesis of F-2,6-BP. F-2,6-BP activates PFK-1 by: (1) increasing its affinity for fructose 6-phosphate and (2) diminishing the inhibitory effect of ATP. This is another example of FEEDFORWARD STIMULATION.

How does CITRATE inhibit PFK-1 in the liver?

CITRATE is an intermediate of the CITRIC ACID CYCLE. High cytoplasmic citrate signals that BIOSYNTHETIC PRECURSORS are abundant → no need to degrade more glucose. Citrate ENHANCES the inhibitory effect of ATP on PFK-1, slowing glycolysis.

How does GLUCAGON regulate pyruvate kinase in the liver?

When blood glucose is LOW, GLUCAGON triggers a CYCLIC AMP CASCADE that leads to PHOSPHORYLATION of pyruvate kinase L isozyme. Phosphorylation DIMINISHES its activity, preventing the liver from consuming glucose it needs to send to the BRAIN and MUSCLES.

How is FRUCTOSE converted into a glycolytic intermediate (in the LIVER)?

1. Fructose → Fructose 1-phosphate (by FRUCTOKINASE, uses ATP).

2. Fructose 1-phosphate → Glyceraldehyde + DHAP (by FRUCTOSE 1-PHOSPHATE ALDOLASE).

3. Glyceraldehyde → GAP (by TRIOSE KINASE, uses ATP).

DHAP continues into Stage 2 of glycolysis.

In OTHER tissues: hexokinase phosphorylates fructose to FRUCTOSE 6-PHOSPHATE directly.

How is GALACTOSE converted to a glycolytic intermediate?

1. Galactose → Galactose 1-phosphate (by GALACTOKINASE, ATP consumed).

2. Galactose 1-phosphate + UDP-glucose → Glucose 1-phosphate + UDP-galactose (by GALACTOSE 1-PHOSPHATE URIDYL TRANSFERASE).

3. UDP-galactose → UDP-glucose (by UDP-GALACTOSE 4-EPIMERASE — epimerizes galactose to glucose; UDP-glucose is REGENERATED and NOT consumed overall).

4. Glucose 1-phosphate → Glucose 6-phosphate (by PHOSPHOGLUCOMUTASE).

What are UDP-monosaccharides and why are they important?

UDP-monosaccharides (e.g., UDP-glucose, UDP-galactose) are ACTIVATED INTERMEDIATES used as substrates for the synthesis of GLYCOSIDIC LINKAGES (bonds between monosaccharides in polysaccharides and glycoproteins). They are a key example of activated intermediates in biochemistry.

What is insulin and what triggers its secretion?

INSULIN is a polypeptide hormone secreted by PANCREATIC β CELLS in response to INCREASED BLOOD GLUCOSE concentration. Its function is to STIMULATE GLUCOSE UPTAKE by tissues.

How do β cells detect blood glucose levels?

Through a "glucose sensor" consisting of TWO components working together:

1. GLUT2 transporter (high KM ~15–20 mM) → glucose enters β cells only when blood glucose is ABUNDANT.

2. GLUCOKINASE (high KM ~50 mM for glucose) → only phosphorylates glucose when glucose is ABUNDANT, trapping it as G-6P.

Together, they ensure insulin is secreted ONLY when glucose is truly high.

Describe the full mechanism by which glucose triggers insulin secretion from β cells.

1. Glucose enters β cell via GLUT2 (only when glucose is abundant).

2. Glucokinase phosphorylates glucose → G-6P.

3. G-6P enters glycolysis → pyruvate → CITRIC ACID CYCLE + electron transport chain → generates large amounts of ATP.

4. Rising ATP/ADP ratio CLOSES ATP-sensitive K⁺ channels (which normally allow K⁺ to flow OUT of the cell).

5. K⁺ channel closing causes DEPOLARIZATION of the cell membrane.

6. Depolarization OPENS VOLTAGE-SENSITIVE Ca²⁺ channels → Ca²⁺ flows INTO the cell.

7. Ca²⁺ influx causes insulin-containing SECRETORY VESICLES to FUSE with the cell membrane.

8. INSULIN is released into the blood → stimulates glucose uptake by tissues.

What type of ATP-sensitive channel is involved in insulin secretion?

An ATP-sensitive K⁺ CHANNEL. When ATP is LOW, the channel is OPEN (K⁺ flows out). When ATP rises (after glucose metabolism), the channel CLOSES, causing membrane depolarization and subsequent Ca²⁺ influx and insulin release.

List all 10 steps of glycolysis with enzyme, product, and ATP/NADH changes.

STAGE 1 (Energy Investment):

Step 1: Glucose → G-6P | HEXOKINASE | −1 ATP | IRREVERSIBLE

Step 2: G-6P → F-6P | PHOSPHOGLUCOSE ISOMERASE | no change

Step 3: F-6P → F-1,6-BP | PHOSPHOFRUCTOKINASE-1 | −1 ATP | IRREVERSIBLE

Step 4: F-1,6-BP → GAP + DHAP | ALDOLASE | no change

Step 5: DHAP → GAP | TRIOSE PHOSPHATE ISOMERASE | no change

(Net: −2 ATP invested; 2 GAP molecules now enter Stage 2)

STAGE 2 (Energy Harvest — per GAP, so × 2):

Step 6: GAP → 1,3-BPG | GAP DEHYDROGENASE | +2 NADH total | NAD⁺ consumed

Step 7: 1,3-BPG → 3-PG | PHOSPHOGLYCERATE KINASE | +2 ATP total (substrate-level)

Step 8: 3-PG → 2-PG | PHOSPHOGLYCERATE MUTASE | no change

Step 9: 2-PG → PEP | ENOLASE | no change (−H₂O)

Step 10: PEP → Pyruvate | PYRUVATE KINASE | +2 ATP total (substrate-level) | IRREVERSIBLE

NET PER GLUCOSE: 2 ATP, 2 NADH, 2 PYRUVATE.

Which TWO steps of glycolysis require ATP investment?

Step 1: Glucose + ATP → G-6P (by HEXOKINASE). Step 3: F-6P + ATP → F-1,6-BP (by PFK-1). You must invest 2 ATP to "prime" the glucose so it can be cleaved and metabolized into 4 ATP in Stage 2. Without this investment, the activation energy cannot be overcome. Net gain = 4 produced − 2 invested = 2 ATP.

At which steps is substrate-level phosphorylation occurring in glycolysis?

Step 7 (PHOSPHOGLYCERATE KINASE: 1,3-BPG → 3-PG + ATP) and Step 10 (PYRUVATE KINASE: PEP → Pyruvate + ATP). Each occurs TWICE per glucose (once per 3-carbon unit). Total: 4 ATP produced.

True or false: Glucose is the only monosaccharide that can be funneled into the glycolytic pathway.

False

What is the definition of glycolysis?

The conversion of one molecule of glucose to 2 molecules of pyruvate and two molecules of ATP

What is gluconeogenesis?

The synthesis of glucose from NONCARBOHYDRATE precursors. It essentially runs in the opposite direction of glycolysis but is NOT a simple reversal. It occurs primarily in the LIVER, with smaller amounts in the KIDNEY.

Why is gluconeogenesis vital?

The BRAIN depends on glucose as its PRIMARY fuel, and RED BLOOD CELLS use glucose as their ONLY fuel. During fasting or starvation, gluconeogenesis ensures these tissues have enough glucose to survive.

Where does gluconeogenesis take place?

Primarily in the LIVER. Small amounts in the KIDNEY and other tissues. Little to none in the BRAIN, SKELETAL MUSCLE, or HEART MUSCLE.

What does gluconeogenesis produce?

It converts pyruvate into GLUCOSE 6-PHOSPHATE or FREE GLUCOSE (depending on the tissue). In most tissues, it ends at G-6P, which is converted to glycogen or used for biosynthesis. In the liver, G-6P is hydrolyzed to free GLUCOSE and released into the blood.