MCAT Pathways

Glycolysis purpose

glucose breakdown for ATP/NADH

Glycolysis location

cytosol

Glycolysis rate-limiting enzyme

PFK-1

Glycolysis inputs → outputs

glucose → 2 pyruvate + 2 ATP + 2 NADH

Glycolysis irreversible enzymes

hexokinase; PFK-1; pyruvate kinase

Glycolysis hormonal regulation

insulin ↑; glucagon ↓

Glycolysis connections

PDH → TCA; lactate fermentation; gluconeogenesis

Glycolysis intermediates

glucose → G6P → F6P → F1,6BP → DHAP/G3P → 1,3-BPG → 3-PG → 2-PG → PEP → pyruvate

Glycolysis enzymes

hexokinase; phosphoglucose isomerase; PFK-1; aldolase; triose phosphate isomerase; G3P dehydrogenase; phosphoglycerate kinase; phosphoglycerate mutase; enolase; pyruvate kinase

Glycolysis products mnemonic

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Glycolysis enzymes mnemonic

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PDH purpose

pyruvate → acetyl-CoA

PDH location

mitochondrial matrix

PDH rate-limiting enzyme

PDH complex

PDH inputs → outputs

pyruvate + CoA + NAD⁺ → acetyl-CoA + NADH + CO₂

PDH regulation

inhibited by ATP, NADH, acetyl-CoA; activated by ADP, pyruvate

PDH connections

glycolysis → TCA

PDH enzymes

E1 pyruvate dehydrogenase; E2 dihydrolipoyl transacetylase; E3 dihydrolipoyl dehydrogenase

PDH cofactors

TPP (B1); lipoic acid; CoA (B5); FAD (B2); NAD⁺ (B3)

PDH cofactor mnemonic

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TCA purpose

generate NADH/FADH₂/GTP

TCA location

mitochondrial matrix

TCA rate-limiting enzyme

isocitrate dehydrogenase

TCA inputs → outputs

acetyl-CoA → CO₂ + NADH + FADH₂ + GTP

TCA irreversible enzymes

citrate synthase; isocitrate dehydrogenase; α-KG dehydrogenase

TCA regulation

ATP/NADH inhibit; ADP stimulates

TCA connections

ETC; amino acid metabolism; β-oxidation

TCA intermediates

acetyl-CoA + oxaloacetate → citrate → isocitrate → α-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate

TCA enzymes

citrate synthase; aconitase; isocitrate dehydrogenase; α-KG dehydrogenase; succinyl-CoA synthetase; succinate dehydrogenase; fumarase; malate dehydrogenase

TCCA intermediates mnemonic

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ETC purpose

ATP production via oxidative phosphorylation

ETC location

inner mitochondrial membrane

ETC inputs → outputs

NADH/FADH₂ + O₂ → ATP + H₂O

ETC regulation

oxygen availability; ADP levels

ETC connections

TCA; β-oxidation; glycolysis

ETC flow

NADH/FADH₂ → Complex I/II → CoQ → Complex III → cytochrome c → Complex IV → O₂

ETC complexes

Complex I = NADH dehydrogenase; Complex II = succinate dehydrogenase; Complex III = cytochrome bc1; Complex IV = cytochrome c oxidase; Complex V = ATP synthase

ETC mnemonic

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Gluconeogenesis purpose

glucose synthesis during fasting

Gluconeogenesis location

liver (mostly); cytosol + mitochondria

Gluconeogenesis rate-limiting enzyme

fructose-1,6-bisphosphatase

Gluconeogenesis inputs → outputs

pyruvate/lactate/glycerol → glucose

Gluconeogenesis regulation

glucagon ↑; insulin ↓

Gluconeogenesis connections

glycolysis; Cori cycle

Gluconeogenesis bypass steps

pyruvate → oxaloacetate → PEP → … → F1,6BP → F6P → G6P → glucose

Gluconeogenesis enzymes

pyruvate carboxylase; PEPCK; fructose-1,6-bisphosphatase; glucose-6-phosphatase

Glycogenesis purpose

glycogen storage

Glycogenesis location

cytosol (liver + muscle)

Glycogenesis rate-limiting enzyme

glycogen synthase

Glycogenesis inputs → outputs

glucose → glycogen

Glycogenesis regulation

insulin ↑; glucagon ↓

Glycogenesis connections

glycogenolysis; glycolysis

Glycogenesis intermediates

glucose → G6P → G1P → UDP-glucose → glycogen

Glycogenesis enzymes

hexokinase; phosphoglucomutase; UDP-glucose pyrophosphorylase; glycogen synthase; branching enzyme

Glycogenolysis purpose

glycogen breakdown

Glycogenolysis location

cytosol

Glycogenolysis rate-limiting enzyme

glycogen phosphorylase

Glycogenolysis inputs → outputs

glycogen → G1P

Glycogenolysis regulation

glucagon ↑; epinephrine ↑; insulin ↓

Glycogenolysis connections

glycolysis

Glycogenolysis intermediates

glycogen → G1P → G6P

Glycogenolysis enzymes

glycogen phosphorylase; debranching enzyme; phosphoglucomutase; glucose-6-phosphatase (liver)

PPP purpose

NADPH production; ribose-5-phosphate synthesis

PPP location

cytosol

PPP rate-limiting enzyme

G6PD

PPP inputs → outputs

G6P → NADPH + ribose-5-phosphate

PPP regulation

NADP⁺ activates

PPP connections

fatty acid synthesis; nucleotide synthesis

PPP intermediates

G6P → 6-phosphogluconolactone → 6-phosphogluconate → ribulose-5-phosphate

PPP enzymes

G6PD; lactonase; 6-phosphogluconate dehydrogenase; transketolase; transaldolase

β-oxidation purpose

fatty acid breakdown for ATP

β-oxidation location

mitochondrial matrix

β-oxidation rate-limiting step

carnitine shuttle

β-oxidation inputs → outputs

fatty acids → acetyl-CoA + NADH + FADH₂

β-oxidation regulation

glucagon ↑; insulin ↓

β-oxidation connections

TCA; ketogenesis; ETC

β-oxidation intermediates

fatty acyl-CoA → trans-Δ²-enoyl-CoA → L-3-hydroxyacyl-CoA → 3-ketoacyl-CoA → acetyl-CoA

β-oxidation enzymes

acyl-CoA dehydrogenase; enoyl-CoA hydratase; hydroxyacyl-CoA dehydrogenase; thiolase

β-oxidation transport enzymes

carnitine acyltransferase I; carnitine acyltransferase II

Fatty acid synthesis purpose

fatty acid production

Fatty acid synthesis location

cytosol

Fatty acid synthesis rate-limiting enzyme

acetyl-CoA carboxylase

Fatty acid synthesis inputs → outputs

acetyl-CoA → palmitate

Fatty acid synthesis regulation

insulin ↑; glucagon ↓

Fatty acid synthesis connections

PPP (NADPH source)

Fatty acid synthesis intermediates

acetyl-CoA → malonyl-CoA → palmitate

Fatty acid synthesis enzymes

acetyl-CoA carboxylase; fatty acid synthase

Ketogenesis purpose

ketone body production during fasting

Ketogenesis location

liver mitochondria

Ketogenesis rate-limiting enzyme

HMG-CoA synthase

Ketogenesis inputs → outputs

acetyl-CoA → ketone bodies

Ketogenesis regulation

fasting/glucagon ↑

Ketogenesis connections

β-oxidation; TCA

Ketogenesis intermediates

acetyl-CoA → acetoacetyl-CoA → HMG-CoA → acetoacetate → β-hydroxybutyrate/acetone

Ketogenesis enzymes

thiolase; HMG-CoA synthase; HMG-CoA lyase

Urea cycle purpose

ammonia detoxification

Urea cycle location

mitochondria + cytosol

Urea cycle rate-limiting enzyme

CPS I

Urea cycle inputs → outputs

ammonia → urea

Urea cycle regulation

activated during amino acid breakdown