Chapter 5: Metabolism—Enzymes, Reactions, Regulation, and Core Pathways
Enzymes, Regulation, and Central Metabolic Pathways
Enzyme identification
Enzymes typically have names ending in “-ase” which signals they are enzymes.
Some enzymes are 100% protein; others are apoenzymes (protein portion) plus a non-protein portion called a cofactor.
If the cofactor is inorganic (e.g., Ca^{2+}, Fe^{2+}), the apoenzyme plus the inorganic cofactor forms a holoenzyme.
If the cofactor is organic, the enzyme is called a coenzyme.
Enzyme structure and active site
Enzymes have sites on the enzyme, with the major one being the active site where the reaction occurs.
Enzymes are highly specific: they work with particular substrates and act as catalysts to lower the activation energy of reactions.
Activation energy: the energy required for a reaction to proceed; enzymes lower this energy, increasing reaction rate.
Enzyme classifications by location and role
Endoenzymes: manufactured within the cell and function inside the cell (e.g., many metabolic enzymes).
Exoenzymes: manufactured within the cell but secreted outside the cell (e.g., defense enzymes).
Constitutive vs induced enzymes
Constitutive enzymes are constantly produced in the cell.
Induced enzymes are produced only when needed.
Example: many bacteria rely on glucose; glucose metabolism enzymes are constitutive. If glucose is depleted and lactose is available, lactose-metabolizing enzymes are induced and then shut off when glucose becomes available again, showing regulation by nutrient availability.
Major reaction types in metabolism
Catabolic reactions (decomposition): larger substrate broken down into smaller products.
Anabolic reactions (synthesis): smaller subunits joined to form macromolecules.
Redox chemistry in metabolism
Oxidation is the loss of electrons; reduction is the gain of electrons.
Redox reactions occur together (redox couple) in metabolic pathways.
Hydrogen ions (protons) often shuttle electrons during redox reactions.
Coenzyme NAD^+ (often written NAD^+) plays a major role as an organic cofactor in redox reactions, becoming NADH when reduced.
NADH is a key carrier of electrons in many steps of metabolism.
Example coenzyme: nicotinamide is part of NAD; NAD is a coenzyme (organic cofactor).
Turnover number (k_{cat})
Enzyme turnover number is the maximum number of substrate-to-product conversions per second.
Typical range: about k_{cat} ext{ ~} 10^{3} ext{ to } 10^{4} ext{ s}^{-1}, though it can vary widely by enzyme.
Factors affecting enzyme activity
Temperature: high temperatures can denature enzymes by disrupting hydrogen bonds, altering shape and function.
pH: shifts in pH can denature enzymes, affecting their structure and activity.
Substrate concentration: at some point, all enzyme active sites are occupied (saturation); adding more substrate does not increase turnover.
Enzyme regulation and inhibition
Competitive inhibition: inhibitor binds to the active site, blocking substrate binding. Often reversible; once product level falls and substrate consumption occurs, inhibitor can dissociate and normal activity resumes.
Noncompetitive inhibition: inhibitor binds to a separate allosteric site, not the active site. This binding can cause a conformational change that alters the active site, often irreversibly, reducing or eliminating catalytic activity.
Allosteric site: a regulatory site separate from the active site used in noncompetitive inhibition.
Feedback inhibition: accumulation of product feeds back to inhibit earlier steps to prevent overproduction; activity is downregulated when product is abundant.
Ribozymes: RNA-based catalysts
Not proteins; ribozymes are RNA enzymes and will be discussed in the context of protein synthesis and RNA function later in the course.
Metabolism overview: respiration and its environments
Metabolism (often referred to as respiration) occurs in aerobic (with oxygen) and anaerobic (without oxygen) environments.
Terminal electron acceptor
Aerobic metabolism: terminal electron acceptor is oxygen (O_2).
Anaerobic metabolism: terminal electron acceptors include substances like iron (Fe^{3+}) or sulfur-containing compounds; oxygen is not used in these cases.
The energy currency: ATP (adenosine triphosphate) is produced to drive cellular processes.
ATP hydrolysis and energy release
The terminal phosphate group of ATP is hydrolyzed to release energy for cellular work.
Reaction: ext{ATP}
ightarrow ext{ADP} + ext{P}_i
Phosphorylation types and their contexts
Oxidative phosphorylation: takes place during oxidative energy production (in both aerobic and anaerobic conditions); efficient and tied to electron transport chain and chemiosmosis.
Photophosphorylation: uses light as the energy source to drive ATP synthesis; efficient and can occur in both aerobic and anaerobic conditions.
Substrate-level phosphorylation (fermentation): direct transfer of a phosphate group to ADP from a substrate; occurs primarily in anaerobic conditions but can occur with oxygen present. Less efficient than oxidative or photophosphorylation.
Central metabolic pathways to produce ATP
Focus on four key stages: glycolysis, the Krebs cycle, the electron transport chain (ETC), and chemiosmosis.
Overall goal: convert glucose and other nutrients into ATP to power cellular activities.
Note on context: active transport and other cellular processes require ATP to initiate metabolism.
Glycolysis (cytoplasm; universal starter step)
Location: cytoplasm in both prokaryotes and eukaryotes.
Substrate: glucose, a six-carbon molecule.
Primary products: two three-carbon molecules called pyruvate; ATP generation is not a major feature of glycolysis itself (it’s more of a preparatory step).
Summary: glucose (6C) → 2 pyruvate (3C each) via a sequence of ten enzymatic steps.
Note: the intermediate products are not required to be memorized for this course; focus is on the big picture, where glycolysis fits in and its role as the starting point for further metabolism.
Pyruvate oxidation and the Krebs cycle (mitochondrial involvement in eukaryotes; cytoplasmic in prokaryotes)
Prep step before Krebs cycle
Pyruvate decarboxylation (removal of CO_2) converts pyruvate into acetyl (to form acetyl-CoA for the cycle).
Reaction (pyruvate dehydrogenase step):
ext{Pyruvate} + ext{NAD}^+ + ext{CoA}
ightarrow ext{Acetyl{-}CoA} + ext{CO}_2 + ext{NADH} + ext{H}^+NAD^+ is reduced to NADH in this preparatory step.
Coenzyme A (CoA) acts as a shuttle to bring the acetyl group into the Krebs cycle; acetyl combines with oxaloacetate to form citrate.
Krebs cycle as a cyclic series of reactions
Entry: acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C):
ext{Oxaloacetate} (4 ext{C}) + ext{Acetyl{-}CoA} (2 ext{C})
ightarrow ext{Citrate} (6 ext{C}) + ext{CoA}Key outcomes during the cycle: multiple decarboxylation steps release CO2; multiple redox steps reduce NAD^+ to NADH and FAD to FADH2; a GDP (or ADP) phosphorylation step yields GTP (which can be converted to ATP).
Important coenzymes involved: NAD^+ → NADH, FAD → FADH_2.
For each glucose molecule, the Krebs cycle runs twice (one for each pyruvate derived from the original glucose).
Recycling of CoA and entry into the next stage
Coenzyme A is recycled and reused in subsequent turns of the cycle.
Electron Transport Chain (ETC) and chemiosmosis
Location of ETC
Prokaryotes: along the cytoplasmic (or inner) membrane and related structures.
Eukaryotes: along the inner mitochondrial membrane (mitochondrial cristae).
Purpose: a series of redox reactions that transfer electrons from reduced carriers (NADH, FADH_2) to terminal electron acceptors, releasing energy stored in the electrons.
Electron carriers in the chain
Flavoproteins (containing flavin, derived from vitamin B2, FMN/FAD-related)
Cytochromes (contain iron in heme groups)
Coenzyme Q (non-protein carrier, ubiquinone)
Final electron acceptor
Aerobic respiration: terminal receptor is O_2; reduction produces water.
Anaerobic respiration: terminal acceptors may be other molecules such as sulfur compounds or iron; end products vary.
Proton gradient and chemiosmosis
As electrons move through the chain, protons (H^+) are pumped across the membrane, creating a gradient (proton motive force).
In eukaryotes, proton gradient forms across the inner mitochondrial membrane (cristae).
In prokaryotes, a periplasmic space forms where protons accumulate across the cytoplasmic membrane.
Gradient drives ATP synthesis via ATP synthase, a transmembrane enzyme that phosphorylates ADP to ATP as protons re-enter the cell:
ext{ADP} + ext{P}_i
ightarrow ext{ATP}ATP yield from oxidative phosphorylation
Eukaryotes: about 36 ext{ ATP per glucose}.
Prokaryotes: about 38 ext{ ATP per glucose} (slightly higher efficiency in some bacteria due to lack of mitochondrial ATP cost).
Fermentation: an alternative pathway when respiration is limited or absent
Occurs with or without oxygen; does not use Krebs cycle or ETC.
Mechanism: substrate-level phosphorylation with an organic molecule acting as an electron acceptor.
Consequence: less efficient ATP production; needs to run more cycles to meet energy demands.
Common types (briefly mentioned): lactic acid fermentation and alcohol fermentation; the lesson notes that multiple types exist, but the focus is on the primary four steps (glycolysis, Krebs, ETC, chemiosmosis).
Practical takeaways and exam focus
The core metabolic sequence to understand: glycolysis → pyruvate oxidation → Krebs cycle → ETC → chemiosmosis.
Glycolysis occurs in the cytoplasm for both prokaryotes and eukaryotes and produces pyruvate without producing large amounts of ATP in that step itself.
The Krebs cycle operates in the cytoplasm for prokaryotes and in the mitochondrial matrix for eukaryotes, processing each acetyl-CoA to yield reduced carriers and a substrate-level phosphorylation event (GTP).
The ETC, coupled with chemiosmosis, is where most ATP is generated via the proton motive force driving ATP synthase activity.
Remember: enzyme activity is regulated to prevent wasteful overproduction; competitive and noncompetitive inhibition are two primary regulatory strategies.
The concept of redox balance and the role of NAD^+/NADH is central to moving electrons through metabolism and to coupling energy extraction to ATP generation.
Ribozymes introduce the idea that not all catalytic activity is protein-based; RNA molecules can also act as catalysts, with special relevance to later discussions of protein synthesis.
Quick reference equations from the content
NAD^+ reduction during glycolysis prep step (pyruvate oxidation):
ext{Pyruvate} + ext{NAD}^+ + ext{CoA}
ightarrow ext{Acetyl{-}CoA} + ext{CO}_2 + ext{NADH} + ext{H}^+Pyruvate decarboxylation and NADH formation (partial view):
ext{NAD}^+ + 2e^- + ext{H}^+
ightarrow ext{NADH}Oxaloacetate combines with acetyl-CoA to form citrate in the Krebs cycle:
ext{Oxaloacetate} (4C) + ext{Acetyl{-}CoA} (2C)
ightarrow ext{Citrate} (6C) + ext{CoA}Substrate-level phosphorylation example (GDP to GTP):
ext{GDP} + ext{P}_i
ightarrow ext{GTP}Overall ATP production per glucose (yield differences by organism):
Eukaryotes: ext{ATP}_{/glucose} ext{ (approx)} = 36
Prokaryotes: ext{ATP}_{/glucose} ext{ (approx)} = 38
ATP synthesis by ATP synthase (phosphorylation step):
ext{ADP} + ext{P}_i
ightarrow ext{ATP}
Important terms to recall for the exam
Apoenzyme, Cofactor, Holoenzyme, Coenzyme
Active site, Substrate specificity, Catalysis, Activation energy
Endoenzyme, Exoenzyme
Constitutive vs Induced enzymes
Competitive vs Noncompetitive inhibition; Allosteric site; Feedback inhibition
Ribozymes (RNA enzymes)
Oxidation, Reduction, Redox reactions, NAD^+/NADH, FAD/FADH_2
Substrate-level phosphorylation, Oxidative phosphorylation, Photophosphorylation
Glycolysis, Pyruvate oxidation, Krebs cycle, Electron Transport Chain, Chemiosmosis
Proton motive force, Proton gradient, ATP synthase
Terminal electron acceptor (O_2 in aerobic, others in anaerobic) and water production in aerobic respiration
Connections and context
The material ties enzyme structure and regulation to the flow of metabolism, showing how cells control energy production via feedback mechanisms and environmental nutrient availability.
The pathway progression provides a framework for understanding energy yield differences between aerobic and anaerobic conditions and how bacteria may adapt to different environments.
The concept of ribozymes hints at the broader scope of catalytic molecules beyond proteins, foreshadowing discussions on genetic information flow (RNA world ideas) and protein synthesis.
Practical considerations and real-world relevance
Temperature and pH sensitivity of enzymes underpin why fever and pH imbalances affect metabolism and organismal function.
In industrial and medical contexts, manipulating fermentation pathways or regulation can influence energy yield, metabolite production, and the behavior of microbes in various environments.
Lab and application note
In lab discussions, citrate utilization and cycle participation will be explored to illustrate the Krebs cycle steps and carbon flow through metabolism.
The lecture emphasizes understanding the big picture of where each pathway occurs (location in cell) and the major products, rather than memorizing every intermediate.
1. Enzymes and Their Regulation
1.1 Enzyme Identification and Structure
Naming Konvention: Enzymes typically end in “-ase.”
Composition:
100% Protein: Some enzymes are entirely protein.
Apoenzymes + Cofactor: Others consist of an apoenzyme (protein portion) and a non-protein cofactor.
Inorganic Cofactor: If the cofactor is inorganic (e.g., ext{Ca}^{2+}, ext{Fe}^{2+}), the complex is a holoenzyme.
Organic Cofactor: If the cofactor is organic, it's called a coenzyme (e.g., NAD).
Active Site: The region on the enzyme where the catalytic reaction takes place.
Specificity: Enzymes are highly specific, interacting with particular substrates.
Catalysis: Enzymes act as catalysts, lowering the activation energy of reactions, thereby increasing the reaction rate.
Activation Energy: The energy required for a chemical reaction to proceed.
1.2 Enzyme Classifications
By Location and Role:
Endoenzymes: Function inside the cell where they are manufactured (e.g., most metabolic enzymes).
Exoenzymes: Secreted outside the cell to function externally (e.g., defense enzymes).
By Production:
Constitutive Enzymes: Constantly produced in the cell (e.g., glucose metabolism enzymes).
Induced Enzymes: Produced only when needed, often in response to nutrient availability (e.g., lactose-metabolizing enzymes when glucose is depleted).
1.3 Factors Affecting Enzyme Activity
Temperature: High temperatures can denature enzymes by disrupting hydrogen bonds, altering their shape and function.
pH: Shifts in pH can also denature enzymes, affecting their structure and activity.
Substrate Concentration: Activity increases with substrate concentration until enzyme active sites become saturated; further substrate addition does not increase turnover.
1.4 Enzyme Regulation and Inhibition
Competitive Inhibition:
Inhibitor binds directly to the active site, blocking substrate access.
Often reversible.
Noncompetitive Inhibition:
Inhibitor binds to an allosteric site (separate from the active site).
Causes a conformational change that alters the active site, reducing or eliminating catalytic activity.
Can often be irreversible.
Allosteric Site: A regulatory site on the enzyme distinct from the active site.
Feedback Inhibition:
Accumulation of the final product inhibits an enzyme in an earlier step of the pathway.
Prevents overproduction and downregulates activity when the product is abundant.
1.5 Ribozymes
RNA-based Catalysts: Not proteins; ribozymes are RNA molecules with catalytic activity.
2. Major Reaction Types in Metabolism
2.1 Catabolic and Anabolic Reactions
Catabolic (Decomposition): Larger substrates are broken down into smaller products (energy-releasing).
Anabolic (Synthesis): Smaller subunits are joined to form macromolecules (energy-requiring).
2.2 Redox Chemistry
Oxidation: Loss of electrons.
Reduction: Gain of electrons.
Redox Reactions: Occur together (redox couple) in metabolic pathways.
Electron Shuttling: Hydrogen ions (protons) often shuttle electrons.
Coenzyme NAD^+:
Major organic cofactor in redox reactions.
Becomes NADH when reduced (gains electrons and a proton).
NADH is a key electron carrier.
Turnover Number (k_{ ext{cat}}): Maximum number of substrate-to-product conversions per second.
Typical range: k_{ ext{cat}} \sim 10^3 ext{ to } 10^4 ext{ s}^{-1}.
3. Metabolism Overview: Respiration and Energy Currency
3.1 Respiration Environments
Aerobic: Occurs with oxygen.
Anaerobic: Occurs without oxygen.
3.2 Terminal Electron Acceptor
Aerobic Metabolism: Oxygen ( ext{O}_2) is the terminal electron acceptor, producing water.
Anaerobic Metabolism: Other substances like iron ( ext{Fe}^{3+}) or sulfur-containing compounds act as terminal electron acceptors; oxygen is not used.
3.3 ATP: The Energy Currency
ATP (Adenosine Triphosphate): Produced to drive cellular processes.
ATP Hydrolysis: The terminal phosphate group is hydrolyzed to release energy for cellular work.
Reaction: ext{ATP} \rightarrow ext{ADP} + ext{P}_i
3.4 Phosphorylation Types
Oxidative Phosphorylation:
Takes place during oxidative energy production (aerobic and anaerobic conditions).
Efficient; tied to the electron transport chain and chemiosmosis.
Photophosphorylation:
Uses light as the energy source for ATP synthesis.
Efficient; can occur in both aerobic and anaerobic conditions.
Substrate-Level Phosphorylation (Fermentation):
Direct transfer of a phosphate group from a substrate to ADP.
Primarily in anaerobic conditions, but can occur with oxygen.
Less efficient than oxidative or photophosphorylation.
4. Central Metabolic Pathways to Produce ATP
4.1 Overall Goal
Convert glucose and other nutrients into ATP to power cellular activities.
4.2 Glycolysis (Universal Starter Step)
Location: Cytoplasm (in both prokaryotes and eukaryotes).
Substrate: Glucose (a six-carbon molecule).
Primary Products: Two pyruvate molecules (three carbons each).
ATP Generation: Not a major ATP-generating step itself; primarily preparatory.
Summary: Glucose (6C)
ightarrow 2 Pyruvate (3C each) via ten enzymatic steps.
4.3 Pyruvate Oxidation and the Krebs Cycle
Location:
Prokaryotes: Cytoplasm.
Eukaryotes: Mitochondrial matrix.
4.3.1 Pyruvate Oxidation (Prep Step):
Pyruvate undergoes decarboxylation (removal of { ext{CO}_2}) to form acetyl.
Acetyl combines with Coenzyme A (CoA) to form Acetyl-CoA.
Reaction (Pyruvate Dehydrogenase Step): ext{Pyruvate} + ext{NAD}^+ + ext{CoA} \rightarrow ext{Acetyl-CoA} + ext{CO}_2 + ext{NADH} + ext{H}^+
{ ext{NAD}^+} is reduced to NADH.
CoA acts as a shuttle, bringing the acetyl group into the Krebs cycle.
4.3.2 Krebs Cycle (Citric Acid Cycle):
Entry: Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).
Reaction: ext{Oxaloacetate} (4 ext{C}) + ext{Acetyl-CoA} (2 ext{C}) \rightarrow ext{Citrate} (6 ext{C}) + ext{CoA}
Key Outcomes Per Cycle:
Multiple decarboxylation steps release { ext{CO}_2}.
Multiple redox steps reduce { ext{NAD}^+} to NADH and FAD to ext{FADH}_2.
One substrate-level phosphorylation step yields GTP (which converts to ATP).
Per Glucose Molecule: The Krebs cycle runs twice (one for each pyruvate).
Recycling: Coenzyme A is recycled and reused.
4.4 Electron Transport Chain (ETC) and Chemiosmosis
Location:
Prokaryotes: Cytoplasmic (or inner) membrane.
Eukaryotes: Inner mitochondrial membrane (cristae).
Purpose: A series of redox reactions that transfer electrons from reduced carriers (NADH, ext{FADH}_2) to terminal electron acceptors, releasing stored energy.
Electron Carriers:
Flavoproteins (contain flavin, derived from vitamin { ext{B}_2}, FMN/FAD-related).
Cytochromes (contain iron in heme groups).
Coenzyme Q (ubiquinone, a non-protein carrier).
Final Electron Acceptor:
Aerobic Respiration: Oxygen ( ext{O}_2); reduction produces water.
Anaerobic Respiration: Molecules like sulfur compounds or iron; end products vary.
Proton Gradient and Chemiosmosis:
As electrons move through the ETC, protons ( ext{H}^+) are pumped across the membrane, creating a proton motive force (a gradient).
Eukaryotes: Gradient forms across the inner mitochondrial membrane.
Prokaryotes: Protons accumulate in the periplasmic space across the cytoplasmic membrane.
This gradient drives ATP synthesis via ATP synthase, a transmembrane enzyme that phosphorylates ADP to ATP as protons re-enter the cell.
Reaction: ext{ADP} + ext{P}_i \rightarrow ext{ATP}
ATP Yield (Oxidative Phosphorylation):
Eukaryotes: Approximately 36 ATP per glucose.
Prokaryotes: Approximately 38 ATP per glucose (due to lack of mitochondrial ATP cost).
5. Fermentation (Alternative Pathway)
Conditions: Occurs with or without oxygen; does not use the Krebs cycle or ETC.
Mechanism: Uses substrate-level phosphorylation with an organic molecule as the electron acceptor.
Efficiency: Less efficient ATP production; requires more cycles to meet energy demands.
Common Types: Lactic acid fermentation and alcohol fermentation.
6. Key Takeaways and Exam Focus
Core Metabolic Sequence: Glycolysis
ightarrow Pyruvate Oxidation
ightarrow Krebs Cycle
ightarrow ETC
ightarrow Chemiosmosis.Glycolysis: Cytoplasmic; produces pyruvate; not a major ATP-generating step itself.
Krebs Cycle: Cytoplasmic (prokaryotes) / Mitochondrial matrix (eukaryotes); processes acetyl-CoA to yield reduced carriers (NADH, ext{FADH}_2) and one GTP/ATP.
ETC & Chemiosmosis: Most ATP is generated here via the proton motive force driving ATP synthase.
Enzyme Regulation: Prevent wasteful overproduction; competitive and noncompetitive inhibition are primary strategies.
Redox Balance: Central role of { ext{NAD}^+}/NADH in electron transfer and energy coupling.
Ribozymes: RNA molecules can also act as catalysts.
7. Quick Reference Equations
Pyruvate Oxidation: ext{Pyruvate} + ext{NAD}^+ + ext{CoA} \rightarrow ext{Acetyl-CoA} + ext{CO}_2 + ext{NADH} + ext{H}^+
NAD{ ext{^+}} Reduction: ext{NAD}^+ + 2e^- + ext{H}^+ \rightarrow ext{NADH}
Krebs Cycle Entry: ext{Oxaloacetate} (4 ext{C}) + ext{Acetyl-CoA} (2 ext{C}) \rightarrow ext{Citrate} (6 ext{C}) + ext{CoA}
Substrate-Level Phosphorylation (GTP): ext{GDP} + ext{P}_i \rightarrow ext{GTP}
Overall ATP Yield (per glucose):
Eukaryotes: ext{ATP}_{ ext{/glucose}} ext{ (approx)} = 36
Prokaryotes: ext{ATP}_{ ext{/glucose}} ext{ (approx)} = 38
ATP Synthesis (ATP Synthase): ext{ADP} + ext{P}_i \rightarrow ext{ATP}
8. Important Terms for the Exam
Apoenzyme, Cofactor, Holoenzyme, Coenzyme
Active site, Substrate specificity, Catalysis, Activation energy
Endoenzyme, Exoenzyme
Constitutive vs Induced enzymes
Competitive vs Noncompetitive inhibition; Allosteric site; Feedback inhibition
Ribozymes (RNA enzymes)
Oxidation, Reduction, Redox reactions, { ext{NAD}^+}/NADH, FAD/ ext{FADH}_2
Substrate-level phosphorylation, Oxidative phosphorylation, Photophosphorylation
Glycolysis, Pyruvate oxidation, Krebs cycle, Electron Transport Chain, Chemiosmosis
Proton motive force, Proton gradient, ATP synthase
Terminal electron acceptor ( ext{O}_2 in aerobic, others in anaerobic) and water production in aerobic respiration.
9. Connections and Context
Enzyme structure and regulation are tied to the flow of metabolism and cellular energy control.
Pathway progression explains energy yield differences in aerobic vs. anaerobic conditions and microbial adaptation.
Ribozymes highlight RNA's catalytic role, key for understanding genetic information flow.
10. Practical Considerations
Enzyme sensitivity to temperature and pH explains why imbalances (e.g., fever) affect metabolism.
Metabolic pathways and regulation are manipulated in industrial and medical contexts (e.g., fermentation, microbial behavior).
11. Lab and Application Notes
Lab discussions will explore citrate utilization for the Krebs cycle and carbon flow.
Focus on the big picture: pathway locations and major products, not every intermediate.