Waste Disposal Lysosomes and Proteasomes

Slide 1 · Cellular Waste Disposal:

Lysosomes and Proteasomes

Intracellular catabolism, protein quality control, and cellular recycling

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

By the end of the session, explain:

How lysosomal acid hydrolases reach their destination

How endocytosis, phagocytosis, and autophagy deliver cargo

How ubiquitin targets proteins for proteasomal degradation

Why defects in intracellular disposal cause cellular dysfunction

How these mechanisms connect to selected diseases

Slide 3 · Cellular housekeeping is an active process

Cells must remove, remodel, and reuse material continuously.

Catabolism supports proteostasis, organelle renewal, and resource recovery.

Waste can originate outside or inside the cell

Distinct pathways handle particles, organelles, and proteins

Slide 4 · What needs to be degraded?

Substrate origin and physical scale help determine the disposal route.

Extracellular cargo

Internalized molecules, fluid and debris

Cellular structures

Aged or damaged organelles

Proteins

Denatured, misfolded and short-lived regulatory proteins

Slide 5 · Two complementary intracellular degradation systems

Lysosomal degradation

Membrane-delimited cargo; Endosomes, phagosomes, autophagosomes

Proteasomal degradation

Predominantly selected cytosolic proteins; Ubiquitin-dependent targeting

Hydrolytic enzymes

Acid hydrolases

Proteolytic core

Proteasome

Both systems support cellular quality control and nutrient recycling.

Slide 6 · Three routes converge on lysosomal digestion

Endocytosis

External molecules / fluid

Phagocytosis

Large particles / microbes

Autophagy

Internal organelles / complexes

Lysosome

Hydrolysis → breakdown products → salvage

Slide 7 · Lysosomes contain acid hydrolases

Approximately 40 types of hydrolases operate in the lysosomal lumen.

These enzymes break complex substrates into smaller recoverable products.

A membrane maintains a distinct digestive compartment

An acidic lumen optimizes enzyme activity

Slide 8 · The lysosome: An acidic digestive compartment

LYSOSOMAL LUMEN

Acidic environment (≈ pH 5)

Multiple classes of acid hydrolases

Membrane-enclosed catabolism

Products can be recycled by the cell

Slide 9 · Major families of lysosomal hydrolases

Different substrate chemistries require different hydrolytic activities.

Proteases

Digest proteins

Nucleases

Digest nucleic acids

Lipases

Digest lipid substrates

Glycosidases

Cleave carbohydrate bonds

Phosphatases

Remove phosphate groups

Sulfatases

Remove sulfate groups

Slide 10 · Why an acidic lumen matters

Lysosomal enzymes function optimally at approximately pH 5.

Endosomal compartments become progressively more acidic along the transport route.

Enzyme activation is coupled to organelle maturation

Compartmentalization helps localize digestive activity

Slide 11 · Lysosomal enzymes begin in the secretory pathway

Rough ER

Hydrolases synthesized in the ER lumen

Golgi

Enzymes undergo M6P modification

Trans-Golgi vesicles

M6P receptor-mediated sorting

Lysosome

Cargo delivered for digestion

Lysosomal hydrolases have to be synthesized and correctly trafficked before they can act.

Slide 12 · Mannose-6-phosphate is a sorting address

M6P marks newly synthesized lysosomal hydrolases.

This modification is placed on the enzymes in the Golgi apparatus.

The M6P tag provides a recognition signal

A defect in the address can disrupt delivery despite enzyme synthesis

Slide 13 · M6P receptors mediate lysosomal enzyme delivery

M6P-tagged enzyme

Hydrolase in Golgi

M6P receptor

Recognition and binding

Transport vesicle

Delivery from trans-Golgi

Acidic compartment

Cargo released; Receptor recycled

Correct protein sorting is as important as catalytic enzyme synthesis.

Slide 14 · Two different lysosomal failure mechanisms

Defect

Immediate event

Result

Catalytic enzyme defect

Hydrolase missing or inactive

Specific substrate cannot be digested

M6P tagging defect

Hydrolases misdirected

Many lysosomal enzymes fail to reach target

Slide 15 · Clinical principle: Sorting defects mimic enzyme deficiencies

M6P pathway failure can deprive lysosomes of multiple hydrolases.

Illustrative correlation: I-cell disease (mucolipidosis II).

Failure to create the recognition marker causes enzyme misdirection

The enzymes may be secreted instead of targeted to lysosomes

Slide 16 · Heterophagy begins with material outside the cell

The cell internalizes external cargo and transfers it to lysosomes.

Endocytosis and phagocytosis are complementary routes.

Endocytosis conveys soluble cargo and small vesicular contents

Phagocytosis handles large particles and microorganisms

Slide 17 · Endocytosis routes soluble material toward lysosomes

Plasma membrane

Fluid-phase or receptor-mediated uptake

Early endosome

Cargo enters endosomal pathway

Late endosome

Increased acidification

Lysosome

Hydrolytic digestion

The material has crossed the plasma membrane before reaching the lysosome.

Slide 18 · Fluid-phase versus receptor-mediated endocytosis

Fluid-phase uptake

Receptor-mediated uptake

Bulk sampling of surrounding fluid

Cell-surface receptor binds selected ligand

Internalization does not require a specific cargo receptor

Allows selective concentration of cargo

Material enters endosomal compartments

Material enters endosomal compartments

Slide 19 · Endosome maturation couples transport to acidification

Early endosome → late endosome → lysosome.

Proteolytic enzymes become increasingly active in later acidic compartments.

Transport and maturation are linked events

The lysosome is the final digestive destination

Slide 20 · Heterophagic cargo reaches an endolysosomal compartment

Internalize

Vesicle buds from plasma membrane

Traffic

Early to late endosomes

Fuse / mature

Interact with lysosomal system

Digest

Enzymes degrade cargo

The endolysosomal pathway coordinates uptake, sorting and degradation.

Slide 21 · Phagocytosis handles large particles

Professional phagocytes engulf microorganisms and bulky debris.

Macrophages and neutrophils are principal examples.

The engulfed material first resides in a phagosome

The phagosome subsequently interacts with lysosomes

Slide 22 · Phagocytosis ends in a phagolysosome

Engulfment

Particle internalized

Phagosome

Membrane-bound cargo

Fusion

Lysosome joins phagosome

Phagolysosome

Digestion of ingested material

Vesicle fusion exposes internalized organisms or debris to acid hydrolases.

Slide 23 · Phagocytic cells couple uptake to degradation

Macrophages

Neutrophils

Recognize and ingest debris and microorganisms

Engulf microorganisms

Contain a developed lysosomal system

Deliver phagosome contents to degradative compartments

Clear extracellular material

Support immediate cellular defense

Slide 24 · Endocytosis versus phagocytosis

Feature

Endocytosis

Phagocytosis

Typical cargo

Fluid or ligand-bound molecules

Large debris / microorganisms

Intermediate

Endosome

Phagosome

Terminal digestion

Lysosomal compartment

Phagolysosome

Typical cells

Many cell types

Macrophages; Neutrophils

Slide 25 · Autophagy is lysosome-directed self-digestion

Cytoplasmic structures are sequestered for subsequent lysosomal degradation.

Unlike heterophagy, the cargo initially resides inside the cell.

Aged or damaged organelles can be removed

Large cytosolic complexes can also be eliminated

Slide 26 · Signals associated with autophagy

The pathway serves both routine quality control and stress adaptation.

Nutrient depletion

Recycling helps support survival

Intracellular stress

Damaged structures require turnover

Infection-related stress

Autophagy contributes to protective responses

Slide 27 · Autophagy begins with an isolation membrane

A phagophore grows around selected cytosolic material.

The ER is an important membrane source description.

Other cellular membranes may contribute to phagophore formation

Sequestration precedes lysosomal fusion

Slide 28 · Atg proteins coordinate autophagosome formation

Initiation

Cell stress or turnover signal

Nucleation

Isolation membrane appears

Elongation

Atg-dependent expansion

Closure

Autophagosome is formed

Autophagy-related gene products organize the membrane remodeling steps.

Slide 29 · An autophagosome has a double membrane

The cargo becomes enclosed before lysosomal digestion begins.

The double-membrane vesicle is a defining morphologic stage.

Cytosolic components lie within the enclosed compartment

Fusion with a lysosome supplies degradative capacity

Slide 30 · Autophagy: Encapsulate, fuse, digest

Phagophore

Isolation membrane

Autophagosome

Double membrane; LC3

Autolysosome

Lysosomal digestion

Slide 31 · LC3 is associated with the autophagic membrane

Microtubule-associated protein light chain 3 (LC3) participates in autophagosome formation.

The textbook scheme depicts LC3 on the autophagic compartment.

LC3 is a mechanistic marker in the source figure

Its presence alone does not define the downstream endpoint of degradation

Slide 32 · Autophagosome maturation enables digestion

Autophagosome

Cytosolic cargo enclosed

Lysosome

Acid hydrolases available

Fusion

Autolysosomal compartment

Products

Metabolites for reuse

The autophagosome does not digest cargo efficiently in isolation.

Slide 33 · Autophagic degradation releases reusable metabolites

Lysosomal hydrolysis converts sequestered cargo into smaller molecules.

These can re-enter the cell’s metabolic pools.

Autophagy is therefore both degradative and salvage-oriented

Recycling becomes especially relevant during nutrient deprivation

Slide 34 · Starvation activates a nutrient-salvage response

Autophagy helps maintain cellular viability under nutrient stress.

Cells mobilize constituents that are no longer essential or functional.

The pathway supplies metabolites from internal sources

Survival benefit depends on the cellular context

Slide 35 · Autophagy supports intracellular quality control

Defunct organelles and bulky protein complexes require organized clearance.

This removal limits persistence of damaged cellular structures.

Turnover is part of ordinary maintenance

Impaired clearance can compound cellular injury

Slide 36 · Mitophagy targets dysfunctional mitochondria

Mitochondrial turnover is a specialized form of autophagy.

Damaged mitochondria are delivered to lysosomes for degradation.

Mitophagy is a mitochondrial quality-control pathway

Defective clearance can allow dysfunctional mitochondria to persist

Slide 37 · How mitophagy contributes to cellular health

Mitochondrion

Aged or dysfunctional organelle

Sequestration

Autophagic enclosure

Fusion

Lysosomal delivery

Renewal

Damaged component removed

Autophagy is not limited to protein digestion: Entire organelles can be recycled.

Slide 38 · Autophagy also contributes to host defense

Some intracellular infections activate the autophagic pathway.

Infection response as one of its protective functions.

Cargo is enclosed within an autophagic compartment

Digestion ultimately requires lysosomal activity

Slide 39 · Three lysosomal delivery routes: Final comparison

Route

Where cargo originates

Delivery intermediate

Endocytosis

Outside cell: Fluid or ligands

Endosome

Phagocytosis

Outside cell: Large particles

Phagosome

Autophagy

Inside cell: Organelles / complexes

Autophagosome

Slide 40 · The lysosome is both a disposal and a recovery site

Hydrolytic products can return to the cytosol for reuse.

Lysosomal digestion does not necessarily mean permanent nutrient loss.

Metabolite salvage supports cellular metabolism

Some material can exit through lysosomal exocytosis

Slide 41 · Lysosomal exocytosis provides an outward route

Lysosomal contents can be discharged into the extracellular space.

Both intracellular salvage and lysosomal exocytosis.

Exocytosis differs from degradation occurring inside a lysosome

Lysosomes participate in membrane dynamics as well as catabolism

Slide 42 · Choose the lysosomal route by cargo origin

Cargo characteristics determine the entrance pathway.

Fluid / ligand

Endocytosis

Microbe / debris

Phagocytosis

Damaged organelle

Autophagy

All three pathways converge on acid hydrolase–mediated digestion.

Slide 43 · Proteasomes degrade selected intracellular proteins

The proteasome is a cytosolic multisubunit proteolytic complex.

It handles misfolded proteins and proteins whose lifespan must be tightly controlled.

Protein turnover is selective rather than indiscriminate

Unlike autophagy, the canonical substrate is an individual protein

Slide 44 · What proteins enter proteasomal degradation?

The pathway connects protein quality control with precise regulation.

Denatured proteins

Altered by stress or chemical injury

Misfolded proteins

Failed to achieve a stable conformation

Regulatory proteins

Molecules whose concentrations control signaling

Slide 45 · A stressed protein can be rescued or degraded

Chaperone-assisted folding

Proteasomal disposal

Chaperones assist nascent proteins in achieving useful conformation

Persistent abnormal proteins are targeted for elimination

Successful folding permits continued protein function

Covalent ubiquitin labeling often precedes degradation

Quality control prevents unnecessary loss

Proteolysis avoids accumulation of defective proteins

Slide 46 · Sources of protein denaturation

The degradation machinery disposes of proteins that cannot remain functional.

Age-related damage

Senescence-associated protein alterations

Heat or UV

Extrinsic physical stress

Reactive oxygen species

Oxidative injury

Chemical stress

Destabilization of folded conformations

Slide 47 · Ubiquitin serves as a degradation signal

A small protein, ubiquitin, is covalently attached to selected substrates.

Many proteasomal targets are recognized through polyubiquitin chains.

Labeling is regulated by dedicated enzyme machinery

Substrate selection helps control protein lifespan

Slide 48 · Ubiquitin conjugation requires E1, E2 and E3

E1

Ubiquitin-activating enzyme

E2

Ubiquitin-conjugating enzyme

E3

Substrate-selective ligase

Substrate

Protein receives ubiquitin

A multistep enzyme system allows regulated protein targeting.

Slide 49 · E1 activates ubiquitin

The first stage prepares ubiquitin for transfer through the conjugation machinery.

E1 participates upstream of substrate-specific tagging.

Activation is an enabling step rather than the final degradation event

The proteasome acts after the targeting cascade

Slide 50 · E2 conveys activated ubiquitin

E2 conjugating enzymes operate between E1 and E3 in the tagging system.

The sequential organization underlies ubiquitin transfer to selected targets.

E2 contributes to ubiquitin transfer

Targeting specificity depends substantially on the associated E3 machinery

Slide 51 · E3 ligases help select the protein substrate

E3 ligase activity links ubiquitin to the protein chosen for removal.

This step connects cell state and signaling to selective destruction.

Different proteins can have regulated lifespans

Errors in targeting can alter steady-state protein abundance

Slide 52 · Polyubiquitination licenses proteasomal turnover

Protein

Selected client

E1–E2–E3

Ubiquitin-transfer system

Polyubiquitin

Degradation-marked substrate

Proteasome

Recognition and proteolysis

The ubiquitin tag acts upstream of protein unfolding and proteolysis.

Slide 53 · The proteasome: A proteolytic cylinder

Polyubiquitinated protein

Peptide fragments

Catalytic sites face the internal channel

Proteins are unfolded and threaded into the core

Slide 54 · Proteasomal substrates are unfolded before digestion

Polyubiquitinated proteins are directed into the proteasome core.

Their polypeptide chains are threaded into the proteolytic cylinder.

Catalytic sites face the hollow central core

The compartment generates small peptide products

Slide 55 · The proteasome produces short peptides

The source describes fragments of approximately 6–12 amino acids.

Further breakdown can release free amino acids for reuse.

Proteolysis supports amino-acid recycling

Fragment generation can also provide peptides for immune processing

Slide 56 · Proteasomal protein turnover closes a recycling loop

Unneeded protein

Misfolded or short-lived

Ubiquitin label

Selective targeting

Proteasome

Peptide generation

Recycling

Amino acid recovery

This is primarily a protein-centered degradation route.

Slide 57 · Proteasomal turnover regulates signaling

Destroying a regulatory protein can terminate or change a cell signal.

Transcription factors and signaling molecules are prominent examples.

Protein half-life can modulate cellular responses

Degradation is therefore a mechanism of regulation, not only waste removal

Slide 58 · Proteasomal peptides contribute to antigen processing

Proteasome-derived peptides can enter the MHC class I presentation pathway.

This links cytosolic protein turnover to adaptive immune recognition.

Antigen presentation is an additional use of generated peptides

The processing pathway is conceptually distinct from bulk lysosomal digestion

Slide 59 · Lysosomes and proteasomes: Distinct mechanisms

Characteristic

Lysosome

Proteasome

Primary cargo

Endocytic / phagocytic contents; Organelles

Selected intracellular proteins

Sorting route

Vesicle trafficking; Autophagy

Often ubiquitin labeling

Digestive machinery

Acid hydrolases

Core proteolytic activities

Physical context

Membrane-bound compartment

Multisubunit protein complex

Typical end products

Metabolites for salvage

Short peptides → amino acids

Slide 60 · How to assign a substrate to its pathway

Cellular problem

Predominant route

Rationale

Ingested bacterium

Phagolysosome

Particle engulfment

Damaged mitochondrion

Autophagy–lysosome

Whole organelle removal

Short-lived transcription factor

Ubiquitin–proteasome

Regulated protein turnover

Internalized soluble ligand

Endosome–lysosome

Endocytic entry

Slide 61 · Clinical extension: Lysosomal storage disorders

A missing or nonfunctional lysosomal component causes incomplete substrate degradation.

Storage results when substrates accumulate within lysosomes.

This is a failure of catabolism, not excess substrate production alone

Accumulation can compromise cellular function

Slide 62 · Primary accumulation in lysosomal storage disorders

Deficient enzyme

A step in catabolism fails

Undigested material

Incomplete breakdown

Lysosomal storage

Substrate builds up

Dysfunction

Enlarged compartments impair cell

Stored material depends on which degradative step is impaired.

Slide 63 · Secondary injury follows defective autophagic clearance

Lysosomal defect

Inefficient degradation

Autophagy impaired

Cargo turnover reduced

Damaged mitochondria

Poor quality control

Cell injury

Oxidative stress / death

The source links primary lysosomal storage with secondary organelle and protein accumulation.

Slide 64 · Why different storage disorders affect different organs

Stored substrate / function

Tissue consequence

Gangliosides enriched in neural tissue

Neuronal storage can predominate

Widely distributed mucopolysaccharides

Multisystem involvement

Macrophage-rich organs with stored substrates

Hepatic and splenic enlargement may occur

Slide 65 · I-cell disease: A protein-sorting defect

Golgi M6P defect

Recognition tag missing

Hydrolases misdirected

Enzymes may be secreted

Lysosomes under-supplied

Substrates remain undigested

Cell injury

Material accumulates

This illustrates why intracellular trafficking is integral to lysosomal function.

Slide 66 · Three mechanistic strategies in lysosomal storage disease

Therapeutic approaches illustrate the different failure points; Applicability is disease-specific.

Enzyme replacement

Supply functional activity

Substrate reduction

Lower the degradative load

Molecular chaperones

Stabilize eligible misfolded enzyme variants

Slide 67 · Clinical extension: Proteasome inhibitors in myeloma

Myeloma plasma cells synthesize large quantities of immunoglobulin chains.

Disturbed protein folding increases reliance on proteasomal disposal.

Blocking the proteasome can cause misfolded proteins to accumulate

The resulting stress can trigger apoptosis in susceptible cells

Slide 68 · Application 1: Loss of lysosomal targeting

CLINICAL APPLICATION

A cell synthesizes several acid hydrolases normally, but many fail to reach lysosomes and are secreted instead.

Which intracellular step is most directly defective?

A. Tagging or recognition of mannose-6-phosphate

B. E3-mediated ubiquitination

C. Formation of phagosomes

Slide 69 · Application 1: Mechanism explained

Correct answer: A — mannose-6-phosphate targeting

Golgi defect

Recognition marker absent

Enzyme misdirection

Hydrolases secreted

Lysosomal deficit

Catabolism impaired

M6P-mediated trafficking is essential even when hydrolase synthesis is intact.

Slide 70 · Application 2: Turnover of a regulatory protein

CLINICAL APPLICATION

A transcription factor must be rapidly removed once a cellular signal terminates.

Which pathway best provides selective protein turnover?

A. Phagocytosis followed by lysosomal fusion

B. E1/E2/E3-directed proteasomal degradation

C. Autophagy of an entire mitochondrion

Slide 71 · Application 2: Mechanism explained

Correct answer: B — ubiquitin–proteasome pathway

E3 substrate choice

Target selected

Polyubiquitin

Target labeled

Proteasome

Protein degraded

Selective turnover is a key mechanism for adjusting signaling-protein abundance.

Slide 72 · Application 3: Damaged mitochondria

CLINICAL APPLICATION

A stressed cell needs to remove a dysfunctional mitochondrion while recycling its constituent molecules.

Which pathway directly handles this whole-organelle cargo?

A. Autophagy followed by lysosomal fusion

B. Endocytosis of extracellular fluid

C. Direct proteasomal entry of the organelle

Slide 73 · Application 3: Mechanism explained

Correct answer: A — autophagic lysosomal turnover

Mitophagy

Mitochondrion enclosed

Fusion

Autophagosome + lysosome

Digestion

Components salvaged

Proteasomes act mainly on selected proteins, not intact organelles.

Slide 74 · Essential points to retain

Cellular waste disposal is selective and compartmentalized:

Lysosomes digest endocytic, phagocytic and autophagic cargo

Acid hydrolases require correct M6P-dependent trafficking

Autophagy supports organelle renewal and nutrient salvage

Ubiquitin–proteasome degradation controls protein quality and lifespan

Disrupted catabolism or targeting can produce cellular disease

Slide 75 · Sources and scope

Primary teaching source

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. 2026. Chapter 1, pp. 13–14; Fig. 1.10.

Correlative sections from the same textbook

Chapter 2, pp. 34–35: autophagy and isolation-membrane stages

Chapter 5, pp. 134–137: M6P trafficking and storage disorders

Chapter 13, p. 555: proteasome vulnerability in myeloma