17/4 Protein Structure & The ATP/ADP Translocase

Lecture Road-Map

• Recap: Lipids & membranes covered previously.

• Today’s focus: Proteins, especially a membrane transporter crucial for ATP production – the ATP/ADP translocase (ANT).

• Key objectives

  • Review all four levels of protein structure.

  • Identify the weak forces that stabilise folding.

  • Link structural elements to function.

  • Examine a real-world membrane protein example in depth.

Membrane Proteins: Big Picture

• Human genome can code for $\approx 20\,000$ proteins; a typical cell expresses $\approx 10\,000$ simultaneously.

• About $\approx 2\,000$ (~20 %) are membrane-associated.

• Functional classes

  • Receptors – detect extracellular signals (e.g. glucagon receptor).

  • Channels – passive ion flow.

  • Transporters – controlled, often energy-coupled movement of solutes (topic of today).

Levels of Protein Structure

Primary Structure

• Linear sequence of 20 canonical amino acids.

• Peptide bond

  • Formed via condensation (water released).

  • Partial double-bond character → planar and highly stable.

  • Broken only by strong acid (hydrolysis) or proteases (to be discussed in later lectures).

• SDS-PAGE detergents/emdenaturants disrupt higher-order folding but do not cleave peptide bonds.

Amino-Acid Categories & Chemical Forces

• Polar, uncharged (e.g. serine, threonine).

• Polar, charged

  • Positive: lysine, arginine, histidine.

  • Negative: aspartate, glutamate.

• Hydrophobic (non-polar): leucine, valine, isoleucine, phenylalanine, etc.

Key weak interactions that collectively drive folding:

• Hydrogen bonds.

• Electrostatic (ionic) interactions.

• van der Waals forces (temporary dipoles).

• Hydrophobic effect – burial of non-polar side chains away from solvent.

Secondary Structures

Alpha Helix

• Backbone hydrogen bonds (C=O(i) – H–N(i+4)).

• $3.6$ residues per turn; side chains project radially.

Beta Sheet

• Composed of beta strands (≈ 5–10 residues each).

• Parallel or antiparallel orientation; inter-strand H-bonding.

• Alternating side-chain orientation (up/down) across sheet.

Loops & Turns

• Connect secondary elements; variable length; often contain active-site or recognition residues.

Tertiary Structure

• 3-D arrangement of secondary motifs.

• Stabilising forces

  • Hydrophobic core.

  • Backbone/side-chain H-bonds & ionic pairs.

  • Disulphide bridges (Cys–Cys); covalent, formed by oxidation, common in extracellular or harsh environments. Broken in lab with $\beta$-mercaptoethanol or DTT.

• Motifs/repeats: helix–loop–helix, beta-barrels, etc.

  • Example: mechanosensitive E. coli channel with repeating helix-loop-helix units.

Quaternary Structure

• Association of multiple polypeptide chains.

• Homo-oligomers – identical subunits (e.g. aldolase tetramer studied in prac; bacterial GroEL chaperone).

• Hetero-oligomers – different subunits (e.g. $\mathrm{F}_1$-ATPase rotor/stator complex).

Structural Motifs in Membrane Proteins

• Transmembrane α-helices enriched in hydrophobic residues interact with lipid tails.

• Beta-barrel outer-membrane proteins; many bacterial toxins/pore-formers adopt this fold.

• Example shown: Cytochrome c oxidase – multiple helices span lipid bilayer; aqueous loops protrude into either side of membrane.

ATP/ADP Translocase (ANT)

Cellular Energy Context

• Human body stores only $\approx 250\,\text{g}$ ATP at any moment, yet turns over ≈ body mass worth ((\sim 70 \text{ kg}) per day.

• Majority of ATP synthesized in mitochondrial matrix; bulk demand is in cytosol → necessitates efficient nucleotide exchange.

Location & Abundance

• Integral protein of inner mitochondrial membrane (IMM).

• Constitutes ~10 % of total IMM protein.

Transport Stoichiometry & Driving Force

• 1 ADP(^{3-}) in ⇌ 1 ATP(^{4-}) out (strict 1:1 exchange).

• Electrogenic: exploits membrane potential (matrix negative, intermembrane space positive due to $H^+$ gradient).

  • Extra negative charge on ATP favours export.

• Separate phosphate carrier symports $\text{HPO}_4^{2-}$ + $H^+$ into matrix to replenish substrate for ATP synthase.

Structural Features

• Six transmembrane helices (H1–H6) form a barrel-like cavity.

• Two pseudo-symmetric repeats.

• Hydrophobic belt outward facing → contacts lipid core.

• Hydrophilic cavity interior → binds nucleotides.

• Key residues

  • Lysine/Arginine clusters at entrance grip phosphate groups.

  • Aromatic (Tyr) positions the adenine base.

• Two alternating conformations

  • m-state (matrix-open) – cavity accessible from matrix, closed to intermembrane space.

  • c-state (cytosol-open) – cavity accessible from intermembrane space, closed to matrix.

• Alternating-access rocker-switch mechanism

  1. ADP binds in c-state → conformational change → flips to m-state.

  2. ADP released in matrix; ATP binds same site.

  3. Second conformational change flips back to c-state; ATP released to intermembrane space/cytosol.

  4. Repeat – guarantees strict 1:1 exchange.

Lipid Interactions & Pathology

• Cardiolipin (tetra-acyl phospholipid enriched in IMM)

  • Stabilises ANT architecture.

  • Provides flexible annulus allowing large hinge motions.

  • Prevents misfolding/aggregation.

• Cardiolipin deficiency → severe energy shortage, muscle wasting (e.g. Barth syndrome).

Dynamic Evidence

• Recent X-ray & cryo-EM structures captured both m- and c-states (paper cited by lecturer).

• Molecular-dynamics movie shows ~$30\,\text{Å}$ translocation path, large domain motions, necessity for surrounding lipids.

Experimental Techniques & Advances

• Membrane protein structures historically difficult – require extraction into detergents or artificial membranes (micelles, nanodiscs, liposomes).

• X-ray crystallography demands high purity & crystals; could take years.

• Cryo-EM now allows lower purity, fewer particles, faster throughput.

• AlphaFold & other AI predictors provide accurate models (~95 % proteome coverage) – revolutionising preliminary structural insights.

Key Functional Principles Illustrated

• Structure ⇌ Function: precise 3-D arrangement of helices & charged pockets dictates selectivity and stoichiometry.

• Weak non-covalent forces cumulatively stabilise folded state; covalent disulphides add extra rigidity when needed.

• Alternating-access transporters prevent free diffusion, ensuring directional, regulated flux.

• Lipid–protein synergy: Specific phospholipids (cardiolipin) are co-factors for structural integrity and dynamics.

• Positively charged side chains (Lys, Arg) are almost universally employed for nucleotide phosphate binding.

Summary / Exam Pointers

• Be able to define and differentiate primary→quaternary structures, including forces involved.

• Recognise common secondary motifs (α-helix, β-sheet) and their residue preferences.

• Explain why hydrophobic residues localise within membranes or protein cores.

• Describe the mechanism and energetic basis of ATP/ADP exchange via ANT.

• Relate cardiolipin’s role to transporter stability and disease.

• Appreciate experimental challenges and modern solutions (cryo-EM, AlphaFold).

• Administrative: quiz details, documentation for clash, lecturer contact protocol.