Temperature Effects on Marine Animals P2

The Effect of Temperature on Proteins

A. Protein Structure and Stability

• Proteins are made of polypeptide chains that fold into specific 3D shapes.

• Four levels of structure:

  1. Primary – Linear amino acid sequence.

  2. Secondary – Local folding into α-helices and β-sheets.

  3. Tertiary – Further folding into a 3D shape.

  4. Quaternary – Multiple polypeptide chains forming a functional protein.

• Protein stability is maintained by different bonds:

  • Covalent bonds (e.g., disulfide bridges).

  • Non-covalent bonds (e.g., hydrogen bonds, hydrophobic interactions).

B. How Heat Affects Protein Structure

• Higher temperatures break these bonds, leading to denaturation.

• Denaturation can be:

  • Partial or complete.

  • Reversible or irreversible, depending on the degree of disruption.

• Stages of Denaturation:

  1. Loss of quaternary structure – Protein subunits separate.

  2. Loss of tertiary structure – Protein starts unfolding.

  3. Loss of secondary structure – α-helices and β-sheets break down.

  4. Complete denaturation – No remaining functional protein.

C. Heat Shock Proteins (HSPs) – Protection Against Heat Stress

• HSPs help prevent or reverse protein denaturation.

• Produced rapidly (within minutes to hours) when an animal faces thermal stress.

• Function:

  • Assist in protein refolding (molecular chaperones).

  • Stabilize proteins during transport in the body.

  • Prevent protein aggregation and degradation.

• Different types of HSPs (named by size):

  • HSP60, HSP70, HSP90, HSP100 (common in animals).

  • HSPs in warm-adapted species have higher activation thresholds.

4. Enzyme Adaptation to Temperature

A. Isozymes – Temperature Adaptation at the Molecular Level

• Isozymes are different forms of the same enzyme that function optimally at different temperatures.

• Cold-adapted species use more flexible enzymes:

  • Weaker bonds → More flexibility → Function at lower temperatures.

  • Lower activation energy needed for reactions.

• Warm-adapted species have more rigid enzymes:

  • Stronger bonds → More stability at high temperatures.

B. Evolution of Thermal Stability

• Thermal stability of enzymes matches the animal’s environment.

• Example: Myofibrillar ATPase stability in fish

  • Measured in fish from different thermal environments:

    • Antarctic fish (-2 to 2°C) → Enzymes break down quickly at 37°C.

    • North Sea fish (2–18°C) → More stable but still degrades.

    • Tropical fish (5–25°C) → More heat-tolerant.

    • Hot spring fish (up to 50°C) → Most stable enzymes.

C. Counter-Gradient Variation Hypothesis

• Cold-adapted species often outperform warm-adapted species when moved to warmer waters.

• Example: Striped Bass Growth Rates

  • Fish from colder regions grew faster at all temperatures.

  • Suggests cold-adapted enzymes are inherently more efficient.

5. The Effect of Temperature on Biomembranes

A. Biomembrane Structure

• Composed of a phospholipid bilayer + membrane proteins.

• Functions of the biomembrane:

  • Defines the cell boundary.

  • Regulates transport of substances.

  • Maintains cell stability.

B. Homeoviscous Adaptation – Adjusting Membrane Fluidity

• Membranes must remain in a liquid-crystalline state to function.

• Cold temperatures → Membranes become rigid → Reduced function.

• Warm temperatures → Membranes become too fluid → Loss of integrity.

C. How Animals Maintain Membrane Fluidity

1. Changing fatty acid composition

   • Cold-adapted species: More unsaturated fatty acids → Increases fluidity.

   • Warm-adapted species: More saturated fatty acids → Increases stability.

2. Increasing sterol content (e.g., cholesterol)

   • Fills gaps in membranes, stabilizing structure.

3. Using enzymes (Desaturases) to modify lipids

   • Delta-9 desaturase introduces double bonds in fatty acids.

   • More enzyme activity at low temperatures → Increases unsaturated lipids.

D. Experimental Evidence of Homeoviscous Adaptation

• Example: Carp acclimation to cold water

  • Fish moved from 30°C to 10°C.

  • Rapid 8–10x increase in desaturase enzyme activity.

  • Helps modify membrane lipids to maintain fluidity.

6. Summary & Key Takeaways

A. How Temperature Affects Proteins & Enzymes

• High temperatures cause denaturation by breaking bonds.

• Heat shock proteins help prevent damage.

• Isozymes allow adaptation to different temperatures.

B. Enzyme Adaptations to Temperature

• Cold-adapted enzymes have higher binding affinity and turnover rates.

• Warm-adapted enzymes are more stable at high temperatures.

C. Biomembrane Adaptations

• Membranes must stay fluid for function.

• Cold-adapted species use more unsaturated fats to prevent freezing.

• Warm-adapted species use more saturated fats to maintain stability.

D. Climate Change Implications

• Warming oceans may exceed optimal enzyme temperatures.

• Cold-adapted species could be at risk if they cannot evolve quickly.