Understanding Protein Structure
Proteins have four levels of structure:
- 🧬 Primary: Straight chain of amino acids. The first amino acid is called N-terminal, and the last is C-terminal.
- 🌀 Secondary: Parts of the chain twist into a right-handed helix (like a spiral staircase) or fold into beta-pleated sheets (held by hydrogen bonds).
- 🎯 Tertiary: The whole chain folds into a 3D shape (like a crumpled ball). This is crucial for the protein’s job! Bonds like disulphide bonds stabilize it.
- 🧩 Quaternary: Multiple protein chains/subunits assemble together. Example: Hemoglobin has 4 subunits (2 α + 2 β).
What Are Enzymes?
Most enzymes are proteins (some special RNA called ribozymes act like enzymes too!). They:
- ⚡️ Speed up chemical reactions (called metabolic reactions).
- 🔑 Have an active site (a pocket where the reactant, called substrate (S), fits).
- 🌡️ Work best at mild temperatures (unlike metal catalysts). Enzymes from hot-springs microbes stay active even at 80–90°C!
How Enzymes Work
Enzymes lower the activation energy (energy needed to start a reaction):
- Substrate (S) enters the enzyme’s active site → forms enzyme-substrate complex (ES).
- S transforms into a transition state (unstable halfway structure).
- Bonds break/remake → product (P) forms → released. Enzyme is reused! 🔄
\[ \text{E} + \text{S} \rightleftharpoons \text{ES} \rightarrow \text{EP} \rightarrow \text{E} + \text{P} \]
Real-World Example
Reaction: \( \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \) (carbonic acid)
⏳ Without enzyme: 200 molecules/hour
⚡️ With enzyme (carbonic anhydrase): 600,000 molecules/second! (10 million times faster)
What Affects Enzyme Activity?
- 🌡️ Temperature:
- Peak activity at optimum temperature.
- Too cold → enzyme sleeps ❄️. Too hot → enzyme dies 🔥 (denatures).
- ⚖️ pH: Works best at optimum pH (e.g., stomach enzymes love acid!).
- 🧪 Substrate concentration:
- Activity ↑ as substrate ↑… until all enzymes are busy (saturation).
- Max speed = \( V_{\max} \) (see graph).
Enzyme Stoppers (Inhibitors)
Chemicals called inhibitors can block enzymes:
- 🥈 Competitive inhibitor:
- Fakes being the substrate 🤥 → blocks active site.
- Example: Malonate blocks succinate dehydrogenase (because they look alike!).
Enzyme Classes
Enzymes are grouped by the reactions they catalyze:
| Class | Reaction Type | Example Reaction |
|---|---|---|
| Oxidoreductases | Oxidation-reduction | \( \text{S}_{\text{reduced}} + \text{S’}_{\text{oxidized}} \rightarrow \text{S}_{\text{oxidized}} + \text{S’}_{\text{reduced}} \) |
| Transferases | Group transfer | \( \text{S-G} + \text{S’} \rightarrow \text{S} + \text{S’-G} \) |
| Hydrolases | Bond cleavage (using water) | Breaking peptides, sugars, etc. |
| Lyases | Remove groups (no water) | \( \ce{X-C-C-Y} \rightarrow \ce{X-Y + C=C} \) |
| Isomerases | Isomer conversion | e.g., Glucose ⇄ Fructose |
| Ligases | Join molecules | Form C-O, C-S, etc. bonds |
Enzyme Helpers (Co-factors)
Some enzymes need non-protein partners:
- 🔗 Prosthetic groups: Tightly bound helpers (e.g., haem in catalase).
- 🤝 Co-enzymes: Temporary helpers (e.g., NAD contains vitamin niacin).
- ⚡️ Metal ions: Zinc in carboxypeptidase.
Key terms: Enzyme protein = apoenzyme. Apoenzyme + co-factor = active enzyme!
Important Concepts for NEET
- 🔥 Activation energy & transition state: How enzymes speed up reactions by lowering energy barriers.
- 🎯 Enzyme inhibition (competitive): Malonate vs. succinate dehydrogenase example.
- 🧬 Protein structure levels: Especially tertiary (3D shape = functional enzyme!).
- ⚖️ Factors affecting activity: Temperature/pH optima, substrate saturation (\( V_{\max} \)).
- 🧪 Co-factors: Roles of prosthetic groups, co-enzymes (e.g., NAD), and metal ions.
Keep practicing enzyme graphs (Figure 9.5) and reaction cycles – you’ve got this! 💪
