Describe effect of pH on enzyme activity — Biology notes

WHAT is happening?

WHY does pH matter at all? Enzymes are proteins made of amino acids with ionisable side chains (e.g. –COOH and –NH₂ groups, plus residues like His, Asp, Glu, Lys). Whether these groups carry a charge depends on the surrounding $[\text{H}^+]$ :

An acidic side chain (–COOH) can lose a proton → –COO⁻ (negative) at high pH.
A basic side chain (–NH₂) can gain a proton → –NH₃⁺ (positive) at low pH.

These charges do two jobs:

Hold the 3D shape — opposite charges form ionic bonds and H-bonds that maintain the fold.
Do the catalysis — the active site often needs one group charged and another uncharged to grab the substrate and react with it.

So the right pH = right pattern of charges = right shape + right chemistry.

HOW pH changes the rate — step by step (Derivation of the bell curve)

We can reason the famous bell-shaped curve from first principles. Suppose catalysis needs:

one group that must be deprotonated (e.g. –COO⁻), and
one group that must be protonated (e.g. –NH₃⁺).

Call the active group's protonation a simple equilibrium: $\text{EH} \rightleftharpoons \text{E}^- + \text{H}^+, \qquad K_a = \frac{[\text{E}^-][\text{H}^+]}{[\text{EH}]}$

The fraction of enzyme in the catalytically active charge state for a single ionisable group follows the Henderson–Hasselbalch relation: $f_{\text{deprotonated}} = \frac{1}{1 + 10^{(pK_a - pH)}}, \qquad f_{\text{protonated}} = \frac{1}{1 + 10^{(pH - pK_a)}}$

Formula Why two groups → a peak

If activity needs BOTH a group deprotonated (with $pK_{a1}$ ) AND a group protonated (with $pK_{a2}$ ), the active fraction is the product: $\text{Activity} \propto \underbrace{\frac{1}{1 + 10^{(pK_{a1} - pH)}}}_{\text{rises with pH}} \times \underbrace{\frac{1}{1 + 10^{(pH - pK_{a2})}}}_{\text{falls with pH}}$ Why this step? One factor increases as pH rises (group gets deprotonated), the other decreases (group gets protonated less). Multiplying a rising term by a falling term gives a single hump — the bell curve. The peak sits between $pK_{a1}$ and $pK_{a2}$ , which is the pH optimum.

Figure — Describe effect of pH on enzyme activity

WHY enzymes have different optima (Dual coding: location → optimum)

The optimum matches where the enzyme normally works:

Enzyme	Location	Optimum pH	Why
Pepsin	Stomach	~2 (acidic)	Folded to work in gastric HCl
Salivary amylase	Mouth	~6.8 (near neutral)	Matches saliva
Trypsin	Small intestine	~8 (alkaline)	Matches bile-neutralised gut
Most cell enzymes	Cytoplasm	~7.0–7.4	Matches blood/cell pH

Worked Examples

Common Mistakes (Steel-man + Fix)

Active Recall

Recall What shape is the activity-vs-pH graph and why?

A bell curve with a single peak (the optimum). Because catalysis needs some groups protonated and others deprotonated — multiplying a rising charge-fraction by a falling one gives one hump.

Recall Why does extreme pH cause irreversible loss of activity?

Extreme $[\text{H}^+]$ disrupts the ionic and hydrogen bonds holding the tertiary structure → the protein unfolds (denatures) → active site shape destroyed → substrate can no longer bind.

Recall Two enzymes both labelled "active at pH 7" — does that mean same optimum?

No. They could share some activity at 7 but have different optima (e.g. 6.8 vs 8). Optimum = the peak, not just any point of activity.

What is an enzyme's pH optimum?

The pH at which it catalyses its reaction at maximum rate.

Why does pH affect enzyme activity?

It changes the charge on ionisable amino acid side chains, altering active-site shape and catalytic chemistry.

What shape is the activity vs pH graph?

A bell-shaped curve peaking at the optimum.

What is pH mathematically?

pH = −log10[H+]; each unit is a 10× change in [H+].

Optimum pH of pepsin and where it works?

~pH 2, in the stomach (gastric acid).

Optimum pH of trypsin and where it works?

~pH 8, in the small intestine.

Is a small pH change near the optimum reversible?

Yes — it only changes charge states, not the fold.

Why is extreme pH usually irreversible?

It breaks bonds holding tertiary structure → denaturation → active site destroyed.

Why does activity RISE as pH approaches the optimum from below?

Groups reach their correct protonation state, improving binding/catalysis.

Why do different enzymes have different optima?

Each is folded to be most active in its natural environment's pH.

Recall Feynman: explain to a 12-year-old

An enzyme is like a tiny machine made of beads (amino acids). Some beads can grab or drop little electric charges depending on how "sour" (acidic) or "soapy" (alkaline) the water is. The machine only works when the beads have just the right charges to hold its shape and grab its food. If the water is exactly right, it works super fast (that's the "best pH"). A little off, it slows down — but it can recover. WAY too sour or too soapy, the machine crumples up and is broken for good.

Connections

Enzyme structure and active site
Effect of temperature on enzyme activity (also a peaked curve, different cause)
Denaturation of proteins
Henderson–Hasselbalch equation
Digestive enzymes pepsin and trypsin
Lock and key vs induced fit model
Maintaining blood pH homeostasis

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, enzyme ek protein hota hai jo precise 3D shape mein folded rehta hai, aur uske active site par kuch amino acid ke side chains hote hain jinpar charge (+ ya −) baithta hai. Yeh charge depend karta hai surrounding [H+] par, yaani pH par. Jab pH bilkul sahi hota hai (optimum pH), tab charges perfect pattern mein hote hain, active site ki shape sahi rehti hai, aur enzyme maximum speed se kaam karta hai. Optimum se idhar-udhar jao to rate girta hai — isliye graph ek bell-shaped curve banta hai, single peak ke saath.

WHY bell curve? Kyunki catalysis ke liye ek group ko protonated (H+ ke saath) chahiye aur dusre ko deprotonated (H+ ke bina). pH badhao to ek factor badhta hai, dusra ghatta hai — dono ka product ek single hump deta hai. Peak un dono pKa ke beech mein aata hai, aur wahi optimum pH hai.

Important point: optimum ke aaspaas chhote pH changes sirf charge badalte hain, isliye yeh reversible hote hain — pH wapas sahi karo to activity wapas aa jaati hai. Lekin bahut zyada acidic ya alkaline pH protein ke ionic/H-bonds tod deta hai, protein unfold ho jata hai (denature), aur yeh usually irreversible hota hai — activity wapas nahi aati.

Har enzyme ka apna optimum hota hai jo uske ghar ke environment se match karta hai: pepsin stomach ke acid mein (pH ~2), trypsin intestine mein (pH ~8), aur zyadatar cell enzymes neutral (~7). Exam tip: "pH 7 hi best hota hai sabke liye" — yeh galat hai. Hamesha environment dekho aur peak dekho, na ki absolute pH.

Test yourself — Enzymes & Bioenergetics Basics

Connections