For a protein to have a quaternary structure, it must :
(a) consist of two or more polypeptide subunits
(b) consist of four polypeptide subunits
(c) have four domains
(d) exist in several alternative conformational states

Options

A. consist of two or more polypeptide subunits
B. consist of four polypeptide subunits
C. have four domains
D. exist in several alternative conformational states

Explanation

To determine the correct answer to the question regarding the quaternary structure of proteins, we can analyze each option step by step.

### Step 1: Understand the Definition of Quaternary Structure
Quaternary structure refers to the complex arrangement of multiple polypeptide chains (subunits) that come together to form a functional protein. This structure is stabilized by various interactions among the subunits.

**Hint:** Remember that quaternary structure involves multiple polypeptide chains interacting with each other.

### Step 2: Evaluate Option (a)
Option (a) states that a protein must consist of two or more polypeptide subunits to have a quaternary structure. This aligns with the definition of quaternary structure, as it specifically involves multiple polypeptide chains coming together.

**Hint:** Look for the minimum number of polypeptide chains needed for quaternary structure.

### Step 3: Evaluate Option (b)
Option (b) states that a protein must consist of four polypeptide subunits. While some proteins do have four subunits, this is not a requirement for quaternary structure. Proteins can have two, three, or more than four subunits.

**Hint:** Consider whether the number of subunits is fixed or variable in quaternary structures.

### Step 4: Evaluate Option (c)
Option (c) states that a protein must have four domains. However, the presence of four domains does not necessarily indicate a quaternary structure, as domains can exist within a single polypeptide chain. Therefore, this option is not correct.

**Hint:** Distinguish between domains and subunits in protein structure.

### Step 5: Evaluate Option (d)
Option (d) states that a protein must exist in several alternative conformational states. While some proteins can adopt multiple conformations, this is not a defining characteristic of quaternary structure. Quaternary structure specifically refers to the arrangement of subunits, not their conformational states.

**Hint:** Focus on the structural aspect of quaternary structure rather than conformational flexibility.

### Conclusion
After evaluating all options, the correct answer is (a) consist of two or more polypeptide subunits. This is the defining characteristic of a protein's quaternary structure.

**Final Answer:** (a) consist of two or more polypeptide subunits.