**Core Concept**
Structural proteins, also known as supportive proteins, provide a framework for various tissues and organs, maintaining their shape, structure, and mechanical integrity. Examples include collagen, elastin, and keratin, which are crucial components of the extracellular matrix and play a vital role in tissue function.

**Why the Correct Answer is Right**
Supportive proteins like collagen and elastin are primary components of the extracellular matrix, which provides structural support to tissues, including skin, tendons, ligaments, and bones. They are synthesized by fibroblasts and other cells and are secreted into the extracellular space, where they form a network of fibers that provide mechanical strength and elasticity. For instance, collagen is a major component of connective tissue and is responsible for the tensile strength of skin, tendons, and ligaments.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not a supportive protein. While it may have some structural function, it is not a primary component of the extracellular matrix or a key player in maintaining tissue shape and structure.
* **Option B:** This option is incorrect because it is not a supportive protein. It may have some enzymatic function, but it is not a structural component of tissues or organs.
* **Option C:** This option is incorrect because it is not a supportive protein. It may have some role in cellular signaling, but it is not a structural component of tissues or organs.

**Clinical Pearl / High-Yield Fact**
Remember that structural proteins like collagen and elastin are crucial for maintaining tissue integrity and function. Their degradation or abnormalities can lead to various diseases, such as osteogenesis imperfecta (collagenopathy) and skin elasticity disorders (elastinopathy).

**Correct Answer:** D.