## Core Concept
Hemoglobin and myoglobin are both proteins found in the body that bind to oxygen, but they have different structures and functions. Hemoglobin is primarily found in red blood cells and is responsible for transporting oxygen from the lungs to the body's tissues. Myoglobin, on the other hand, is found in muscles and stores oxygen for use during periods of high energy demand.

## Why the Correct Answer is Right
The correct answer, **D. Contain heme group**, characterizes both hemoglobin and myoglobin. Both of these proteins contain a **heme group**, which is a crucial component for their oxygen-binding capabilities. The heme group in both proteins consists of an iron ion (Fe2+) coordinated to a porphyrin ring, allowing them to reversibly bind oxygen.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but any statement not related to the common characteristics of hemoglobin and myoglobin would be incorrect.
- **Option B:** This option is not provided, but if it suggested a characteristic unique to one of the proteins, it would be incorrect.
- **Option C:** This option is not provided, but similar to option B, if it described a feature not shared by both proteins, it would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that while both hemoglobin and myoglobin contain a heme group and bind oxygen, their oxygen-binding curves are different. Hemoglobin has a sigmoid-shaped oxygen-binding curve, which allows for cooperative binding of oxygen and facilitates the release of oxygen to tissues. Myoglobin, however, has a hyperbolic oxygen-binding curve, indicating non-cooperative binding and a higher affinity for oxygen, which allows it to store oxygen effectively.

## Correct Answer: D. Contain heme group