Many proteins can be classified into two distinct subtypes - globular or fibrous. These two types differ in their shapes and solubilities. Globular proteins are also known as spheroproteins and typically are approximately round in shape. They contain a mix of amino acid types and contain differing sequences in their primary structures. Globular proteins have many different functions, such as enzymes, cellular messengers, and molecular transporters. These roles often require the proteins to be soluble in the aqueous cellular environment. They are also sensitive to changes in their environment, such as pH and temperature. Hemoglobin, immunoglobulin, and protein kinase A are examples of globular proteins. Fibrous proteins are either long and narrow proteins or assemble to form long and thin structures. They are may contain repetitive units and usually consist either of alpha helices or beta sheets and, in rare cases, a mix of both. The amino acids in the primary structure often consist of repeating amino acid sequences. The role of fibrous proteins is primarily structural. Many are located in the extracellular matrix and are present in connective tissues to impart strength and joint mobility. They are not typically soluble in water; however, they may be soluble in strong acids or bases. Collagen, keratin, elastin, silk, and fibrin are examples of fibrous proteins.