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Functional Roles of Proteins in Humans
Page 24 Polyacrylamide Gel Electrophoresis in the Presence of a Detergent 72 HPLC Chromatographic Techniques Separate Amino Acids, Peptides, and Proteins 72 Affinity Chromatography 73 General Approach to Protein Purification 73 Determination of Amino Acid Composition of a Protein 74 Techniques to Determine Amino Acid Sequence of a Protein 74 XRay Diffraction Techniques Are Used to Determine the Three Dimensional Structure of Proteins 76 Various Spectroscopic Methods Are Employed in Evaluating Protein Structure and Function 79 Ultraviolet Light Spectroscopy 79 Fluorescence Spectroscopy 79 Optical Rotatory Dispersion and Circular Dichroism Spectroscopy 80 Nuclear Magnetic Resonance 81 Bibliography 82 Questions and Answers Clinical Correlations 83 2.1 Plasma Proteins in Diagnosis of Disease 37 2.2 Differences in Primary Structure of Insulins used in Treatment of Diabetes Mellitus 41 2.3 A Nonconservative Mutation Occurs in Sickle Cell Anemia 42 2.4 Symptoms of Diseases of Abnormal Collagen Synthesis 50 2.5 Hyperlipidemias 56 2.6 Hypolipoproteinemias 59 2.7 Glycosylated Hemoglobin, HbA1c 62 2.8 Use of Amino Acid Analysis in Diagnosis of Disease 74 2.1— Functional Roles of Proteins in Humans Proteins perform a surprising variety of essential functions in mammalian organisms. These may be grouped into dynamic and structural. Dynamic functions include transport, metabolic control, contraction, and catalysis of chemical transformations. In their structural functions, proteins provide the matrix for bone and connective tissue, giving structure and form to the human organism. An important class of dynamic proteins are the enzymes. They catalyze chemical reactions, converting a substrate to a product at the enzyme's active site. Almost all of the thousands of chemical reactions that occur in living organisms require a specific enzyme catalyst to ensure that reactions occur at a rate compatible with life. The character of any cell is based on its particular chemistry, which is determined by its specific enzyme composition. Genetic traits are expressed through synthesis of enzymes, which catalyze reactions that establish the phenotype. Many genetic diseases result from altered levels of enzyme production or specific alterations to their amino acid sequence. Transport is another major function for proteins. Particular examples discussed in greater detail in this text are hemoglobin and myoglobin, which transport oxygen in blood and in muscle, respectively. Transferrin transports iron in blood. Transport proteins bind and carry steroid hormones in blood from their site of synthesis to their site of action. Many drugs and toxic compounds are transported bound to proteins. Proteins participate in contractile mechanisms. Myosin and actin function in muscle contraction. Proteins have a protective role through a combination of dynamic functions. Immunoglobulins and interferon are proteins that protect the human against bacterial or viral infection. Fibrin stops the loss of blood on injury to the vascular system. Many hormones are proteins or peptides. Protein hormones include insulin, thyrotropin, somatotropin (growth hormone), luteinizing hormone, and folliclestimulating hormone. Many diverse polypeptide hormones have a low molecular weight (<5000) and are referred to as peptides. In general, the term protein is used for molecules composed of over 50 amino acids and the term peptide is used for molecules of less than 50 amino acids. Important peptide hormones include adrenocorticotropin hormone, antidiuretic hormone, glucagon, and calcitonin. Proteins control and regulate gene transcription and translation. These include histones that are closely associated with DNA, repressor and enhancer transcription factors that control gene transcription, and proteins that form a part of the heteronuclear RNA particles and ribosomes.