Covalent Regulation of Enzyme Activity

There are two basic types of covalent control of enzyme activity reversible and irreversible. An example of irreversible activation is the proteolytic cleavage of proenzymes in the digestive tract that lead to the active forms of trypsin and chymotrypsin. The activation of these proteolytic enzymes outside the cell is critical, as the presence of active enzymes within the cell could lead to the unwanted degradation of cellular components, or to the complete digestion of the contents of the...

Clinical Box 61 In Vivo Labeling of Cells

Cells monitor their environment through receptor proteins embedded in the cell membrane. Receptor proteins have several attached molecules of sugar or oligosaccharide chains protruding into the extracellular space. These oligosaccharide chains or glycans serve as binding points for ligands to the receptor protein. Developing the ability to detect glycans specific to a particular type of receptors or cells in the living patient would be a major advance in diagnosis and therapy. A recent study...

Nuclear Receptors

Endocrine glands secrete products called hormones into the circulatory system and these are eventually brought into contact with their target tissues. Certain types of hormones, for example steroid hormones, can freely pass the plasma membrane of the target cell and then act by binding to intracellular receptors. The complex of hormone and receptor is then translocated into the nucleus of target cells, where it affects the transcription of genes by binding to certain DNA sequences called...

Signaling Molecules

Neurotransmitters vary greatly in their composition. Acetylcholine, the biogenic amines, and some amino acids form the so-called classical neurotransmitters (because they were first to be discovered and studied extensively see Sec. 6.3.1). Other neurotransmitters are the neuropeptides and neurosteroids (see Sec. 6.3.2). Finally, recent research has revealed that neurons can also release gaseous transmitters such as nitric oxide or carbon monoxide to affect nearby nerve cells. They constitute...

Clinical Box 31 Alcohol Sensitivity in East Asian Populations

The majority of beverage-derived ethanol is metabolized by two enzyme systems in the liver alcohol dehydrogenase and aldehyde dehydrogenase. Fthanol l Acetaldehyde I Acetate Alcohol Dehydrogenase Aldehyde Dehydrogenase Both enzymes utilize a coenzyme, NAD+, as the electron acceptor in order to catalyze the oxidation of their substrates. Humans possess multiple forms of both enzymes these are termed isoenzymes. Isoenzymes are related gene products that catalyze similar reactions, but do so with...

Carbohydrates and Their Polymers 141 Monosaccharides

Like amino acids there are a large number of significant carbohydrates found in nature. Carbohydrates are known either as aldoses or as alde-hydic polyols because of the aldehydic carbonyl at carbon atom 1 (C1) and extensive hydroxyl (OH) substitution. Carbohydrates also exist as ketoses having an internal carbonyl or keto group ( O) generally at C2, and again, many alcohol (OH) substituents on the carbon chain. Figure 1.9 illustrates these structures for a 6-carbon aldohexose and a 5-carbon...

Nonmembrane Bound Organelles 231 Ribosomes

As mentioned above, ribosomes are organelles used to help form proteins. Instead of being composed of, or contained in, a membrane, ribosomes are proteins combined with RNA molecules. The ribosome is made of two subunit pieces which come together to facilitate formation of a chain of amino acids into a peptide and then often into a larger protein. Ribosomes have a large subunit and a small subunit. A ribosome that is attached to the endoplasmic reticulum (ER) is called bound ribosome, while...

Cell Membrane

From the early days of the microscope, the cell has been differentiated as having an outer boundary membrane (the cell or plasma membrane) containing a heterogeneous soup (cytoplasm) and a nucleus. As microscopy has improved, more details have emerged from the cellular morass to be seen as distinct objects. Through study of pathologic cells and experimentation, we have come to learn about the function of these objects called organelles. For instance, a pathologic cell deficient in the organelle...