Terminology

Aerobic - Refers to the presence of oxygen. Organisms that use oxygen are called aerobic.

Aerobic Glycolysis - Refers to glycolysis which is occuring under conditions where the electron transport and oxidative phosphorylation are occurring. This means oxygen must be present.

Anabolic - Refers to metabolic processes which build larger molecules from smaller ones. Anabolic pathways require energy to be input. Gluconeogenesis is an anabolic process because it makes glucose from pyruvate.

Anaerobic - Refers to the absence of oxygen or the absence of a need for it; processes that must or can occur without oxygen are called anaerobic processes.

Anaerobic Glycolysis - Refers to glycolysis which is occurring under conditions in the absence of oxygen. When anaerobic glycolysis is occurring, NADH must be recycled to NAD+. This usually occurs by production of lactate from pyruvate (animals and some bacteria) or by production of ethanol from pyruvate (yeast).

Catabolic - Refers to metabolic processes which break down large molecules into smaller ones, releasing energy. Glycolysis is a catabolic pathway.

Covalent Modification - A means of regulating the activity of some enzymes. In the case of glycogen phosphorylase or pyruvate kinase, it involves putting on or taking off a phosphate group. When glycogen phosphorylase has phosphates on it, it is in the active form. On the other hand, when pyruvate kinase is phosphorylated, it is less active (Figure 13.18).

Feedforward Activation - Refers to metabolites in a pathway allosterically activating enzymes that are "ahead" of them in the pathway in which they are metabolized. For example, fructose-1,6-bisphosphate allosterically activates pyruvate kinase by a feedforward mechanism.

Fermentation - An energy-yielding metabolic pathway that involves no net change in oxidation state. Anaerobic glycolysis is an example of a fermentation.

Glycosidic bonds - Sugars, such as glucose that form rings create an asymmetric carbon called an anomer at the site of closing of the ring. This is carbon #1 for aldohexoses and carbon #2 for ketohexoses. If the hydroxyl group on the anomeric carbon is joined to another molecule, such as a sugar, so as to split out water, the resulting bond is called a glycosidic bond. Glycogen is composed of many glycosidic bonds between glucose units, because the bonds are formed through carbon #1 (the anomeric carbon of glucose).

Isozymes - Isozymes are molecules that are different forms of the same enzyme. For example, lactate dehydrogenase has several isozymes.

Kinase Cascade - The kinase cascade refers to the regulatory cascade involved in hormonal control of cellular actions mediated through cAMP. It is called a kinase cascade because the ultimate regulation is brought about as a result of the action of a series of kinase enzymes (which add phosphates to other molecules) (Figure 13.18).

Lactose Intolerance - Refers to a condition in humans that results from disappearance of the enzyme lactase from the intestinal mucosal cells after age 4 to 6. In these people, ingestion of milk products causes intestinal distress, because of bacterial action on the lactose that accumulates.

Limit Dextrin - Refers to the end product of digestion of amylopectin or glycogen by -amylase. -amylase is unable to cleave maltose units off at 1->6 branch point in the polysaccharides. The result of this digestion with the many branches of glucoses from branch points is called a limit dextrin.

Oxidative Phosphorylation - Refers to one of the three ways in which ATP is made from ADP in cells. Oxidative phosphorylation occurs in the mitochondria and requires both electron transport and oxygen. In oxidative phosphorylation, ATP synthesis is powered by a proton gradient created by the electron transport system. See here.

Pasteur Effect - Refers to an observation of Louis Pasteur that when anaerobic yeast cultures that are metabolizing glucose are exposed to air, the rate of glucose utilization decreases dramatically. This is due to the fact that in air, aerobic glycolysis is favored and it is much more efficient at producing ATP than anaerobic glycolysis.

Phosphorylation - Refers simply to the addition of a phosphate to a molecule. Enzymes that add phosphates to molecules are called kinases. Phosphorylation of ADP to form ATP is an important reaction for cells. Phosphorylation of enzymes, such as glycogen phophorylase, is important for controlling their activity.

Photophosphorylation - One of three cellular mechanisms for making ATP from ADP. Phosphorylation occurs in plants and photosynthetic bacteria. Photophosphorylation of ADP to ATP depends directly on energy from sunlight. The light energy is captured by a pigment such as chlorophyll and is passed in the form of excited electrons to an electron transport chain; the electron transport chain uses energy from the electrons to create a proton gradient across a membrane, which drives the synthesis of ATP.

Regulation - The term regulation has many possible meanings. With respect to enzymatic action, regulation refers to anything that affects the activity of the enzyme.

Respiration - With respect to energy metabolism, the process in which cellular energy is generated through the oxidation of nutrient molecules with O 2 as the ultimate electron acceptor. This type of respiration is also called cellular respiration to distinguish it from respiration in the sense of breathing.

Substrate level phosphorylation - One of three cellular mechanisms for making ATP from ADP. Involves a high energy phosphorylated intermediate which tranfers a phosphate to ADP to form ATP. The reaction catalyzed by pyruvate kinase is a substrate level phosphorylation because phosphate is transferred from PEP to ADP to form ATP and pyruvate (Table 13.1).