Info

Calories/hour/ pound of body weight

Figure 2.8 Energy expenditures for various activities. This bar graph can assist you in determining how many calories you burn.

where the substrate binds is called the enzyme's active site. The enzyme binds its substrate, helps convert it to a reaction product, and then resumes its original shape so it can perform the reaction again (Figure 2.9).

Lactose intolerance is a common dietary problem caused by an enzyme deficiency. People with lactose intolerance are unable to digest large amounts of lactose (the most common sugar in milk) which is the result of a shortage of the enzyme lactase, typically produced by the cells of the small intestine. This enzyme breaks down lactose into simpler forms that can then be absorbed into the bloodstream. When there is not enough lactase available to digest lactose, nausea, cramps, bloating, gas, and diarrhea can occur. For reasons scientists do not yet understand, the lactase enzyme becomes less efficient as we age.

The speed and efficiency of the many different enzymes will lead to an overall increase or decrease in metabolic rate. Thus, when you say that your metabolism is slow or fast, you are actually referring to the speed at which enzymes catalyze chemical reactions in your body.

The properties of metabolic enzymes, like those of all proteins, are determined by the genes that encode them. Genes are passed from parents to children; they are large molecules located inside your cells that contain protein-building instructions. This is why the genes you inherit influence your rate of fat storage and utilization. In addition, if an enzyme requires a vitamin to act as a coen-zyme and that vitamin is not available, metabolism may be slowed.

All of these variables help explain why some people seem to eat and eat and never gain an ounce, while others struggle with their weight for their entire lives. To obtain a rough measure of how many Calories you should consume per day, multiply the weight you wish to maintain by 15, and add the number of Calories you burn during exercise. If you are trying to lose weight, decrease your caloric intake or increase your exercise level. Losing one pound of fat requires you to burn 3500 Calories.

Regardless of your metabolic rate, all food must be converted into a form of energy your body can use.

Active site

Enzyme sucrose

Substrate sucrose

Glucose

Fructose

Active site

Enzyme sucrose

Substrate sucrose

Glucose

Fructose

1. The shape of the substrate matches the shape of the enzyme's active site.

2. When the substrate binds to the active site, the enzyme changes shape and the bond between the sugars is stressed.

3. The shape change splits the substrate and releases the two subunits. The enzyme is able to perform the reaction again.

1. The shape of the substrate matches the shape of the enzyme's active site.

2. When the substrate binds to the active site, the enzyme changes shape and the bond between the sugars is stressed.

3. The shape change splits the substrate and releases the two subunits. The enzyme is able to perform the reaction again.

Figure 2.9 Enzymes. The enzyme sucrase is cleaving (splitting) the disaccharide sucrose into its monosaccharide subunits, fructose and glucose. The enzyme can then be recycled to perform the same reaction over and over again.

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